Running head: ATTENTIONAL AND OCULOMOTOR CAPTURE

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## Characteristics of Covert and Overt Visual Orienting:

Evidence from Attentional and Oculomotor Capture

## Shu-Chieh Wu and Roger W. Remington

## NASA Ames Research Center

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## Final Version May 19, 2003

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## Abstract

Five visual search experiments found oculomotor and attentional capture
consistent with predictions of contingent orienting, contrary to claims
that oculomotor capture is purely stimulus driven. Separate saccade and
attend-only conditions contained a color target appearing either singly,
with an onset or color distractor, or both. In singleton mode, onsets
produced oculomotor and attentional capture. In feature mode capture was
absent or greatly reduced, providing evidence for top-down modulation of
both types of capture. Though attentional capture by color distractors
was present throughout, oculomotor capture by color occurred only when
accompanied by transient change, providing evidence for a dissociation
between oculomotor and attentional capture. Oculomotor and attentional
capture appear to be mediated by top-down attentional control settings,
but transient change may be necessary for oculomotor capture.

## Characteristics of Covert and Overt Orienting:

## Evidence from Attentional and Oculomotor Capture

Orienting in the visual world is accomplished by overt shifts of eye
fixation in coordination with covert shifts of spatial attention (e.g.,
Posner, 1980). Several lines of evidence suggest that the two are
controlled by a common set of mechanisms. For example, attention and
saccades are closely coupled in everyday activities, such as reading
(e.g., Rayner & Pollatsek, 1989). Though attention can be shifted while
the eyes remain fixated, it cannot be shifted independently when a
saccade is made. Attention has been observed to move ahead of voluntary
saccades to the saccade target, even when the incentive is to withhold
the shift, or to shift attention to another location (Posner, 1980;
Remington, 1980; Kowler, Anderson, Dosher, & Blaser, 1995; Hoffman &
Subramaniam, 1995; Irwin & Gordon, 1998; see Hoffman, 1998 for a
review). The locus of attention has also been shown to bias saccade
trajectories (Sheliga, Riggio, & Rizzolatti, 1994; Sheliga, Riggio,
Craighero, & Rizzolatti, 1995), and may even play an active role in
saccade target localization (Kowler, et al., 1995). These behavioral
observations converge with neurophysiological and neuropsychological
findings that show overlapping brain areas activated by both attention
and saccades (e.g., Corbetta, 1998; Corbetta, Akbudak, Conturo, Snyder,
Ollinger, Drury, et al., 1998; Nobre, Gitelman, Dias, & Mesulam, 2000;
Bushnell, Goldberg, & Robinson, 1981; Andersen, 1995; Wurtz, Sommer,
Pare, & Ferraina, 2001). The close coupling between attention and
saccades is reflected in pre-motor theories of covert attention, which
argue that spatial attention is controlled by the same mechanisms that
govern saccades (Klein, 1980; Klein & Pontefract, 1994; Rizzolatti,
Riggio, & Sheliga, 1994; Rizzolatti, Riggio, Dascola, & Umilta, 1987).

However, the strong behavioral evidence for common control of saccades
and attention has come essentially from dual-task conditions
demonstrating an inability to attend to one target concurrent with a
saccade to another (e.g., Hoffman & Subramaniam, 1995; Kowler, et al.,
1995). An inability to independently allocate attention and execute
saccades need not imply a common control mechanism, but could reflect
conflict for a common resource, such as a common spatial map, that need
not be involved directly in the control. Indeed, interference is often
observed in dual-task conditions even when the control of the respective
responses are independent to a large degree, as when a vocal response to
one task is paired with a manual response to the other (e.g., McCann,
Remington, & Van Selst, 2000). It is difficult to see how dual-task
methods that require simultaneous, incompatible shifts of the eyes and
attention would allow one to dissociate interference arising from common
control mechanisms as opposed to a shared common resource.

Here we approach the issue of common control by examining whether
involuntary responses of saccades and attention are sensitive to the
same stimulus conditions. Visual search (e.g., Yantis & Jonides, 1984;
Theeuwes, 1991) and spatial cueing (e.g., Folk, Remington, & Johnston,
1992) experiments have shown that under some conditions an irrelevant
peripheral stimulus will involuntary capture spatial attention, causing
it to be shifted to the location of the distracting stimulus. If the
same control mechanism underlies attention and saccades then stimulus
properties that elicit involuntary attention shifts should also elicit
involuntary saccades. Conversely, stimulus conditions that fail to
elicit capture of one should also fail to elicit capture from the other.
Following this logic, the experiments reported here test whether the
same contingencies that affect attentional capture also affect
oculomotor capture. Folk, Remington, & Johnston (1992) have shown that
task goals actively influence which stimuli will capture attention and
which will not. According to Folk et al., capture is not a completely
stimulus-driven process in response to particular stimulus features such
as abrupt onsets (Yantis & Jonides, 1984) or to salience (Theeuwes,
1991), but is contingent on behavioral goals. For example, they showed
that abrupt onsets would capture attention when the target was also
defined by onset, but not when the target was defined as a color
singleton. If saccades and attention are controlled by the same
mechanisms then the same contingencies should be observed when saccades
are involuntarily elicited by a peripheral stimulus. In particular, it
would be expected that top-down and bottom-up factors that affect the
capture of attention would affect the capture of saccades.

### Attentional and Oculomotor Capture

The issue of common control bears directly on the debate over the role
of top-down modulation of attentional capture. Recent results by
Theeuwes and colleagues (Theeuwes, Kramer, Hahn, & Irwin, 1998;
Theeuwes, Kramer, Hahn, Irwin, & Zelinsky, 1999) have been interpreted
as support for the hypothesis that attentional capture is purely
stimulus-driven and not modulated by top-down goals. For example,
Theeuwes et al. (1998) found that, in a visual search task that required
a saccade to a uniquely colored target, half of the initial saccades
were directed instead toward a task-irrelevant onset distractor.
Theeuwes et al. (1999) argued that this oculomotor capture reflected
involuntary shifts of attention to the abrupt onset object, as evidenced
by further increased reaction times when the onset distractor was
associated with a response incompatible with the target response. Irwin,
Colcombe, Kramer, and Hahn (2000) found that new objects defined by
abrupt onsets produced more oculomotor capture than luminance increments
of existing objects, but found no capture by color singletons. Theeuwes
et al. and Irwin et al. argue that abrupt onsets define new objects
which will capture attention and the eyes involuntarily despite a
top-down attentional set for color singletons.

Since the target property in Theeuwes et al. and Irwin et al.'s
experiments was color, not onset, those results pose a challenge for the
contingent orienting theory of Folk et al (1992; see Theeuwes et al.,
1998; 1999; Irwin et al., 2000). This challenge relies on the assumption
that oculomotor capture is a direct reflection of attentional capture.
In particular it rests on the twin assumptions that the eyes are
captured because attention is captured, and that the requirement to
saccade or remain fixated has no effect on the conditions that will
elicit attentional capture. Neither assumption has yet been tested.
Though attention and saccade tasks recruit a highly overlapping network
of cortical areas, there is evidence that these areas are utilized to
different degrees in each task (e.g., Corbetta et al., 1998; Nobre et
al., 2000), suggesting some separation of functional control mechanisms.
If the control mechanisms of saccades and attention were separable, then
it is possible that response characteristics observed under saccade
conditions reflect the behavior of the saccade control system, rather
than the spatial attention control system. This would mean that
properties that elicit involuntary saccades (and accompanying attention
shifts) would not necessarily elicit attentional capture. Thus, patterns
of oculomotor capture would not necessarily be indicative of failure of
top-down control of attention.

The challenge to contingent capture posed by the oculomotor capture
results can be criticized on other grounds as well. The procedures used
by Theeuwes et al. (1998; 1999) and Irwin et al. (2000) are similar to
an earlier study by Theeuwes (1992, Experiment 1) where participants
displayed attentional capture by irrelevant color singletons when
searching for a form singleton. Bacon and Egeth (1994) showed that this
method promotes a singleton detection strategy in which subjects respond
to the presence of any unique singleton stimulus even when in another
dimension. When they removed the singleton status of the target in
Theeuwes's design, color singletons no longer captured attention. Based
on those results, they proposed a distinction between feature search and
singleton detection modes. Not only do Bacon and Egeth's (1994) results
clearly demonstrate that feature singletons do not necessarily capture
attention in a stimulus-driven manner, they may also provide an
explanation for the oculomotor capture by abrupt onsets observed by
Theeuwes et al. (1998; 1999) and Irwin et al. (2000). It is possible
that abrupt onsets captured attention and the eyes in those studies
because they fit an attentional set for singletons, not because they
defined new objects. This possibility may also explain why Theeuwes et
al. (1999) found an attention shift toward oculomotor distractors even
though some evidence suggests that attention may not precede involuntary
saccades such as those characteristic of oculomotor capture (Mokler,
Deubel, & Fischer, 2000). Oculomotor capture has yet to be tested under
feature search conditions as defined by Bacon and Egeth (1994).

### Present Experiments

In the experiments reported here we explore the relationship between
attention and saccades by examining whether oculomotor and attentional
capture respond similarly to the same stimulus conditions. Specifically,
the experiments attempted to answer three related questions:

1)  Do abrupt onsets have a unique ability to elicit attentional and
    oculomotor capture, so that they will do so when conditions for
    singleton detection mode are removed?

2)  Will feature singleton distractors, such as color singletons elicit
    oculomotor capture?

3)  Are the eliciting conditions for oculomotor capture identical to
    those for attentional capture?

Each of these questions is a part of the larger question of whether
oculomotor capture, like attentional capture, is subject to top-down
modulation, or whether it is purely stimulus-driven. Evidence that
oculomotor capture is purely stimulus-driven, and that a clear link
exists between it and attentional capture, would strengthen the case for
stimulus-driven attentional capture as well as for a common control
mechanism for attention and saccades.

To address these issues we compare rates of oculomotor capture in
saccade conditions to the response time effects in no-saccade
conditions. Thus, all experiments reported here included two parallel
task conditions, attention and saccade, with identical designs. The
logic of the experiments was to manipulate the amount of attentional
capture through experimental designs that encourage different modes of
search and then evaluate the impact on the occurrence of oculomotor
capture.

## Experiment 1

Experiment 1 included a saccade task condition that replicated the
experiment of Theeuwes et al. (1998) save for the choice of colors. In
addition, we collected reaction time data from a parallel condition of
identical experimental design that required no saccades.

### Method

[Participants]{.underline}. Twenty undergraduate students recruited from
local colleges near NASA Ames Research Center participated either for
course credit or for moderate monetary compensations. Twelve completed
the attention task condition; eight completed the saccade task
condition. All participants reported normal or corrected-to-normal
vision and no color blindness.

[Apparatus]{.underline}. An IBM compatible Pentium II personal computer
controlled display of stimuli on a 21 inch SVGA monitor, collection of
keyboard responses, and storage of data. Participants were seated in a
comfortable chair with their head secured on a head and chin rest placed
53.5 cm in front of the monitor. They made manual responses with index
and middle fingers of their right hand using the "." and "/" keys on a
PC keyboard. Eye movements were recorded with an infra-red video-based
eye tracking system (ISCAN), which outputs data at a temporal resolution
of 120 Hz and a spatial resolution of approximately 0.5 degree visual
angle. The system tracked eye positions by computing the distance
between the centers of the pupil and cornea reflection of the left eye.
The experiment was carried out in a dedicated, sound-attenuating booth.

