Sensors and Materials, Vol. 24, No. 6 (2012) 347–358
MYU Tokyo
S & M 0890

Mechanisms of Motion-Defined Shape Perception
Elucidated by the Regression Analyses between
Psychophysical and Psychophysiological Data
Shin’ichiro Kamiya*, Toshiyo Tamura and Takao Sato1
Department of Medical System Engineering, Graduate School of Engineering, Chiba University
1Department of Psychology, Graduate School of Humanities and Societies, University of Tokyo(Received September 29, 2011; accepted March 19, 2012)
Key words: motion-defined shape perception, visual field, random dot kinematogram, evoked
potentials, regression analysis

To elucidate the mechanisms of motion-defined shape perception (SP), we carriedout experiments by employing the random-dot kinematogram. Participants wereasked 1) whether a coherently moving dot in a core rectangle was moving “toward theupper left or lower right” (direction discrimination task) and 2) whether the shape of amotion-defined rectangular area composed of the moving dots was “oblong vertically or
horizontally” (pattern discrimination task) with various moving velocities from 14.4 to
126 deg/s. When stimuli were presented in the upper or lower half of the visual field (VF),the rate of correct responses for six participants were significantly higher in the lower
VF than in the upper VF for the pattern discrimination task (lower VF enhancement) at
25 and 40 deg/s (p<0.05) but not for the direction discrimination task. When the visualevoked potential was measured in the psychophysiological experiments with similar
stimuli, the amplitudes of the peak potential at 290 ms were significantly correlatedwith the data of correct-response rates for the pattern discrimination task (p<0.05)
alone. It was suggested that the information was initially treated for motion itself in themagnocellular system and then returned presumably to the parvocellular system in the
primary visual cortex to accomplish the motion-defined SP at approximately 290 ms.

1.

Introduction

Motion-defined shape perception (SP) is realized when we watch an area composedof coherently moving dots surrounded by a “sandstorm” region of incoherently moving
dots. Obviously, SP is an essential element in visual information processing, and is
indispensable for pattern recognition.
Corresponding author: e-mail: skamiya@graduate.chiba-u.jp

*

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Several types of SP have been identified on the basis of their specific visual
dimensions. They are broadly classified as luminance-defined SP,(1–5) motion-definedSP,(3,6–8) disparity-defined SP,(9,10) texture-defined SP,(11,12) orientation- and color-definedSP,(10) and illusory contour perception.(13) In general, these types of SP are processed by
one or both of the two neuronal systems, the parvocellular system, which processes shapeand color, and the magnocellular system, which deals with motion and location.(14,15)
Luminance-defined SP is the most fundamental among them. In psychophysiologicalstudies of this SP, Jeffreys(1) demonstrated that the peak components of the visual-evoked
potential (VEP) revealed the polarity reversal corresponding to upper and lower visual
field (VF) presentations. This polarity reversal was regarded as reflecting the retinotopyof the early visual cortex.(16)
Furthermore, in psychophysical experiments for this luminance-defined SP, McAnany
and Levine(17) reported that only under the parvocellular-dominant condition, did the
responses in the experiments exhibit visual anisotropy, namely, enhanced sensitivity tostimuli presented in the lower VF (lower VF enhancement).
Motion-defined SP is somewhat complicated, as it includes two processes: motion
detection and shape perception. It has been established that motion detection is
performed in the magnocellular system represented by the middle temporal area(MT)(18–24) or V3A.(25–32) Concerning the role of the parvocellular system in this SP,
however, no direct evidence has been presented.
A variety of psychophysical and psychophysiological analyses have also been
applied to the investigation of motion-defined SP. In their psychophysical experiments,
Chang and Julesz(6) employed an apparent motion stimulus made using a random-dot
kinematogram (RDK) of two frames and examined two tasks: the pattern of the area
composed of coherently moving dots and the direction of their motion. When the amountof displacement was increased, the rates of correct responses to both tasks decreased.
However, the threshold of the displacement (Dmax) was much larger for the direction
discrimination task than for the pattern discrimination task. In their psychophysiologicalstudy, Niedeggen and Wist(33) found that the onset of coherent motion per se elicited anegative peak component at around 200 ms in VEP. By contrast, Bach and Meigen(34)emphasized that the VEPs induced by four types of SP (namely, luminance-defined,orientation-defined, motion-defined, and disparity-defined) shared a common peakcomponent appearing at 230–310 ms. However, the results obtained in psychophysicalexperiments have remained isolated from the above psychophysiological findings, andno attempt has been made to clarify the relationship between them. To gain more insight
into motion-defined SP, it is important to perform both types of experiment with thesame stimuli on the same participants and to carry out regression analyses between thepsychophysical and psychophysiological data obtained.
In this study, we provided a series of motion-defined stimuli by employing RDK andcarried out the psychophysical experiments for two tasks of direction discriminationand pattern discrimination as well as the psychophysiological experiment for VEP
measurement around the occipital area induced by the same stimuli. Regression
analyses were performed between the data on the rate of correct responses for each task(acquired in the psychophysical experiments) and the component of VEPs (revealedin the psychophysiological experiment). The results of the analyses were mainlySensors and Materials, Vol. 24, No. 6 (2012)