[Stimuli]{.underline}. The stimulus display consisted of a 0.54° central
fixation cross, a 1.5° circle surrounding the fixation (to serve as a
warning signal), and six circles arranged with equal spacing along the
circumference of a large imaginary circle centered at fixation. The six
circles appeared at 0, 60, 120, 180, 240, and 300 angular degrees from
straight right. Each circle contained a stimulus figure. In the saccade
task, the radius of the large circle was 10.08 degrees of visual angle,
for the color circles 1.48 degrees, and for the stimulus figures 0.4
degrees. In the attention task, the radius of the large circle was 3.4
degrees, for the color circles 0.7 degrees, and for the stimulus figures
0.8 degrees. Colors used in the experiment, green and red, were
pre-matched for luminance using a homemade program, which displays a
spinning wheel consisted of pie pieces in alternating colors (in this
case, red and green) and creates an illusion of apparent spinning motion
when the two colors are unequal in luminance. Equiluminance was achieved
by adjusting the RGB values of each color to annual that illusion. The
wheel configuration was presented foveally.

[Design]{.underline}. There were two task conditions, *attention* and
*saccade*. In both task conditions, half of the trials contained an
abrupt onset distractor. Targets appeared equally often in all six
locations. The onset distractor, when present, could appear at 90, 150,
210, or 270 degrees counterclockwise away from the target, resulting
angular distances 90 and 150 degrees. In Euclidean distances, these
distances corresponded to 14.25 and 19.47 degrees of visual angle in the
saccade task condition and 4.81 and 6.57 degrees in the attention task
condition, respectively. The onset distractor was inserted between two
circles along the large imaginary circle.

All circles contained a letter O as a placeholder when a trial began.
When the target location was revealed through color change, an arc
segment was removed from each of the letter O placeholders. In the
target circle, an arc segment was removed from either the right or left
side of the letter O, resulting in figures resembling a C or a reversed
C. Non-target circles, including the circle appearing with the onset
distractor, had a segment removed from either the top or bottom of the
letter O, resulting in figures resembling a C rotated in 90 or 270
degrees. The two possible probe directions, forward or reversed C,
appeared equally often. There were a total of 384 trials divided into 3
blocks in each task condition. Trials within a task condition across
different trial conditions were completely randomized and presented to
each participant in a different order.

[Procedure]{.underline}. Prior to the first block of the attention and
the saccade task, participants received 36 practice trials. Each of the
following two blocks was preceded by three dummy trials from which data
were discarded. The attention task condition lasted about 15 minutes;
the saccade task condition lasted between 20 to 40 minutes.

A calibration procedure was administered before each block of trials in
the saccade task condition. Participants fixated two sets of nine
calibration targets (crosshairs) presented serially in a 3 x 3 grid in a
random order across the display. After the first set, calibration
parameters that mapped eye positions to screen coordinates in pixels
were computed using a non-linear algorithm. Screen pixel coordinates for
each fixation measured in the second set were then computed using these
parameters and compared against the actual locations of the targets in
view. Calibration continued until no measured position was more than 1.5
degrees away from the actual position.

In both attention and saccade task conditions participants were required
to determine whether the stimulus figure inside the target circle was a
forward or reversed C. For the forward C, subjects responded by pressing
"/" key with the middle finger of the right hand; for the reversed C
they pressed the "." key with the index finger of the right hand. In
order to minimize the tendency to base responses on the distinction
between alphabet and non-alphabet, the reference to letter C was avoided
in the instruction and participants were instead told to determine the
opening of a small circle inside the target circle. Eye movements were
not monitored in the attention task condition, but participants were
asked to maintain fixation at the center cross.

The attention and saccade tasks contained a common sequence of events
(Figure 1) but with different timing. In the attention task condition, a
trial began with an initial display of a white fixation cross, a small
white circle surrounding the cross, and six green circles arranged along
a large imaginary circle centering at fixation. Inside each of the six
circles was a green letter O serving as placeholder for the upcoming
stimulus. The initial display was presented for one second, followed by
the removal of the central circle to signal the forthcoming target
display. After 500 ms five of the six green circles turned red,
revealing the target display. On half the trials an onset distractor (an
additional red circle) was presented simultaneously with the change to
the target display. Simultaneously with the color change, a small arc
segment from each of the letter O's was removed, and the altered
characters remained on for 200 ms before being erased. Participants were
told to report the location of the opening in the character inside the
remaining green circle. They were instructed to shift attention to the
remaining green circle without shifting their gaze from the central
fixation marker.

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Insert Figure 1 here

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In the saccade condition, the initial display remained until
participants had maintained fixation within six degrees radius around
the fixation cross for one full second. Stimulus figures inside the
color circles remained visible until the end of a trial.

### Results and Discussion

For the attention task condition, trials with incorrect responses or
reaction times longer than 1000 ms were excluded from further analyses;
such trials accounted for 6% of total trials. For the saccade task
condition, trials with incorrect manual responses (2%) or of
indeterminate eye movement patterns (8%) were excluded. As a result, 9%
of total trials in the saccade task condition were discarded and not
analyzed further.

To evaluate the influence of onset distractors on saccades, for each
trial, the end point of the initial saccade exiting the central fixation
was categorized with respect to one of the locations of potential
destinations (target, onset distractor, or elsewhere). In general a
saccade was said to be made toward a particular location if the first
fixation landed within +/- 30 degrees angular distance from the direct
path to that location. Based on this coding scheme, we found a
significant proportion, 27%, of first saccades erroneously went to the
onset distractor (plotted in Figure 2 upper left panel for later
comparison), consistent with previous experiments (Irwin et al., 2000;
Theeuwes et al., 1998, 1999). The presence of the onset distractor also
affected manual responses in the saccade condition. Averaged mean manual
reaction time (plotted in the leftmost panel of Figure 3) was
significantly lengthened from 927 to 955 ms by onset distractors, *t*
(7) = 3.49, *p* \< .01.

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Insert Figures 2, 3 about here

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Saccade trajectories are known to curve away from visual stimuli to
which covert attention has been directed (e.g., Sheliga et al., 1994;
1995; Doyle & Walker, 2001, 2002; Godijn & Theeuwes, 2002). If the
oculomotor capture is associated with an attention shift, saccade
trajectories should curve away from onset distractors. To test for
curvature we compared the trajectories of correct saccades to oblique
targets (i.e., targets at 60, 120, 240, or 300 degrees) in the no-onset
condition to trajectories for the same oblique targets in the presence
of an onset at 90 degrees. Saccade curvature was determined for each
trial by first calculating the amplitude of the most direct path from
fixation to the end point of the first saccade. Then, the peak deviation
was calculated by determining the largest perpendicular distance between
any eye sample on the actual trajectory and the direct path. Trials were
excluded if their deviation angle, the angle subtended by the peak
deviation, was greater than 30 degrees, because these saccades likely
underwent a change in course instead. Finally, a curvature index was
calculated by dividing peak deviation by saccade amplitude. This
procedure results in a measure of curvature per unit amplitude, which
could rule out curvature variation pertaining to saccade amplitude only
(Doyle & Walker, 2001). Curvature measures were assigned positive or
negative signs to indicate their direction. Saccades that curved toward
the hemifield where the target was (considered the ipsilateral
hemifield) were given negative signs; those curved toward the opposite
hemifield (considered the contralateral hemifield) from the target were
given positive signs.

Results of averaged median saccade curvatures are summarized in Figure
4. The baseline curvature of saccades made without distractors is
depicted by the position of the vertically aligned reference line. Its
negative value is reminiscent of curvature typically observed for
oblique saccades, which extends first horizontally then vertically
(e.g., Viviani, Berthoz & Tracey, 1977). Because of the existence of
baseline curvature, analysis of saccade curvature focused on the
difference in curvature between conditions with and without distractors.
Differences in curvature caused by ipsilateral and contralateral onset
distractors are represented in Figure 4 by horizontal bars. If onsets
cause saccades to curve away, onsets located ipsilaterally to the target
should result in contralateral curvature deviation, and vice versa.
Indeed, separate one-tailed t-tests results showed that onsets, whether
placed ipsilaterally or contralaterally to a target, significantly
curved saccades away (*ps* \< .05).

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Insert Figure 4 about here

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Other measures associated with onset capture were also computed. Median
latency for the first saccade in each condition was computed for each
participant. The overall averaged latency of erroneous saccades toward
the distractor (207 ms) was significantly shorter than that of correct
saccades (294 ms) in the same condition, *t* (7) = 4.27, *p* \< .01.
Oculomotor capture is also signified by short fixation durations on the
distractor before a corrective saccade is made toward the target; the
durations are usually shorter than what is required to program a saccade
anew (200 ms, see Salthouse & Ellis, 1980). In the present experiment,
averaged median fixation duration was only 112 ms, comparable to similar
measures reported previously (Irwin et al., 2000; Theeuwes et al., 1998,
1999).

In the attention task condition, abrupt onsets significantly delayed
responses to the target, from 604 ms to 612 ms, *t* (11) = 2.03, *p* \<
.05 (Figure 5, leftmost panel). The increase in response time cannot be
attributed to speed-accuracy tradeoffs, because abrupt onsets
correspondingly increased error rate from 4.6% to 7.4%, *t* (11) = 2.97,
*p* \< .01. In sum, results from the saccade condition replicated those
reported by Theeuwes et al. (1998) and showed clear evidence of
oculomotor capture. Results from the attention condition suggest that
the same design also induces attentional capture.

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Insert Figure 5 about here

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## Experiment 2

Experiment 2 examined whether distractors, abrupt onsets and/or color
singletons would elicit attentional and oculomotor capture in a design
that discouraged broad singleton detection mode. Following the logic of
Bacon and Egeth (1994) we discouraged singleton detection mode by
including trials with a color distractor, and tested participants in
saccade and no-saccade conditions as in Experiment 1. If successful,
there should be no attentional capture by abrupt onsets. This outcome
would be consistent with contingent orienting and would strengthen the
case for the distinction between feature and singleton search modes.
Furthermore, since Experiment 2 contained trials in which the only
distractor was an onset singleton, we can compare rates of oculomotor
capture with those in Experiment 1 under the same stimulus conditions.
If both oculomotor and attentional capture are modulated by task goals,
the adoption of feature mode should have corresponding effects on the
rate of oculomotor capture and the amount of attentional capture
measured in no-saccade conditions.

### Method

[Participants]{.underline}. Twenty-six individuals participated in the
experiment, including one of the authors. Eight completed both attention
and saccade task conditions; eighteen completed only the attention
condition. Among those who completed both tasks, five of the
participants were staff members of the lab. All other participants were
undergraduate students recruited from the same subject pool as
Experiment 1. All participants met the same requirements specified in
Experiment 1. Five of the participants in the saccade task condition had
participated in similar experiments but, except the author, they were
naive to the purpose of the present experiment.

[Apparatus and stimuli]{.underline}. The apparatus and stimuli were
identical to those used in Experiment 1 except the addition of the color
blue, whose luminance was matched to those of red and green using the
same procedure described in Experiment 1.