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utilized to identify (1) the time course and (2) the activated areas of the motion-defined
SP. In addition, we tried to examine the interaction between the magnocellular andparvocellular systems in this SP. Finally, the general implication of such regression
analyses in perception studies is discussed.

2.

Materials and Methods

2.1

Participants

2.2

Stimuli

2.3

Procedure of psychophysical experiments

Six students of the same university with normal or corrected-to-normal vision (visualacuity>1.0) participated in the experiments. All of them (four males and two females,
average age 21.4 years old) gave their written informed consent to participate in theexperiments in advance. Participants were all right-handed.
The visual stimuli were generated with a Power Macintosh G4 computer using thePsychophysicstoolbox program library,(35) which, together with Videotoolbox, enabled
the interruption of scanning lines via “VBL task”. Stimuli were presented on a CRT
screen (EIZO, FlexScan T565) at a frame rate of 60 Hz, resolution of 640×480 pixels,
and viewing distance of 57 cm. The stimuli were composed of RDKs. The size of therandom dots was 3.6 arc min; half were white (22.14 cd/m2) and half were black (0 cd/m2) (Fig. 1). The RDK window (240×240 pixels) was placed in the upper half or lower
half of the center of the screen, surrounded by an averaged gray background (11.07 cd/m2). In the center of the RDK window, we positioned the core rectangular region (120×60 pixels), which was elongated either vertically or horizontally. In the core region
presented at 7.2 deg eccentricity, the dots moved coherently in either an upward andleft or a downward and right direction. The velocities of the moving dots varied across
six levels ranging from 14.4 to 126 deg/s (14.4, 18, 25.2, 39.6, 68.4, and 126 deg/s).
The rest of the RDK region was a “sandstorm” area composed of incoherently moving
dots. The duration of stimulation (“ON” period) was 500 ms, and each presentation
was followed by an “OFF” period, during which time the dots in the entire RDK region
moved incoherently for 500 ms. The fixation point, set at the center of the screen as ared point (2×2 pixels), was continuously turned on during both “ON” and ”OFF” periods.
The experiments focused on two independent tasks: (1) pattern discrimination and(2) direction discrimination. The pattern discrimination task consisted of 12 conditions(six moving-dot velocities and two core patterns). The participants were asked to answer
whether the contour of the core rectangular region was oblong vertically or horizontally.
In this task, the moving direction was fixed to the lower right [Fig. 1(a)].
The direction discrimination task also included 12 conditions (six moving-dot
velocities and two moving directions). The participants were asked to answer whether
the direction of the moving dots was toward the upper left or lower right. In this task,
the core region was fixed at a horizontally elongated pattern [Fig. 1(b)].
For both tasks, each condition set was repeated 20 times, giving a total of 240 trials

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(a)

(b)
Fig. 1. Schematic drawings of RDK stimuli used in psychophysical experiments for patterndiscrimination task (a) and direction discrimination task (b). The core rectangular region
composed of coherently moving dots in the direction indicated by arrows is surrounded by the"sandstorm" zone composed of incoherently moving dots. For the pattern discrimination task inpanel (a), the participants were asked whether the pattern of the core rectangular region was oblong
vertically or horizontally, whereas for the direction discrimination task in panel (b), they wereasked whether the direction of moving dots was toward the lower right or upper left. The velocityof the moving dots was varied in 6 levels (14.4, 18, 25.2, 39.6, 68.4, and 126 deg/s). The stimulus
used in the case of the right side of panel B was also used in the psychophysiological experiments.

presented randomly in a series (constant stimuli method). When one trial was initiated,
the response was entered through the keyboard of the PC, using the two-alternative
forced-choice method during either the “ON” or “OFF” period. Then the next trial was
followed immediately. The first sequence of 240 trials was performed with either upperor lower VF presentation, and the VF presentation was reversed for the second sequence.
The participants were instructed to give their responses as accurately as possible.

2.4

Procedure of psychophysiological experiments

In the psychophysiological experiments, the velocity of the RDK stimuli was also
varied across six levels, but the core rectangular shape and motion direction were fixed
to horizontal and lower right, respectively, because preliminary experiments had shownthat no difference in VEPs was elicited by using two shapes or two directions. Each

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participant was exposed to the same RDK stimuli 100 times to record one VEP in either
the upper or lower VF. The active electrode was placed on Oz (the 10% system), above
the early visual cortex, and the reference electrode was placed on the left earlobe.(1) The
impedance of each electrode was confirmed to be less than 5 kΩ at 10 Hz. EEG wasrecorded with Biotop 6R12-2 amplifiers (NEC Corporation, Japan). Data were digitizedwith a sampling rate of 1,000 Hz and stored on a PC. A high-cut filter was used forthe measurement to diminish the high-frequency noise over 30 Hz. Trials containing
eyeblinks were excluded off-line. In our preliminary experiment, we confirmed thatthe time difference between the trigger signal for the recording system and that for thepresentation was less than 5 ms.

3.

Results

3.1

Results of psychophysical experiments

Figure 2 shows the relationship between the moving-dot velocity (abscissa inlogarithmic scale) and the correct-response rate (ordinate) obtained in the psychophysicalexperiments. The correct-response rate data shown are averages of the six participants.
Panel A shows the results of the pattern discrimination task in the upper and lower VF
presentations. Panel B shows the results of the direction discrimination task in each of
the two VFs. The data in each graph were fitted to a sigmoid function curve, and thethreshold (75% correct-response rate) was estimated from each curve. The threshold in
the pattern discrimination task in Panel A was 23.8 deg/s in the upper VF and 34.4 deg/s

(a)

(b)

Fig. 2. Results of psychophysical experiments for pattern-discrimination task (a) and directiondiscrimination task (b). The correct rate is plotted vs the dot velocity in a logarithmic scale. The
dotted line with blank circles (○) indicates the case of the upper-VF presentation, and the solid linewith solid circles (●) shows the case of the lower VF. The significant difference between the upperand lower presentations is indicated by asterisks (*: p<0.05). The horizontal line designates thethreshold level of the correct rate of 0.75.

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in the lower VF. The threshold in the direction discrimination task in Panel B was 67.7
deg/s in the upper VF and 58.1 deg/s in the lower VF. These results suggested that thedirection discrimination task was much easier than the pattern discrimination task in both
VF presentations.
In Fig. 2(a), the effect of VF on the correct-response rate was examined bystatistically analyzing (two-tailed paired t-test) the data of the six participants for eachmoving velocity. The results revealed that the correct-response rate for the patterndiscrimination was significantly higher in the lower VF presentation than in the upper
VF presentation at the velocities of 18 deg/s [t(5)=2.37, p<0.05], 25.2 deg/s [t(5)=3.31,
p<0.05], and 39.6 deg/s [t(5)=2.81, p<0.05]. Figure 2(b), however, shows no significant
difference in the correct-response rate for direction discrimination at any velocity. The
results of these psychophysical experiments demonstrated that lower VF enhancement
was observed for pattern discrimination but not for direction discrimination.

3.2

Results of psychophysiological experiments

3.3

Correlation between psychophysical and psychophysiological data

Panels A–F in Fig. 3 depict the grand means of VEPs induced by RDK stimuli for thesix participants at each velocity (14.4, 18.0, 25.2, 39.6, 68.4, and 126.0 deg/s). The solidlines represent VEPs elicited by lower VF presentation, and the dotted lines represent
VEPs elicited by upper VF presentation. It should be noted that components later than
300 ms involve event-related potentials,(36) which are irrelevant to VEPs.
Within the time range of 300 ms, the lower VF responses, represented by solid lines,
revealed clear negative peaks within the range of 260–290 ms, although they became lessconspicuous as the velocity increased. As for the upper VF responses, represented bydotted lines, the positive peaks were also observed at around 290 ms, except in the caseshown in panel D (39.6 deg/s). The reversed polarities of the peak components elicitedby the upper and lower VF presentation in this study were consistent with the results ofprevious VEP studies obtained by luminance-defined checkerboard stimulation.(1,2)
Changes in the peak amplitudes of the VEPs in relation to moving-dot velocity are
shown in Fig. 4. The solid circles represent the absolute values of the negative peaks
elicited by lower VF presentation, and the empty circles indicate the peak values induced
by upper VF presentation. The dot velocities are plotted in a logarithmic scale. In thevelocity range from 14.4 to 39.6 deg/s, the peak amplitudes of the VEPs elicited by lower
VF stimuli were much larger than those elicited by upper VF stimuli. The ratio of thelower/upper amplitudes was maximally 5.8 at 25.2 deg/s.
To investigate the relationships between the results of the psychophysical andpsychophysiological experiments, we carried out regression analyses of the data on therates of correct responses with the absolute amplitudes of the VEP peak around 290 ms.
As shown in Fig. 5(a), the correct-response rate data for the pattern discrimination task
and the absolute peak amplitudes showed a significant correlation in both the lower VF[r=0.89, t(4)=3.88, p<0.01] and upper VF [r=0.73, t(4)=2.15, p<0.05]. By contrast, no
statistically significant correlation was found for the direction discrimination task (panel B)
for either the lower or upper VF presentation [r=0.67, t(4)=1.81, n.s.; r=0.53, t(4)=1.25, n.s.,Sensors and Materials, Vol. 24, No. 6 (2012)