[Design]{.underline}. The design of Experiment 2 was identical to that
of Experiment 1 except that half the trials contained a blue color
distractor on the target display. As a result, there were four sets of
96 trials: no distractor, onset distractor only, color distractor only,
or both onset and color distractors (Figure 6). Trials of different
types were intermixed and presented to each participant in a different
randomized order. The onset distractor, when present, could appear at
90, 150, 210, or 270 degrees counterclockwise away from the target,
resulting in two possible angular distances, 90 and 150 degrees. The
color distractor, when present, could appear at any one of the five
non-target locations, resulting in three possible angular distances, 60,
120, and 180 degrees. In Euclidean distances, these corresponded to
10.08, 17.46, and 20.16 degrees of visual angle in the saccade task
condition and 3.4, 5.89, and 6.8 degrees in the attention task
condition, respectively. When both color and onset distractors were
present, certain restrictions in their placement was maintained,
depending on the angular distance between the onset distractor and the
target. When the target and the onset distractor were 90 degrees apart,
the color distractor would not appear in between them. When the target
and onset distractor were 150 degrees apart, the color distractor would
appear at neither the location in between them nor 60 degrees away from
the target.

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Insert Figure 6 about here

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[Procedure]{.underline}. The procedure used in Experiment 2 was
identical to that used in Experiment 1 except for those participants who
completed both tasks. To avoid unwanted transfer of eye movement
responses between tasks, the attention task was always done first. Three
participants completed both tasks in the same day; the other five
completed the saccade task condition after one to seven days.

### Results

Location of first fixation and manual reaction time data were analyzed
with respect to two factors, presence of an onset distractor and
presence of a color distractor. For the attention condition, incorrect
trials and reaction times longer than 1000 ms were excluded; such trials
accounted for 7% of total trials. For the saccade condition, trials with
incorrect manual responses (2%) or of indeterminate eye movement
patterns (1.3%) were excluded. As a result, 3.3% of total trials in the
saccade condition were discarded.

[Manual reaction time of the attention task condition]{.underline}.
Response times of correct trials were further trimmed to exclude those
over three standard deviations. Mean response times of the remaining
trials for each trial type in the attention task condition are
summarized in the middle left panel of Figure 5. A 2 x 2 analysis of
variance (ANOVA) with factors of onset and color on data from 26
participants revealed a main effect of color, *F*(1,25) = 18.53, *MSE* =
150.5703, *p* \< .001, and an interaction between color and onset,
*F*(1,25) = 6.98, *MSE* = 124.3538, *p* \< .05. Analyses of simple
effects showed that the interaction was due to a significant delay (8
ms) caused by onsets on trials with no color distractor, *F*(1, 25) =
8.01, *MSE* = 124.3538, *p* \< .01, that was absent on trials with a
color distractor (*p* \> .3). In addition, the presence of a color
distractor also significantly reduced response accuracy, *F*(1,25) =
5.62, *MSE* = 0.0009, *p* \< .01. No other effect was found in the
accuracy results.

[Saccade paths]{.underline}. Saccade paths were computed as in
Experiment 1, and are summarized in the left panel of Table 1 and the
upper middle panel of Figure 2. In the no-distractor condition about 80%
of saccades were correctly made toward the target. In the onset only
distractor condition approximately 10% of first fixations fell near the
path toward the distractor. In the color only distractor condition
approximately 20% of first fixations fell near the path toward the color
distractor. When both onset and color distractors were present, color
distractors attracted significantly more saccades (21%) than onset
distractors (9%), *t* (7) = 3.47, *p* \< .05.^1^

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Insert Table 1 here

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[Saccade latency]{.underline}. Table 2 shows averaged median latency
results by distractor condition and saccade destination. A 2 x 2 ANOVA
with factors of color and onset found a main effect of color, *F* (1,7)
= 6.24, *MSE* = 68.1974, *p* \< .05; the presence of a color distractor
delayed saccades made correctly toward the target. In all conditions
when oculomotor capture by distractors occurred, latencies of the
erroneous saccades regardless of distractor type were always
significantly shorter than those made correctly toward the target; for
the onset only condition, *t* (7) = 4.49, *p* \< .01, for the color only
condition, *t* (7) = 5.40, *p* \> .005. In the condition where both
distractors were present, latencies of saccades toward onset distractors
appeared to be shorter than those of saccades toward color distractors
but the difference did not reach statistical significance, *t* (7) =
2.21, *p* \> .05.

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Insert Table 2 here

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

[Fixation duration following captured saccades]{.underline}. Fixation
duration measured the time elapsed between the end of an erroneous
saccade toward a distractor and the onset of the subsequent correction
saccade toward the target. Averaged median latency results are
summarized next to their respective fixation targets in Table 2. It is
evident that most of these durations were shorter than what usually
required for the programming of a voluntary saccade.

[Manual reaction time of the saccade task condition]{.underline}.
Because manual responses could only be made after gaze has reached the
target, manual reaction times in the saccade task condition are to a
large extent contingent on the pattern of eye movements on a given
trial. Reaction times should be longer when the eyes went first to a
non-target location. This was exactly what happened; manual reaction
times were 830 ms following correct saccades to the target and delayed
by another 100 ms if the eyes initially went to a distractor. No other
difference was found in manual reaction times following correct saccades
in different distractor conditions. Reaction times were also analyzed
according to distractor conditions with no regard to first saccade
destinations, in a way analogous to the analysis of manual reaction
times in the attention task condition. Mean reaction times (three
standard deviation trimmed) of correct responses for each distractor
condition in the saccade task condition are summarized in the middle
left panel of Figure 3. A 2 x 2 ANOVA with factors of onset and color
found only a main effect of color, *F*(1,7) = 17.07, *MSE* = 495.2427,
*p* \< .005. Color distractors not only captured the eyes on a
proportion of trials but also delayed manual responses overall.

### Discussion

Overall, Experiment 2 found little evidence of attentional capture by
abrupt onsets when conditions favored feature search mode. The presence
of an onset distractor failed to produce attentional capture when the
color target lost its singleton status due to the inclusion of trials
containing another uniquely colored item. The contingency on task goals
observed here supports other findings showing that onsets only capture
attention when they fit the current attentional control settings, and
extends Bacon and Egeth's (1996) findings based on color distractors and
form targets. This result suggests that previous findings of attentional
capture by onset distractors (Irwin et al., 2000; Theeuwes, 1994;
Theeuwes et al., 1998) can be attributed to singleton detection mode
rather than pure stimulus-driven capture.

Despite the elimination of attentional capture, results from the saccade
task showed evidence of reduced, but residual oculomotor capture by
abrupt onsets. Compared to Experiment 1, oculomotor capture was reduced,
from 27% (Experiment 1) to 10%, but not completely eliminated.^2^ On the
one hand, this correspondence between the magnitude of attentional
capture and the rate of oculomotor capture indicates that oculomotor
capture is also in part contingent on attentional control settings. On
the other hand, the residual oculomotor capture despite the absence of
attentional capture is evidence of a dissociation between attention and
oculomotor capture. The persistence of oculomotor capture by abrupt
onsets in conditions analogous to those that produce no attentional
capture suggests that the saccade system may be more sensitive to
transients than the attention system. Apparently, top-down modulation
due to the instruction to saccade produces an attentional and oculomotor
set that differs from that produced when the instruction is to attend
only. The relationship of this form of top-down modulation and that
associated with singleton and feature modes is not yet known.

Analysis of the trials that showed oculomotor capture indicates that
characteristics of the erroneous saccades were similar to those observed
previously (Theeuwes et al., 1998; Theeuwes et al., 1999; Irwin et al.,
2000). Latencies of saccades made toward the abrupt onset were uniformly
shorter than those made toward the target. Once a saccadic error
occurred, the gaze generally remained on the onset item for a period of
time shorter than what would be needed for the programming of a new
saccade.

It is assumed that the elimination of attentional capture and the
reduction of oculomotor capture by abrupt onsets were due to the use of
feature search mode. To be qualified as a mode of operation, feature
search should be characteristic of all trials within a block, regardless
of whether the target is a singleton or non-singleton. Empirically, this
held true in the saccade condition, as reflected in the similarly small
proportion of saccades captured by onsets with and without a color
distractor, and in the absence of delays in manual responses by onsets
in the same conditions. However, in the attention condition we observed
capture by onsets on trials with a singleton target (i.e., no color
distractor) that was not present on trials with a non-singleton target
(i.e., with a color distractor). Since the attention task condition was
always presented first, it is possible that attentional capture by
onsets was present only early in practice. We tested for practice
effects by comparing data from the first and second half of the session.
Mean response times are depicted in Figure 7. It is evident that
significant attentional capture by onset distractors in singleton
conditions occurred only in the first half of the session. A 2 x 2 ANOVA
on the second half data that showed only a main effect of color, *F*(1,
25) = 14.94, *MSE* = 227.4051, *p* \< .001, confirmed this observation.
This finding argues that the distinction between singleton and feature
search mode can be generalized to different stimulus properties and
applied to attentional as well as oculomotor capture. In addition, the
adoption of a particular mode of search appears to be a gradual rather
than instantaneous process. The absence of attentional and oculomotor
capture on trials with no color distractors also argues against any
heterogeneity based accounts. With the addition of a color distractor on
non-singleton target trials, increased heterogeneity among non-target
items may have rendered abrupt onsets less salient and consequently
reduced their ability to capture attention and the eyes. No such
argument can be made for non-singleton target trials. Abrupt onsets on
these trials appeared against the same homogenous background, just as in
Experiment 1, and yet produced no capture. This finding supports the
existence of a top-down attentional mode based on features, which is
capable of overriding stimulus-driven attentional and oculomotor
capture, and can be sustained over trials.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Insert Figure 7 about here

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

The logic of encouraging feature search mode applied by Bacon and Egeth
(1994) concerns making singleton detection strategy ineffective. In
theory, singleton strategy would be ineffective if either the target or
singleton distractor lost its singleton status. Empirical evidence shows
however that singleton detection mode could only be rendered ineffective
by removing the singleton status of the target. In an oculomotor capture
experiment with a design similar to that of Theeuwes et al. (1998;
1999), Kramer, Cassavaugh, Irwin, Peterson, and Hahn (2001) asked
participants to search for a color singleton target in the presence of
either zero, one, or two irrelevant onset distractors. They found that
oculomotor capture occurred on 21% to 26% of the trials with two onsets,
comparable to and not statistically different from the 25% to 31% rates
found for trials with one onset. In our Experiment 2, adding a color
singleton greatly reduced the rate of oculomotor capture. Clearly,
adding another onset singleton did not have the same effect as adding a
unique color singleton. This finding suggests that in the present
experiments, it is setting attention for features that prevented capture
by onsets, not simply removing conditions favoring singleton detection.

Insofar as there exists an attentional mode based on features, the
question remains whether it is based on target feature value. The
presence of a color distractor on a proportion of trials meant that
uniqueness in color dimension was an unreliable predictor of target
location. In theory attention would operate more efficiently if set for
the target color value. The finding that the color distractor caused
significant interference in attention and saccade conditions, however,
suggests the set adopted could not always reliably distinguish between
different unique colors. Thus, the feature based mode exhibited in the
present results does not seem to fit the existent definition of feature
search mode, which according to Bacon and Egeth (1994), refers to a mode
wherein observers monitor "a specific feature map that codes for the
presence of the relevant feature" (p. 486). However, it is questionable
whether feature search mode was ever attained in Bacon and Egeth's
results. For example, in their experiment 3, observers searched for a
specific form (circle) in the presence of up to two unique forms (square
and triangle) among a background of diamond forms. Although the search
process appeared to proceed in parallel even with increasing numbers of
diamonds, the overall search process was delayed with increasing numbers
of unique forms. Bacon and Egeth (1994) regarded this effect as a
consequence of increasing distractor heterogeneity (cf. Duncan &
Humphreys, 1989). Nevertheless, if attention was indeed set on the
target feature value (circle), the search process should have remained
parallel even with increasing numbers of unique forms.

Based on the similarity between the findings from the present research
and those of Bacon and Egeth's (1994), we use the term feature search
mode to describe the set adopted for non-singleton targets, even though
the set appears to be for feature category rather than value. It is not
clear why participants were not able to set for the target feature value
in the present research, especially since Folk and Remington (1998)
reported color selectivity in the search for color singleton targets. We
think the discrepancy may be rooted in the different ways in which
distractors are implemented in these two studies. It requires further
research to clarify this inconsistency.

As the feature mode adopted by participants appeared to be based on
feature category, interference caused by color distractors could then
naturally be viewed as capture. Characteristics of the observed
erroneous saccades are also in accord with involuntary capture; saccades
made toward the color distractors were fast with very brief fixations
before subsequent correction saccades, just as those signifying
oculomotor capture by abrupt onsets (cf. Theeuwes et al., 1998). Our
findings that irrelevant color distractors can capture attention and the
eyes are at odds with Irwin et al.'s (2000) results. The discrepancy may
be attributed to the fact that in those experiments where Irwin et al.
failed to observe color capture, the targets were onset singletons.
Since the saccade system appears to be especially susceptible to abrupt
onsets, as the present results suggest, it is possible that the
combination of bottom-up and top-down emphasis on onsets precluded
interference from otherwise salient distractors. The effect of
transients on saccades and attention is explored further in next
experiments.

## Experiment 3

Experiment 2 found attentional and oculomotor capture by color
distractors that we attributed to a match between the stimulus property
of the distractor and the attentional control setting adopted in feature
search mode. There is an alternative explanation, however. The color
distractor involved a transient change in color. There is evidence that
equiluminant chromatic changes capture attention even when the target is
defined by onset (Snowden, 2002). One could also argue that the color
distractor captured attention and the eyes not simply because it fitted
the attentional set for color, but because its color change rendered it
more salient than the target, or the onset. This account is consistent
with Theeuwes (1992, 1994) who has argued that attentional capture is
determined by the relative salience between the distractor and the
target. Theeuwes (1994) reported that when abrupt onsets defined
targets, transient color singleton distractors produced attentional
capture, but static ones did not. Based on these results one could
conclude that transient color singletons and abrupt onsets exude a
higher level of salience than static color singletons.

Experiment 3 was designed to investigate the role of salience by
eliminating transient change as a property of the color distractor. In
Experiment 3, the initial circles were the color of the color distractor
(blue) instead of the color of the target (blue). The target was still
in the green circle among red ones. On some trials a blue color
distractor was present. As a result, the target appeared through a
transient change in color from blue to green whereas the color
distractor, when presented, remained statically blue. Hence their
relative salience should be reversed from that in Experiment 2.

There could be a number of outcomes surrounding the color distractor.
First, if visual salience is essential to the capture of attention and
saccades by color distractors in Experiment 2, this modified procedure
should virtually eliminate that capture. If color contingency is a
critical property of the operative set, color capture should remain. As
for the onset distractor, according to the contingent capture theory, it
should now elicit attentional capture because the emergence of the
target involves change, as does the emergence of the abrupt onset items.
There should also be a correspondingly higher rate of oculomotor capture
than observed in the previous experiment.

### Method

[Participants]{.underline}. Twenty six new individuals participated in
the experiment. Eighteen completed the attention task condition; eight
completed the saccade task condition. All met the same requirements as
in Experiment 1.

[Apparatus and stimuli]{.underline}. The apparatus and stimuli were
identical to those used in Experiment 1.

[Design and procedure]{.underline}. The design and procedure were
identical to those of Experiment 2 except that the circles in the
initial display were blue instead of green (Figure 6, upper middle
panel). As a result, the color target went through a transient change
from blue to green and the color distractor remained statically blue.

### Results

For the attention task condition, trials with incorrect responses or
reaction times longer than 1000 ms were excluded from further analyses;
such trials accounted for 9% of total trials. For the saccade condition,
trials with incorrect manual responses (2%) or of indecipherable eye
movement patterns (3%) were excluded. As a result, 5.4% of total trials
in the saccade condition were discarded and not analyzed further.

[Manual reaction time of the attention task condition]{.underline}. Mean
reaction times of correct responses for each trial type in the attention
task condition are summarized in Figure 5. A 2 x 2 ANOVA with factors of
onset and color distractors on data from 18 participants revealed main
effects of color, *F*(1,17) = 9.18, *MSE* = 118.7813, *p* \< .01, as
well as onset, *F*(1,17) = 5.42, *MSE* = 236.9449, *p* \< .05. No
interaction was found between the two factors. No effect was found
either in the accuracy results, also shown in Figure 5.

[Saccade paths]{.underline}. The middle panel of Table 1 and the lower
middle panel of Figure 2 present, in each distractor condition, the
proportions of first saccades made toward each of the four possible
destinations. The results showed clear oculomotor capture by onset
distractors (23%), none by color.

[Saccade latency]{.underline}. Averaged median latency results by
distractor condition and saccade destination are summarized in Table 3.
There was no difference between latencies of saccade made directly
toward the target in different distractor conditions. Latencies of
saccade directed toward the onset distractor were uniformly shorter than
those toward the target, regardless of the state of the color
distractor.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Insert Table 3 here

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

[Manual reaction time of the saccade task condition]{.underline}. Mean
manual reaction times were 790 ms following correct saccades and delayed
by 90 ms when the eyes went initially to the onset distractor. No
difference was found following correct saccades in different distractor
conditions. When analyzed by distractor conditions without regard to
first saccade destinations (data shown in Figure 3, middle panel),
results of a 2 x 2 ANOVA with factors of onset and color revealed a main
effect of onset, *F* (1, 7) = 7.83, *MSE* = 422.5241, *p* \< .05. The
presence of onset distractors caused oculomotor capture on a significant
proportion of trials, which led to the overall lengthening of manual
reaction times in respective conditions.

### Discussion

Results of the attention task condition challenge the contribution of
visual salience to attentional capture. Attention was captured by an
unchanged unique color item even when the intended target was
prominently signaled by a transient change into another unique color.
Consistent with predictions of contingent capture, abrupt onset items
now produced significant attentional capture. This outcome is similar to
Folk et al.'s (1992) finding that identity discrimination targets
presented as onsets are susceptible to interference from onset
distractors. The color distractor shared color but not change with the
target, while the onset distractor shared change but not color. Since
both color and onset distractors produced capture, we conclude that a
complete match between stimulus and attentional control settings is not
necessary to elicit capture.

The saccade task condition returned some rather unexpected outcomes. In
previous research, oculomotor capture has always been observed in
conjunction with attentional capture, supporting theories of oculomotor
capture which hypothesize that the programming and execution of
reflexive saccades are initiated by reflexive shifts of attention (e.g.,
Theeuwes et al., 1999). The present experiments found positive support
for this hypothesis from the finding that both the eyes and attention
were captured by singleton distractors when they were consistent with
task goals. However, Experiment 2 and Experiment 3 both found
dissociations between attention and saccades. In Experiment 2, onset
distractors produced a residual oculomotor capture in saccade
conditions, but failed to produce attentional capture in no-saccade
conditions. In Experiment 3, static color distractors produced
significant attentional capture, but no oculomotor capture. A similar
dissociation between attention capture and inhibition of return has been
observed by Gibson and Amelio (2000); singleton color cues that fitted
attentional control settings and produced significant attentional
capture failed to elicit inhibition of return. Saccades, and perhaps
inhibition of return, exhibit a dependency on and sensitivity to
transients not characteristic of attention.

## Experiment 4

Experiment 3 showed that unique color distractors captured attention
under an attentional set for color even when they appeared static and
undoubtedly less salient than the target, but the capture of attention
was not accompanied by a corresponding capture of the eyes. We
hypothesize that the triggering of oculomotor capture requires transient
changes in distractors. There is, however, another possibility. If the
saccade system is more susceptible to the influence of salient stimulus
properties, then having a target that was more salient than the
distractor may simply have prevented oculomotor capture by the weaker
color distractor. Experiment 4 was conducted to distinguish between
these two possibilities.

In Experiment 4, salience of the target and distractor was equated by
making both of them static. This was achieved by modifying the initial
display to consist of an equal number of green and blue circles. The
target display emerged when both non-target and non-distractor circles
turned red. The task still required the search for a green target circle
among red ones, in the likely presence of a blue distractor and a red
onset. The target was chosen from one of the original green circles, and
the color distractor from one of the original blue circles. As a result,
on trials with a color distractor, both target and distractor remained
unchanged throughout the trial. Since the target now was a color
singleton but had no transient change associated with it, contingent
capture theory would predict attentional capture by the color distractor
but not by the onset distractor. Conversely, a salience account would
predict strong capture by the onset because with both the target and
color distractors being static, the onset distractor became the most
salient unique item on the display. If oculomotor capture follows the
same pattern as attentional capture, the static color distractor should
produce oculomotor capture. If, however, a transient change is critical
to triggering oculomotor capture, the onset distractor should elicit
involuntary saccades, and we should observe a dissociation between
attention capture and oculomotor capture.

### Method

[Participants]{.underline}. Twenty eight individuals participated in the
experiment. Twenty completed the attention task condition; eight
completed the saccade task condition. All met the same requirements as
in Experiment 1.

[Apparatus and stimuli]{.underline}. The apparatus and stimuli were
identical to those used in Experiment 1.

[Design and procedure]{.underline}. The design and procedure were
identical to those of Experiment 2 except that the circles on the
initial display were mixes of equal numbers of blue and green (Figure 2,
upper right panel). It was constrained so that no three consecutive
circles were of the same color. The target and distractor appeared at
locations previously occupied by the same color. The arrangement allowed
both the target and distractor to remain static. The participants were
only told to locate the green target; they were not informed of its
relation with the three initial green circles.

### Results

For the attention task condition, trials with incorrect responses or
reaction times longer than 1000 ms were excluded from further analyses;
such trials accounted for 8% of total trials. For the saccade condition,
trials with incorrect manual responses (3%) or of indecipherable eye
movement patterns (9%) were excluded. As a result, 12% of total trials
in the saccade condition were discarded and not analyzed further.

[Manual reaction time of the attention task condition]{.underline}. Mean
reaction times of correct responses for each trial type in the attention
task condition are summarized in the right panel of Figure 5. A 2 x 2
ANOVA with factors of onset and color distractors on data from 20
participants revealed a main effect of color, *F* (1, 19) = 6.76, *MSE*
= 161.8963, *p* \< .05. No other effect was found. In the accuracy
results, also shown in Figure 3, a 2 x 2 ANOVA with factors of onset and
color revealed a main effect of onset, *F*(1, 19) = 4.75, *MSE* =
0.0004, *p* \< .05; onset distractors significantly reduced response
accuracy.

[Saccade paths]{.underline}. The lower panel of Figure 4 and the right
panel of Table 1 present, in each distractor condition, the proportions
of first saccades made toward each of the four possible destinations.
The results showed a clear influence by onset distractors and none above
chance by color. The presence of an onset item distracted around 13% of
saccades on average.