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Fig. 3. VEPs elicited by RDK stimuli at six moving-dot velocities. All the solid lines represent
the grand average of VEPs for the six participants evoked by the lower-VF presentation, and thedotted lines show those evoked by the upper-VF presentation. The number in each panel indicatesthe moving-dot velocity. In all the panels, the ordinate is the peak amplitude in μV and theabscissa indicates the elapsed time in ms.

respectively]. These results suggest that the peak deflection of the VEP at around 290 ms
was definitely linked to the information processing of motion-defined SP.

4.

Discussion

4.1

Time course of motion-defined SP

The results in Fig. 5(a) demonstrated that the peak amplitudes of VEPs around 290
ms in the present psychophysiological experiments were significantly correlated withthe correct-response rates for the pattern-discrimination task alone. This result indicatedthat the latency of this shape perception was approximately 290 ms. On the other hand,354

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Absolute peak amplitude (µV)

Absolute peak amplitude (µV)

Fig. 4. Changes in absolute amplitudes of VEP peaks at around 290 ms corresponding to movingdot velocity. The solid points connected with solid lines represent the absolute amplitudes of the
grand average curve for the lower-VF presentation, and the open circles with dotted lines represent
those for the upper VF.

Correct rate

Correct rate

(a)

(b)

Fig. 5. Diagrams of correlation between correct rate and peak amplitude of VEP at around 290
ms. In panel (a), the absolute amplitude of the VEP peak at around 290 ms was significantly
correlated with the rate of correct responses for the pattern discrimination task for both theupper- and lower-VF presentations. The regression line of the peak amplitude (Y) against thecorrect rate (X) was Y=3.20X–0.84 in the lower-VF presentation (r=0.89, p<0.01) and Y= 2.41X–0.86
in the upper-VF presentation (r=0.73, p<0.05). The correlations between the same parameters
for the direction discrimination task, shown in panel (b), were not significant in either the lowerVF (r=0.67, n.s.) or the upper-VF (r=0.53, n.s.) presentation.

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it has been explored in many psychophysical studies that the information processingof coherent motion onset is achieved at about 200 ms.(33,37–41) The difference in latencyfrom 200 to 290 ms may be regarded as the time lag for information transition from thesite for motion detection to that for pattern perception. To confirm this transition model
more precisely, however, it is necessary to carry out experiments with larger numbers ofparticipants.
Bach and Meigen(34) reported, in their psychophysiological study, that a common
component was found in the VEPs elicited by 4 differently defined stimuli (luminance-,orientation-, motion-, or disparity-defined). When the effects of the onset and offset on
VEPs (“low-level” VEP) were cautiously cancelled out, the induced VEPs (“texturesegregation” VEP) all revealed similar waveforms with peaks in the range from 230
to 310 ms. These extensive and important findings, however, must be confirmed inregard to reliability by using other findings acquired from studies of entirely different
approaches. In this aspect, the results in this study can be good counterpart evidence, atleast for motion-defined SP, because the latency (approximately 290 ms) of our VEPs,
of which the specific linkage to SP was certified by the regression analyses between
psychophysical and psychophysiological data, was highly consistent with that of
VEP determined independently of such regression analyses. This agreement suggeststhat similar correlation studies should be applied to other types of SP to elucidate thepsychophysical implication of induced VEPs. In our experiments, the luminance andother static elements of the visual field were kept unchanged at the onset and offset ofthe stimulation. This maneuver was probably effective in minimizing the induction of“low-level” VEPs. Since the term “texture segregation” is nearly identical to our “SP”, it
is not surprising that their findings were highly consistent with the results of the present
study.