[Saccade latency]{.underline}. Averaged median latency results by
distractor condition and saccade destination are summarized in Table 4.
Latencies of saccades made toward the target were affected by the
presence of an onset distractor, revealed by a 2 x 2 ANOVA with factors
of onset and color, *F*(1,7) = 6.76, *MSE* = 349.4179, *p* \< .05.
Correct saccades toward the target were faster in the presence of onset
distractors. Latencies of saccade directed toward the onset distractor
were uniformly shorter than those toward the target, regardless of the
state of the color distractor.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Insert Table 4 here

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

[Manual reaction time of the saccade task condition]{.underline}. Mean
manual response times were 910 ms following correct saccades and delayed
by 180 ms if the eyes went initially to the onset distractor. No
difference was found in manual reaction times following correct saccades
in different distractor conditions. When analyzed according to
distractor conditions with no regard to first saccade destinations,
results of a 2 x 2 ANOVA with factors of onset and color revealed a main
effect of color, *F* (1, 7) = 6.10, *MSE* = 1002.6422, *p* \< .05. The
presence of color distractors, though caused no significant oculomotor
capture, delayed manual responses.

### Discussion

The pattern of reaction time results from the attention task condition
was similar to the second half of Experiment 2. Consistent with the
contingent capture theory, there was attentional capture by color
distractors but not by onset distractors. The pattern for oculomotor
capture differs from the pattern for attentional capture. Color
distractors failed to produce oculomotor capture, replicating Experiment
3, and confirming the crucial involvement of transient changes in
triggering oculomotor capture. Onset distractors produced minimal
oculomotor capture, replicating that of Experiment 2, and demonstrating
again that oculomotor capture is contingent on attentional control
settings. The fact that onset distractors in the present experiment were
most salient on the display and produced little interference in the
absence of any other substantial distraction strengthens the case for
contingent capture even further. The presence of residual oculomotor
capture by abrupt onsets here and in Experiment 2, despite an absence of
attentional capture, suggests that transient change has a heightened
effect on orienting when subjects are prepared to make a saccade.

As shown in Figures 4 and 6, Experiment 4 found faster manual responses
in the attention task condition and slower latencies in the saccade task
condition than observed in our earlier experiments. A possible
explanation for the difference is the reduced uncertainty regarding
target location. While the participants were not explicitly informed of
the special relation between the target and the composition of the
initial display, nothing prevented them from realizing the relation and
utilizing that knowledge. If participants could restrict their attention
to a reduced set of items by focusing on target color, they would be
able to locate and respond to the target faster. This explanation would
be consistent with the finding of faster reaction times in the attention
task condition. However, as uncertainty about target location was
reduced, uncertainty about the location of the color distractor was
reduced as well. If the uncertainty levels associated with target and
distractor locations were to have any effect, they should work to reduce
attentional capture by color distractors. The fact that attentional
capture by color distractors remained at approximately the same level
suggests that this aspect of the design did not cause spurious results.

## Experiment 5

Experiment 5 was designed to ensure that the pattern of results from the
previous experiments would be observed when non-critical design features
were changed. One concern is that in order to avoid confusion over the
categorization of saccade destination, abrupt onsets have so far never
appeared at the 30 degree locations next to a target. This manipulation
may have reduced the uncertainty of target location if onsets were
detected first and therefore inadvertently facilitated target
localization. As a result, attentional and oculomotor capture may have
been reduced not because of the use of feature search mode but rather
the use of voluntary allocation of attention (e.g., Theeuwes et al.,
1998, Experiment 2). To address this, Experiment 5 lifted the
restriction on where abrupt onsets could appear in relation to a target.
A second concern is that in all previous experiments the target circle
contained a letter C rotated in either 0 or 180 degrees while the rest
of the circles contained a letter C rotated either 90 or 270 degrees. It
may be argued that the difference in gap orientation served as a
preattentive cue that helped localize the target, thus reducing the
effectiveness of onset (but apparently not color) cues. In Experiment 5,
all circles contained a letter C rotated 0 or 180 degrees; hence the
target could only be located through its unique color. Finally, in
previous experiments eye positions were not monitored in the attention
task condition, though brief exposure time was used to minimize
potential eye movements. In this experiment we monitored eye positions
in the attention condition to assure that the observed effects in the
attention condition were not due in fact to eye movements.

### Method

[Participants]{.underline}. Twenty eight individuals participated in the
experiment. Twenty completed the attention task condition; eight
completed the saccade task condition. All were recruited from the same
subject pool and met the same requirements as in Experiment 1.

[Apparatus and stimuli]{.underline}. The apparatus and stimuli were
identical to those used in Experiment 1.

[Design and procedure]{.underline}. The design and procedure were
identical to those of Experiment 2 except changes noted below. The onset
distractor could appear 30, 90, 150, 210, 270, or 330 degrees
counterclockwise from the target, resulting angular distances 30, 90 and
150 degrees. The 30 degree distance corresponded to 5.22 degrees of
visual angle in the saccade task condition and 1.76 degrees in the
attention task condition. Second, on target displays, all circles
contained a letter C rotated either 0 or 180 degrees. Third, eye
position was monitored in the attention condition.^3^

### Results

For the attention task condition, trials with incorrect responses or
reaction times longer than 1000 ms were excluded from further analyses;
such trials accounted for 7.7% of total trials. For the saccade
condition, trials with incorrect manual responses (1%) or of
indecipherable eye movement patterns (3%) were excluded. As a result, 4%
of total trials in the saccade condition were discarded and not analyzed
further. In addition, trials with onsets at 30 degrees were excluded
from analyses in both task conditions to maintain comparability of the
current results to those of previous experiments.

[Manual reaction time of the attention task condition]{.underline}. Mean
reaction times of correct responses for each trial type in the attention
task condition are summarized in the right panel of Figure 5. A 2 x 2
ANOVA with factors of onset and color distractors on data from 20
participants revealed a main effect of color, *F* (1, 19) = 15.82, *MSE*
= 241.5857 , *p* \< .001. No other effect was found. An additional 2 x 2
ANOVA was carried out for trials on which fixation was maintained.^4^
Results again showed only a main effect of color, *F* (1, 19) = 17.78,
*MSE* = 202.2948, *p* \< .001. The accuracy results, also shown in
Figure 5, are based on all trials with no regard to eye positions, as in
previous experiments. A 2 x 2 ANOVA with factors of onset and color
revealed a main effect of onset, *F* (1, 19) = 5.21, *MSE* = 0.0010, *p*
\< .05, and an interaction between onset and color, *F* (1, 19) = 4.88,
*MSE* = 0.0005, *p* \< .05. The nature of the interaction was explored
with analyses of the simple effects; results showed that the interaction
was due to significant performance decrement caused by onsets on trials
with no color distractors, *F* (1, 19) = 13.75, *MSE* = 0.0005, *p* \<
.005. This accuracy effect appeared to be confined to three subjects
whose accuracy dropped 10% in the onset only condition. When data from
those three were excluded, accuracy effects disappeared, but the
reaction time data continued to show effects of color, *F* (1, 16) =
12.73, *MSE* = 228.9614, *p* \< .005, with no effect of onset.

[Saccade paths]{.underline}. The results of saccade path analyses are
summarized in the rightmost panel of Table 1 and the lower right panel
of Figure 2. Similar to Experiment 2, about 80% of saccades were
correctly made toward the target in the control conditions. Onset and
color distractors captured approximately 12% of saccades when presented
singly. When presented jointly, onset and color distractors captured
approximately 10% and 14% of saccades respectively and a marginally
significant difference was found between their rates of capture (*p* \<
.09).

[Saccade latency]{.underline}. Averaged median latency results by
distractor condition and saccade destination are summarized in Table 5.
There was no difference between latencies of saccade made directly
toward the target in different distractor conditions. Erroneous saccades
directed toward distractors, whether onset or color, had latencies
significantly shorter than correct saccades in the same conditions;
*t*(7) = 5.40 , *p* \< .001, for the onset only condition, and *t*(7) =
3.72, *p* \< .01, for the color only condition. In the condition where
both distractors were present, latencies of saccades toward onset
distractors appeared to be shorter than those of saccades toward color
distractors but the difference did not reach statistical significance,
*t*(7) = 1.00, *p* \> .15.

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

Insert Table 5 about here

\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\--

[Manual reaction time of the saccade task condition]{.underline}. Mean
manual reaction times were 880 ms following correct saccades and delayed
by approximately 100 ms when the eyes went initially to a distractor. No
difference was found following correct saccades in different distractor
conditions. When analyzed according to distractor conditions with no
regard to first saccade destinations, results of a 2 x 2 ANOVA with
factors of onset and color revealed a marginal effect of color, *F* (1,
7) = 5.32, *MSE* = 591.2767, *p* \< .06.

### Discussion

The results of Experiment 5 essentially replicated those of Experiment
2. In the attention task condition, when analyzed with no regard to eye
positions as in previous experiments, results showed significant
attentional capture by color distractors and none by onset distractors.
The same pattern of results was obtained on those trials on which
fixation was maintained within a criterion surrounding fixation. In the
saccade task condition, oculomotor capture by onset distractors was
again largely reduced, compared to Experiment 1. Color distractors also
produced significant oculomotor capture, though the rate of capture
appeared to be lower than that of Experiment 2. Together, these results
again refute the notion that abrupt onsets invariably capture attention
with no regard to top-down attentional control settings. Instead, the
current attentional control setting determines what feature properties
will capture attention.

## General Discussion

The visual search experiments reported here examined the involuntary
responses of the eyes and spatial attention to abrupt onset and color
singleton distractors. Attentional capture was assessed by response time
differences in no-saccade conditions, oculomotor capture by the
proportion of first saccades to the distractor in saccade conditions.
Across the five experiments, we have shown that there exist
contingencies and dissociations between attentional and oculomotor
capture. While both were mediated by top-down attentional control
settings, oculomotor capture was observed only for stimuli with
transient properties. We discuss these results by focusing on how
different visual properties, specifically abrupt onsets and color
singletons, affect attentional and oculomotor capture under different
search modes and task demands.

### Contingencies in oculomotor and attentional capture

[Capture by onsets]{.underline}*.* According to Irwin et al. (2000) and
Theeuwes et al. (1998, 1999) oculomotor capture occurs because attention
has been captured. Hence, they interpret oculomotor capture by abrupt
onsets but not color to mean that abrupt onsets are special stimuli with
regard to attentional capture. However, our results showed that abrupt
onsets captured attention only when they conformed to behavioral goals.
Specifically, the significant attentional capture observed under
singleton detection mode (Experiment 1) was eliminated simply by the
presence of another uniquely colored singleton distractor (Experiments
2, 4, and 5). Note that the modulating effects of target-distractor
contingencies in Experiments 2, 4, and 5 can be confidently attributed
to a set adopted by the participants; the same onset distractor displays
that elicited capture in Experiment 1 failed to do so when intermixed
with color singleton distractor trials. Further, the extent to which the
additional color distractor modulated attentional capture by abrupt
onsets depended on target properties. The same static color distractor
that eliminated capture in Experiment 4 exhibited no modulating effects
when the color target was accompanied by a transient change in
Experiment 3.

Oculomotor capture for onsets followed a pattern similar to attentional
capture. The rate of oculomotor capture by abrupt onsets in saccade
conditions varied according to the magnitude of attentional capture
measured in no-saccade conditions. When attention was captured by abrupt
onsets (Experiments 1 and 3), oculomotor capture by abrupt onsets was
substantial. When attention was not captured by onsets (Experiments 2,
4, and 5) oculomotor capture by onsets was greatly reduced. These
results suggest that oculomotor capture is also modulated by top-down
attentional control settings. However, saccadic control appears to be
more sensitive than attention to luminance transients; residual
oculomotor capture by abrupt onset distractors persisted despite the
absence of attentional capture in corresponding no-saccade conditions.
We discuss below the failure of static stimuli to elicit oculomotor
capture even when they elicit attentional capture.

Overall, the modulation of the capture produced by abrupt onsets for
both attention and saccades follows a pattern of contingent capture
similar to that observed in spatial cueing experiments (Folk, et al.,
1992). Here we demonstrate the same contingencies in visual search
experiments. The design of our experiments follows closely those
conditions from which claims of stimulus-driven capture have been based
(e.g., Theeuwes et al., 1998; Theeuwes et al., 1999; Irwin et al.,
2000). By altering the composition of trials we found that abrupt onsets
only capture attention when they share relevant properties with the
target, which could be a singleton status (Experiment 1) or a property
reliably associated with the target (Experiment 3) in feature search
mode. By demonstrating contingencies in a visual search paradigm we have
countered possible objections to contingent capture arising from the use
of the spatial cueing paradigm.

[Capture by color singletons]{.underline}. Attentional capture by color
singletons observed in Experiments 2-5 is consistent with contingent
capture predictions, given that feature mode was expressed here not in
terms of feature value, but feature dimension (color). The case for
contingent attentional capture in general is further strengthened by
findings in the present study that salience per se did not determine
whether the distractor would capture attention. We use salience here to
refer to the physical distinctiveness of the stimulus, which includes
the magnitude of luminance change for onset, as well as uniqueness on
the color dimension. Though no metric exists for equating the salience
of onsets versus color singletons, our results show no pattern that
could be easily interpreted as support for salience in determining
capture. For example, it would be reasonable to expect that static color
singletons should be less salient than transient color singletons
produced by changing the color from the previous display. Nonetheless,
in Experiment 3, static color distractors captured attention when the
target was prominently defined by a transient color change. The
luminance change associated with abrupt onsets would be expected to make
them more salient than the static color singletons. Yet, in Experiment
4, the onset distractor failed to elicit any attentional capture, though
again it did produce a residual oculomotor capture. Clearly, the
physical properties of the stimulus alone do not appear sufficient to
produce involuntary covert orienting, at least within the limits of
intensity and color differences used here. Indeed, evidence from single
neuron recording in primate visual cortex (V4) shows that salience may
be determined not simply by physical properties, but also by top-down
set (see Treue, 2001, for a review).

Patterns of oculomotor capture to color singletons did not show such a
clear contingency on task goals as observed with onset singletons.
Oculomotor capture by color singletons occurred only when the color
singleton had a transient property, as when it changed color. Our data
indicate that color alone is not an effective stimulus for eliciting
involuntary saccades. However, color must have some effect in modulating
oculomotor capture. If oculomotor capture responds only to transients
there should have been no effect of the color of a transient stimulus;
capture should occur to the strongest transient, likely the onset. There
is no evidence for this in our data. Oculomotor capture by onsets in
feature search conditions never exceeded that of the transient color
distractor. In Experiment 2, the capture rates for transient color
singletons were \~20% , compared to \~11% for abrupt onsets. In
Experiment 5, the proportion of capture by onsets and transient color
singletons were approximately equal. Why do we not see more oculomotor
capture to the more salient transient onset? One explanation is that the
contingencies associated with attention are modulating the response of
the oculomotor system. Because attention responds to the color, in
accord with task goals, it inhibits the tendency to move the eyes to the
onset.

[Summary]{.underline}. The present results provide answers to the three
questions outline earlier. Our results do not support the claim that
onsets are special in the sense of being sufficient to capture attention
regardless of attentional set. Our results, however, do support a
special role for transient stimulation (luminance transients, or color
change) in producing oculomotor capture. Residual rates of oculomotor
capture by onsets suggest a heightened sensitivity to onsets in eye
movement conditions. The presence of substantial oculomotor capture to
transient color singletons but not static color singletons suggests that
transient change may be a necessary condition. Hence, the stimulus
properties that elicit oculomotor capture are not identical to those
that elicit attentional capture. Though both are contingent on top-down
settings, attention and saccades can be dissociated in the patterns of
their involuntary responses to static and transient stimuli. In summary,
our results do not support the claim that oculomotor capture (and hence
attentional capture) is purely stimulus driven. We argue that earlier
claims for an absence of top-down modulation (Theeuwes et al., 1998;
1999; Irwin et al., 2000) failed to consider the effects of search mode
on attentional control settings.

One issue worth further discussion is the small effect size (around 10
ms) of attentional capture found consistently throughout all the
experiments, especially in Experiment 1. Theeuwes (1994) reported an
effect size of 50 ms for attentional capture by onset distractors in the
search for a static color singleton target. Others have reported effect
sizes around 25 ms for singleton capture conditions that involved form
targets and color distractors (Bacon & Egeth, 1994, Experiment 1;
Theeuwes, 1992, Experiment 1). However, all those effects were found in
singleton detection mode. Effect sizes comparable to the present ones
have been reported by Bacon and Egeth (1994, Experiment 3), which read
between 10 to 20 ms based on their figure depictions. A likely
explanation for reduced capture effects is that the more stringent set
adopted in feature mode is effective in eliminating capture on a greater
proportion of trials. As for the small effect size of singleton capture
reported in Experiment 1, Theeuwes & Godijn (2001) observed that the
effect of attentional capture is attenuated as the distance between
target and distractor increases. Theuuwes and Godijn (2001) estimated
about 60 ms for the nearest distractor (2.3 degrees of visual angle
away) and 20 ms for the farthest distractor (9.2 degrees away). We found
a similar distance effect by onset distractors in the attention task
condition of Experiment 5 (Figure 8).^5^ A one-way ANOVA found an
overall difference in response time to the target related to the
distance between the target and onset, *F* (2, 38) = 18.25, *MSE* =
470.2602, *p* \< .0005; the difference was primarily due to a
significantly increase in reaction time when the onset was at 30 degrees
away (1.76 degrees of visual angle) from the target, *t* (19) = 4.34,
*p* \< .0005. It is possible that the effect size in Experiment 1 was
relatively small because onset distractors never appeared at 30 degrees
away from a target. Since the closest distance between a target and
color distractor was 60 degrees (3.4 degrees of visual angle), shorter
than the closest analyzed distance between a target and onset
distractor, it is possible that the observed color capture effects were
simply due to this distance advantage. The suspicion can be put to rest
as the result of a one-way ANOVA on data from the color only attention
condition of Experiment 5 (also shown in Figure 8) showed no effect of
distance, *F* (2, 38) = 2.08, *MSE* = 385.2256, *p* \> .1. Although
Theeuwes & Godijn have offered an explanation of the distance effect in
terms of local inhibition surrounding a distractor that captures
attention, it is not clear why attending to the distractor should
produce only a local inhibitory effect instead of the expected cost when
attention is diverted from the target regardless of the distance. In any
case, the phenomenon has not been thoroughly explored. We must leave it
to future research to reconcile distance effects with top-down and
bottom-up accounts of attentional capture.

### The control of attention and saccades in involuntary orienting 

It is clear from the different responses to transient and static
distractors that oculomotor capture is not simply an overt form of
attentional capture, in which the eyes are captured because spatial
attention has been captured (e.g., Theeuwes et al., 1998, 1999; Irwin et
al., 2000). This dissociation is also inconsistent with the claim of
some premotor theories (e.g., Rizzolatti et al., 1994) that spatial
attention is simply a saccade whose motor output has been inhibited. If
only the motor act were inhibited then the stimulus properties that
elicit capture should have been the same for saccade and no-saccade
conditions. Instead, our results suggest an involuntary orienting system
whose behavior results from an interaction between attention and saccade
control centers. This interaction is mediated by task-related processes
that determine whether or not to move the eyes (*task set*), and for
which stimulus properties attention should be set (*target set*).

The contingencies and dissociations we observe here must have some
correspondence with activity in neural circuits associated with saccades
and attention. Some indication of how ***target sets*** modulate
orienting as described by contingent orienting, is found in research on
the brain mechanisms underlying saccades and attention in. For
attention, goal related top-down modulation is observed in area V4 (see
Treue, 2001, for a review). Sensitivity of V4 neurons to a particular
stimulus is increased when the stimulus is attended, either by located
within an attended spatial region (Reynolds, Pasternak, & Desimone,
2000) or by possessing attended features (Motter, 1994). Response of a
V4 neuron is driven by the neuron's response to the attended stimulus,
even when the attended stimulus is otherwise ineffective and presented
along with an effective (preferred) and salient stimulus in the
receptive field (Reynolds, Chelazzi, & Desimone, 1999; Reynolds &
Desimone, 2003). These findings demonstrate that, at the level of V4,
visual salience appears to be an interactive product of top-down and
bottom-up factors, and its ability to elicit neuronal responses is
modulated by surrounding stimuli and current attentional settings.

For saccades, behavioral relevance of target sets is reflected in at
least two cortical areas, the intraparietal area (LIP) and the frontal
eye fields (FEF). Cells in LIP (Wurtz, et al., 2001; Colby, Duhamel, &
Goldberg, 1996; Pare & Wurz, 2001) and FEF (Schall, Hanes, Thompson,
King, 1995) show enhanced activity time-locked to the presentation of a
stimulus to which a saccade is intended. The firing rate of neurons in
FEF is higher to distractors that share features with the target (Bichot
& Schall, 1999a, 1999b). This selective activation mirrors the
attentional control settings proposed by Folk et al. (1992). Both FEF
and LIP interact with sub-cortical neurons in the superior colliculus
(SC) to generate saccades. Given the interconnections between FEF and
LIP (e.g., Stanton, Bruce, & Goldberg, 1995) and between LIP and V4
(e.g., Blatt, Andersen, & Stoner, 1991), it is likely that all of these
cortical areas have a role in determining relevant stimuli for shifting
spatial attention and saccadic eye movements that corresponds to the
contingency on target sets observed here.

Patterns of connectivity and interaction between FEF, LIP, and SC lay
out the basis of underlying brain mechanisms that could give rise to the
observed patterns of results related to task sets. It is generally
thought that the superior colliculus (SC) is responsible for
involuntary, reflexive saccades (see e.g., Bichot, 2001). Superficial
areas receive direct input from visual cortex as well as from the retina
that presumably provides rapid information of the location of transient
events (e.g., abrupt onsets) in the visual world (Lee, Helms, Augustine,
Hall, 1997; Bichot, 2001; Munoz et al., 2000). In contrast, color
information is processed in areas that receive input from striate cortex
but do not have direct connection to SC (cf. Ottes, Van Gisbergen, &
Eggermont, 1987). Consistent with this, we found the latency of saccades
to onset distractors reliably shorter than to color distractors.

How can ***task sets*** (instructions to fixate or saccade) alter the
effectiveness of transient stimuli to capture attention or elicit
involuntary eye movements? In the case of instructions to fixate or
saccade there can be no preparation for particular stimulus properties.
One way for the same stimulus property to elicit differing responses
according to instructions, is to modulate the response differently in
each condition. Assuming that transient stimuli can elicit involuntary
responses via the direct retinal-collicular pathways, we conjecture that
such bottom-up influences are countered by cortical pathways operating
via FEF, which convey top-down information specified by task demands. In
SC, eye movements are generated by saccade neurons, each tuned to a
specific direction and amplitude. The task goal of maintaining fixation
could lead to strong cortical input from FEF to fixation neurons in SC
whose firing inhibits saccade neurons in SC (e.g., Munoz & Wurtz, 1993),
as well as from FEF to omnipause neurons in the pons whose tonic
discharge during active fixation inhibits the brain stem saccade burst
generator (e.g., Segraves, 1992). To trigger an involuntary saccade,
sensory inputs from the retina and striate cortex that signal the abrupt
onset would have to overcome the low resting level of activation in
saccade cells as well as the gating by omnipause neurons on the burst
generator. Indeed, Tse, Sheinberg, and Logothetis (2002) showed that
abrupt onsets caused no systematic saccades, microsaccades, or
deviations in fixational eye movements when the intent was to remain
fixated. This could explain why, in the no-saccade condition, transient
stimuli do not elicit saccades despite direct input. Target related
feature information, however, is processed by the ventral visual
pathway, which presumably would not affected by the dampened activity of
the saccade system. As a result, abrupt onsets will be able to elicit
involuntary attention shifts when they are part of the target set.

In the saccade condition, the goal of making a rapid saccade would
discourage strong inhibition of saccade neurons in SC, which
consequently would have higher activation levels than in no-saccade
conditions. The increased activity of SC neurons increases the
likelihood that direct input from early visual processing would result
in an occasional erroneous saccade to transient events. There is also
evidence that LIP neurons respond to transients in their receptive field
even when those onsets are not the saccade target (Colby, Duhamel, &
Goldberg, 1996; Gottlieb, Kusunoki, & Goldberg, 1998). Given the delays
in processing and communicating information about the presence of color
singletons, SC could be preparing to make a saccade to the onset before
information about the target location has accumulated. The differential
response of FEF neurons to relevant and irrelevant distractor properties
does not appear until 50-100 ms following the first initial FEF activity
(Bichot & Schall, 1999a, 1999b). The same general considerations suggest
why static color singletons fail to capture the eyes. The lack of a
direct link between color centers and SC deprives saccade neurons in SC
with the rapid source of activation provided by abrupt onsets. Static
singletons should also fail to excite LIP, since cells in LIP do not
fire to pre-existing stimuli unless they are behaviorally significant
(Colby et al., 1996; Gottlieb et al., 1998), and so cannot activate SC
through that pathway.

The above account maintains the tight coupling of attention and saccades
posited by premotor theories. It differs in that the inhibition is not
just of the movement, controlled by brain stem mechanisms, but of the
control center for initiating saccades. As a result, the inhibition
changes the response patterns by effectively damping the signal from low
level visual processes. Damping the saccade response exposes a mode of
control that is more purely attentional. In our view, the behavioral and
neurophysiological evidence suggests that the cortical inputs to SC
represent a later attention system whose job is to modulate the behavior
of a more primitive orienting system exclusive to saccades.

The neurophysiology may also provide insight into the debate over
top-down or stimulus-driven attentional capture. Essentially there have
been four proposals for what will capture attention: abrupt onsets
(Yantis & Jonides, 1984), salience (Theeuwes, 1992), new objects (Yantis
& Hillstrom, 1995), and contingencies established as attentional control
settings (Folk et al., 1992). The neurophysiology provides some evidence
for each of these. Luminance transients are conveyed directly to SC, LIP
responds to new objects even if not relevant, LIP and V4 also reflect
stimulus salience, and FEF, LIP, and V4 reflect the target sets that
determine which stimulus properties are relevant. We suggest that
differences in methods and stimuli can cause one or more pathways to
dominate. Our experiments likely produce complex activity in all areas
where at times one or another of these underlying pathways dominate
performance. A deeper understanding of the relationship between the
behavioral paradigms and the underlying brain activity may reveal modes
of orienting consistent with each of the classes of explanation.

## References

Andersen, R. A. (1995). Encoding of intention and spatial location in
the posterior parietal cortex. *Cerebral Cortex, 5*, 457-469.

Bacon, W. F., & Egeth, H. E. (1994). Overriding stimulus-driven
attentional capture. *Perception and Psychophysics, 55,* 485-496.

Bichot, N. P. (2001). Attention, eye movements, and neurons: Linking
physiology and behavior. In M. Jenkin & L. Harris (Eds.), *Vision and
Attention* (pp. 209-232). New York, NY: Springer.

Bichot, N. P., & Schall, J. D. (1999a). Saccade target selection in
macaque during feature and conjunction visual search. *Visual
Neuroscience, 16,* 81-89.

Bichot, N. P., & Schall, J. D. (1999b). Effects of similarity and
history on neural mechanisms of visual selection. *Nature Neuroscience,
2,* 549-554.

Blatt, G. J., Andersen, R. A., & Stoner, G. R. (1990). Visual receptive
field organization and cortico-cortical connections of the lateral
intraparietal area (are LIP) in the macaque. *Journal of Comparative
Neurology, 299,* 421-445.

Bushnell, M. C., Goldberg, M. E., & Robinson, D. L. (1981). Behavioral
enhancement of visual responses in monkey cerebral cortex. I. Modulation
in posterior parietal cortex related to selective visual attention.
*Journal of Neurophysiology, 46,* 755-787.

Colby, C. L., Duhamel, J-R., & Goldberg, M. E. (1996). Visual,
presaccadic, and cognitive activation of single neurons in monkey
lateral intraparietal area. *Journal of Neurophysiology, 76*, 2841-2852.

Corbetta, M. (1998). Frontoparietal cortical networks for directing
attention and the eye to visual locations: Identical, independence, or
overlapping neural systems? *Proceedings of the National Academy of
Sciences of the United States of America, 95,* 831-838.

Corbetta, M., Akbudak, E., Conturo, T. E., Snyder, A. Z., Ollinger, J.
M., Drury, H. A., et al. (1998). A common network for functional areas
for attention and eye movements. *Neuron, 21,* 761-773.

Doyle, M., & Walker, R. (2001). Curved saccade trajectories: Voluntary
and reflexive saccades curve away from irrelevant distractors.
*Experimental Brain Research, 139,* 333-344.

Doyle, M. C., & Walker, R. (2002). Multisensory interactions in saccade
target selection: Curved saccade trajectories. *Experimental Brain
Research, 142,* 116-130.

Duncan, J., & Humphreys, G. W. (1989). Visual search and stimulus
similarity. *Psychological Review, 96,* 433-458.

Folk, C. L., Remington, R. W., & Johnston, J. C. (1992). Involuntary
covert orienting is contingent on attentional control settings. *Journal
of Experimental Psychology: Human Perception and Performance, 18,*
1030-1044.

Gibson, B. S., & Amelio, J. (2000). Inhibition of return and attentional
control settings. *Perception and Psychophysics, 62,* 496-504.

Godijn, R., & Theeuwes, J. (2002). Programming of endogenous and
exogenous saccades: Evidence for a competitive integration model.
*Journal of Experimental Psychology: Human Perception and Performance,
28,* 1039-1054*.*

Gottlieb, J. P., Kusunoki, M., & Goldberg, M. E. (1998). The
representation of visual salience in monkey parietal cortex. *Nature,
391,* 481-484.

Hoffman, J. E. (1998). Visual attention and eye movements. In H. Pashler
(Ed.), *Attention*. East Sussex, UK: Psychology Press.

Hoffman, J. E., & Subramaniam, B. (1995). The role of visual attention
in saccadic eye movements. *Perception and Psychophysics, 57,* 787-795.

Irwin, D. E., Colcombe, A. M., Kramer, A. F., & Hahn, S. (2000).
Attentional and oculomotor capture by onset, luminance, and color
singletons. *Vision Research, 40,* 1443-1458.

Irwin, D. E., & Gordon, R. D. (1998). Eye movements, attention, and
trans-saccadic memory. *Visual Cognition, 5,* 127-155.

Klein, R. (1980). Does oculomotor readiness mediate cognitive control of
visual attention? *Attention and performance VIII* (pp. 259-276).
Hillsdale, NJ: LEA.

Klein, R., & Pontefract, A. (1994). Does oculomotor capture readiness
mediate cognitive control of visual attention? Revisited! In C. Umiltà &
M. Moscovitch (Eds.), *Attention and Performance XV: Conscious and
Nonconscious Information Processing* (pp. 333-350). Cambridge, MA: MIT
Press.

Kramer, A. F., Cassavaugh, N. D., Irwin, D. E., Peterson, M. S., & Hahn,
S. (2001). Influence of single and multiple onset distractors on visual
search on singleton targets. *Perception and Psychophysics, 63,*
952-968.

Kowler, E., Anderson, E., Dosher, B., & Blaser, E. (1995). The role of
attention in the programming of saccades. *Vision Research, 35,*
1897-1916.

Kustov, A. A., & Robinson, D.L. (1996). Shared neural control of
attentional shifts and eye movements. *Nature, 384,* 74-77.

Lee, P. H., Helms, M. C., Augustine, G. J., & Hall, W. C. (1997). Role
of intrinsic synaptic circuitry in collicular sensorimotor integration.
*Proceedings of the National Academy of Science of the United States of
America, 94,* 13299-13304.

McCann, R. S., Remington, R. W., & Van Selst, M. (2000). A dual-task
investigation of automaticity in visual word processing. *Journal of
Experimental Psychology: Human Perception and Performance, 26,*
1352-1370.

Mokler, A. Deubel, H. & Fischer, B. (2000, August). Unintended saccades
can be executed without presaccadic attention shift. Poster presented at
the 23rd European Conference on Visual Perception. Groningen, The
Netherlands.

Motter, B. C. (1994). Neural correlates of attentive selection for color
or luminance in extrastriate area V4. *Journal of Neuroscience, 14,*
2178-2189.

Munoz, D. P., Dorris, M. C., Pare, M., & Everling, S. (2000). On your
mark, get set: Brainstem circuitry underlying saccadic initiation.
*Canadian Journal of Physiological Pharmacology, 78,* 934-944.

Munoz, D. P., & Wurtz, R. H. (1993). Fixation cells in monkey superior
colliculus. I. Characterisitcs of cell discharge. *Journal of
Neurophysiology, 70,* 559-575.

Nobre, A. C., Gitelman, D. R., Dias, E. C., & Mesulam, M. M. (2000).
Covert visual spatial orienting and saccades: Overlapping neural
systems. *NeuroImage, 11,* 210-216.

Ottes, F. P., Van Gisbergen, J. A. M., & Eggermont, J. J. (1987).
Collicular involvement in a saccadic colour discrimination task.
*Experimental Brain Research, 66,* 465-478.

Pare, M., & Wurtz, R.H. (2001). Progression in neuronal processing for
saccadic eye movements from parietal cortex area LIP to superior
colliculus. *Journal of Neurophysiology, 85*, 2545-2562.

Posner, M. I. (1980). Orienting of Attention. *The Quarterly Journal of
Experimental Psychology, 32,* 3-25.

Rayner, K., & Pollatsek, A. (1989). *The Psychology of Reading*.
Englewood Cliffs, NJ: Prentice Hall.

Remington, R. W. (1980). Attention and saccadic eye movements. *Journal
of Experimental Psychology: Human Perception and Performance, 6,*
726-744.

Remington, R. W., Folk, C. L., & McLean, J. P. (2000). Contingent
attentional capture or delayed allocation of attention? *Perception and
Psychophysics, 63,* 298-307.

Remington, R. W., & Folk, C. L. (2001). A dissociation between attention
and selection. *Psychological Science, 12,* 511-515.

Reynolds, J. H., Chelazzi, L., & Desimone, R. (1999). Competitive
mechanisms subserve attention in macaque areas V2 and V4. *Journal of
Neurophysiology, 19,* 1736-1753.

Reynolds, J. H., & Desimone, R. (2003). Interacting roles of attention
and visual salience in V4. *Neuron, 37,* 853-863.

Reynolds, J. H., Pasternak, T., & Desimone, R. (2000). Attention
increases sensitivity of V4 neurons. *Neuron, 26,* 703-714.

Rizzolatti, G., Riggio, L., Dascola, I., & Umiltà, C. (1987).
Reorienting attention across the horizontal and vertical meridians:
evidence in favor of a premotor theory of attention. *Neuropsychologia,
25,* 31-40.

Rizzolatti, G., Riggio, L., & Sheliga, B. M. (1994). Space and selective
attention. In C. Umiltà & M. Moscovitch (Eds.), *Attention and
performance XV: Conscious and Nonconscious Information Processing* (pp.
231-265). Cambridge, MA: MIT Press.

Salthouse, T. A., & Ellis, C. L. (1980). Determinants of eye-fixation
duration. *American Journal of Psychology, 93,* 207-234.

Schall, J. D., Hanes, D. P., Thompson, K. G., King, D. J. (1995).
Saccade target selection in frontal eye field of macaque. I. Visual and
premovement activation. *Journal of Neuroscience, 15,* 6905-6918.

Segraves, M. A. (1992). Activity of monkey frontal eye field neurons
projecting to oculomotor regions in the pons. *Journal of
Neurophysiology, 68,* 1967-1985.

Sheliga, B. M, Riggio, L., & Rizzolatti, G. (1994). Orienting of
attention and eye movements. *Experimental Brain Research, 98*, 507-522.

Sheliga, B. M., Riggio, L., Craighero, L., Rizzolatti, G. (1995).
Spatial attention-determined modifications in saccade trajectories.
*NeuroReport, 6,* 585-588.

Snowden, R. J. (2002). Visual attention to color: Parvocellular guidance
of attentional resources? *Psychological Science, 13,* 180-184.

Stanton, G. B., Bruce, C. J., & Goldberg, M. E. (1995). Topography of
projections to posterior cortical areas from the macaque frontal eye
fields. *Journal of Comparative Neurology, 353,* 291-305.

Theeuwes, J. (1991). Exogenous and endogenous control of attention: The
effect of visual onsets and offsets. *Perception and Psychophysics, 49,*
83-90.

Theeuwes, J. (1992). Perceptual selectivity for color and form.
*Perception and Psychophysics, 51,* 599-606.

Theeuwes, J. (1994). Stimulus-driven capture and attentional set:
Selective search for color and visual abrupt onsets. *Journal of
Experimental Psychology: Human Perception and Performance, 20,* 799-806.

Theeuwes, J. (1996). Perceptual selectivity for color and form: On the
nature of the interference effect. In A. F. Kramer, M. G. H. Coles, & G.
D. Logan (Eds.), *Converging Operations in the Study of Visual Selective
Attention* (pp. 297-314). Washington, DC: American Psychological
Association.

Theeuwes, J., & Godijn, R. (2001). Attentional and oculomotor capture.
In C. L. Folk & B. S. Gibson (Eds.), *Attraction, Distraction and
Action: Multiple Perspectives on Attentional Capture* (pp. 121-149).
Elsevier.

Theeuwes, J., Kramer, A. F., Hahn, S., & Irwin, D. E. (1998). Our eyes
do not always go where we want them to go: Capture of the eyes by new
objects. *Psychological Science, 9,* 379-385.

Theeuwes, J., Kramer, A. F., Hahn, S., Irwin, D. E., & Zelinsky, G. J.
(1999). Influence of attentional capture on oculomotor control. *Journal
of Experimental Psychology: Human Perception and Performance, 25,*
1595-1608.

Treue, S. (2001). Neural correlates of attention in primate visual
cortex. *Trends in Neuroscience, 24,* 295-300.

Tse, P. U., Sheinberg, D. L., & Logothetis, N. K. (2002). Fixational eye
movements are not affected by abrupt onsets that capture attention.
*Vision Research, 42,* 1663-1669.

Viviani, P., Berthoz, A., & Tracey, D. (1977). The curvature of oblique
saccades. *Vision Research, 17,* 661-664.

Wurtz, R. H., Sommer, M.A., Pare, M., & Ferraina, S. (2001). Signal
transformations from cerebral cortex to superior colliculus for the
generation of saccades. *Vision Research, 41,* 3399-3412.

Yantis, S., & Hillstrom, A. P. (1994). Stimulus-driven attentional
capture: Evidence from equiluminant visual objects. *Journal of
Experimental Psychology: Human Perception and Performance, 20,* 95-107.

Yantis, S., & Jonides, J. (1984). Abrupt visual onsets and selective
attention: Evidence from visual search. *Journal of Experimental
Psychology: Human Perception and Performance, 10,* 601-620.

## Author Note

Shu-Chieh Wu and Roger W. Remington, NASA Ames Research Center.

This research was supported by a post-doctoral fellowship from the
National Research Council and by funding from the Airspace Operations
Systems (AOS) Project of NASA\'s Airspace Systems Program. We thank Ray
Klein, Brad Gibson, Richard Godijn, Joel Lachter, Jim Johnston, and an
anonymous reviewer for helpful comments on earlier versions of this
article. We also thank Jeff Mulligan and Lee Stone for their technical
assistance. Portions of this article were presented at the 42nd Annual
Meeting of the Psychonomic Society, Orlando, Florida, November 2001.

Correspondence concerning this article should be addressed to Shu-Chieh
Wu, NASA Ames Research Center, MS 262-4, Moffett Field, California
94035. Electronic mail may be sent to <scwu@mail.arc.nasa.gov> or
<rremington@mail.arc.nasa.gov>.

## Footnotes

1\. Due to the addition of color distractor conditions, it was not
possible to obtain sufficient observations among oblique saccades to
provide reliable trajectory information. Saccade trajectories were
therefore not analyzed in subsequent experiments.

2\. In the control condition the eyes went elsewhere on about 20% of the
trials. That is, each non-target location has a base rate of 4 to 5% to
be the destination of a saccade. Elimination of oculomotor capture means
that the proportion of saccades that goes to a particular distractor
item does not exceed chance level.

3\. As it occurred during the course of the study that some participants
had difficulty maintaining fixation at the central cross, the last five
of the twenty participants in the attention condition were run in a
modified version of the experimental program that issued warnings of
deviated eye fixation should such instance occurred three trials in a
row.

4\. Fixation was considered maintained when no saccadic eye movement was
detected, or when detected, the endpoint of the first movement from
fixation did not go beyond one half of the display radius (3.4 degrees).
The proportions of fixated trials varied from 55% to 95% among
participants. We analyzed the distribution of eye movement endpoints on
fixated (included) and moved (excluded) trials in the onset only
condition to determine whether the excluded trials consisted of mostly
trials with eye movements to onsets. The analysis was conducted based on
the collective data from all participants. On fixated trials with
detected saccadic eye movements (\~31%), 53% of the eye movements went
to target, 7% to onset, and 40% to elsewhere. On moved trials, 49% of
the eye movements went to target, 14% to onset, and 37% to elsewhere. It
is evident that on most excluded trials the eyes went to the target.

5\. The distance effect analysis was based on all valid trials with no
regard to eye position in order to attain a workable number of
observations at each angular distance.

Table 1

Proportions of first saccades directed toward the target and elsewhere
by distractor conditions in Experiment 2-5

  ------------ ------------ ------- ------------ ------- ------------ ------- ------------ -------
  Distractor   Experiment 2         Experiment 3         Experiment 4         Experiment 5 
  conditions                                                                               

               Target       Else    Target       Else    Target       Else    Target       Else

  Control      0.81         0.19    0.83         0.17    0.78         0.22    0.79         0.21

  Onset only   0.69         0.20    0.69         0.10    0.68         0.18    0.72         0.15

  Color only   0.60         0.20    0.84         0.14    0.78         0.18    0.71         0.18

  Both         0.53         0.17    0.64         0.08    0.68         0.15    0.60         0.16
  ------------ ------------ ------- ------------ ------- ------------ ------- ------------ -------

Table 2

Saccade latency (ms) by direction of first saccades and distractor
conditions and fixation duration (in parenthesis) by distractor type in
Experiment 2

  ---------------- ------------- ------------- ------------- -------------
  Distractor       Direction of                              
  conditions       first                                     
                   saccades                                  

                   Target        Onset         Color         Elsewhere

  Control          286           \-\--         \-\--         174

  Onset only       271           203 (73)      \-\--         209

  Color only       289           \-\--         223 (50)      236

  Both             283           194 (87)      220 (74)      225
  ---------------- ------------- ------------- ------------- -------------

*Note*. Dashes indicate the saccade latency was not measured.

Table 3

Saccade latency (ms) by direction of first saccades and distractor
conditions and fixation duration (in parenthesis) by distractor type in
Experiment 3

  ---------------- ------------- ------------- ------------- -------------
  Distractor       Direction of                              
  conditions       first                                     
                   saccades                                  

                   Target        Onset         Color         Elsewhere

  Control          241           \-\--         \-\--         218

  Onset only       240           204 (69)      \-\--         223

  Color only       242           \-\--         \-\--         218

  Both             239           189 (71)      \-\--         223
  ---------------- ------------- ------------- ------------- -------------

*Note*. Dashes indicate the saccade latency was not measured.

Table 4

Saccade latency (ms) by direction of first saccades and distractor
conditions and fixation duration (in parenthesis) by distractor type in
Experiment 4

  ---------------- ------------- ------------- ------------- -------------
  Distractor       Direction of                              
  conditions       first                                     
                   saccades                                  

                   Target        Onset         Color         Elsewhere

  Control          334           \-\--         \-\--         313

  Onset only       315           264 (114)     \-\--         271

  Color only       338           \-\--         \-\--         260

  Both             323           265 (121)     \-\--         264
  ---------------- ------------- ------------- ------------- -------------

*Note*. Dashes indicate the saccade latency was not measured.

Table 5

Saccade latency (ms) by direction of first saccades and distractor
conditions and fixation duration (in parenthesis) by distractor type in
Experiment 5.

  ---------------- ------------- ------------- ------------- -------------
  Distractor       Direction of                              
  conditions       first                                     
                   saccades                                  

                   Target        Onset         Color         Elsewhere

  Control          300           \-\--         \-\--         277

  Onset only ^a^   296           222 (81)      \-\--         266

  Color only       292           \-\--         245 (70)      268

  Both ^a^         294           230 (76)      234 (71)      271
  ---------------- ------------- ------------- ------------- -------------

*Note*. Dashes indicate the saccade latency was not measured.

^a^ Trials with onsets at 30 degrees were not included.

## Figure Captions

*Figure 1.* Sequence of events within a trial for Experiment 1. The
target display shown here contains an abrupt onset distractor.

*Figure 2.* Proportions of saccades made toward onset and color
distractors by distractor conditions in Experiments 1 to 5. Asterisks
indicate proportions that exceeded chance level (*p* \< .05, one-tailed
t-tests).

*Figure 3.* Manual response times and error rates of the saccade task
condition in Experiments 1 to 5.

*Figure 4.* Differences in the curvature of saccade trajectories caused
by ipsilateral and contralateral distractors in Experiments 1. Black
asterisks indicate the results of one-tailed t-tests with *p* values
below .05.

*Figure 5.* Manual response times and error rates of the attention task
condition in Experiments 1 to 5. In Experiment 5 response times were
based on fixated trials only.

*Figure 6.* Fixation and target displays for Experiments 2 to 4. Note
that Experiments 2 to 4 had identical target displays; they only
differed in the composition of the fixation display.

*Figure 7.* Manual response times of the attention task condition in
Experiment 2, broken down by first and second halves of session.

*Figure 8*. Distance effect of manual response times of the attention
task condition in Experiment 5.

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