4.2

Elicited areas in process of motion-defined SP

In this study, the VEP waveforms at around 290 ms in Fig. 3 exhibited polarity
reversal, which has been regarded as proof of information processing in the early visualcortex, including V1, V2, and V3.(1,16)
Several areas in the early visual cortex have been reported to be concerned withpattern perception in motion-defined SP. In human imaging experiments, the kinetic
occipital (KO) area located between MT and the V3 complex was found to be activatedby a motion-defined shape rather than MT.(42–45) Furthermore, the lateral occipitalcomplex (LOC) area covering the extrastriate visual cortex was also activated inmotion-defined SP as well as luminance-defined SP.(46) In electrophysiological studies
in Macaque monkeys, the V3 complex (V3, VP) area has also been suggested to be an
important area for the integration and transformation of visual signals for motion andpattern perception.(47,48)
In a clinical study in patients with parieto-occipital cortex lesions including
Brodmann’s areas 18 and 19, Regan et al.(49) reported that several patients, who wereintact in motion detection itself, were not able to discriminate motion-defined letter
shapes. Consequently, it was concluded that the area for motion detection must be
clearly isolated from the area for motion-defined shape perception, and must be involved

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in the injured regions of the patients. Note that areas 18 and 19 are largely overlappedwith such areas as V3 complex, KO, and LOC mentioned above. These findings thenwell reinforce our idea that the motion-defined SP is accomplished in the early visualcortex after being treated in the magnocellular system (MT or V3A) for the informationof motion itself.

4.3

Roles of magnocellular and parvocellular systems

4.4

Significance of regression analyses between psychophysical andpsychophysiological data

This is the first report demonstrating the lower-VF enhancement in motion-defined
SP. The results of our psychophysical experiments in Fig. 2(a) revealed that the correctresponse rate for the pattern discrimination task was significantly higher for the lowerVF presentation than for the upper-VF presentation at two velocities (25.2 and 39.6
deg/s), whereas no significant differences in the correct-response rates were found forthe direction discrimination task [Fig. 2(b)]. This result suggests that the discrimination
processes for these two tasks are differently treated in information processing.
It is conceivable that the VEP amplitude difference between the upper- and lowervisual field (VF) presentations is induced by the different locations of activated areas forthe individual presentations. However, according to Portin et al.,(50) although the depthsof dipoles for lower left and upper right stimuli were about equal, the equivalent current
dipole amplitudes were stronger for the lower left than the upper right stimuli, making itvery improbable that their results were due to differences in source depths.
It has been established that direction discrimination is processed in the magnocellular
system in MT(51,52) or V3A.(25,27) In contrast, various studies have suggested that theparticipation of the parvocellular system in image processing is reflected as lower VF
enhancement. For instance, it has been reported in the micro-electrode study(15) that the
parvocellular system plays dominant roles in the treatment of high-spatial-frequency
images. In addition, Skrandies(53) and Carrasco et al.(54) have also demonstrated thatthe increased spatial frequency in the sine wave grating makes the enhancement more
prominent. By combing these two kinds of findings, the lower-VF enhancement may
be assumed to be a sign representing the parvocellular system participation. McAnany
and Levine(17) have shown that the lower-VF enhancement takes place only underparvocellular-dominant conditions in luminance-defined SP. It is then likely that evenin motion-defined SP, the parvocellular system is incorporated in its shape perception
process. To examine such a model, however, we require further detailed spatial
information about excited areas associated with reliable temporal resolution of latencies.

One of the noteworthy findings in this study is, as shown in Fig. 5, a close correlation
between the data on the rate of correct responses for pattern discrimination and the peak
amplitude of VEP at around 290 ms. This means that the more accurate the perception,
the more prominent the peak. In other words, the magnitude of the VEP peak reflects
the accuracy of motion-defined SP. The question then arises whether such a quantitative
relationship between the psychological reaction and the physiological response isconfined to motion-defined SP or can be extended to other types of visual SP or different

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types of perception by any other sensory modality. To the best of our knowledge, thistype of regression analysis between psychophysical and psychophysiological data has
not been reported in conventional perception studies.
The time course of the perception processes in motion-defined SP was identified
with the aid of regression analyses, as shown in Fig. 5(a). A similar approach would bepossible for other types of perception with complex processes, as long as a significant
correlation is established between the correct-response rate for a psychophysical task,
which explicitly represents a process, and a VEP component with a stable latency.
The regression study proposed here may be regarded as a new approach for the timecourse analyses of perception. If such temporal information is combined with spatial
information of excited areas by virtue of noninvasive measurement methods withhigher spatial resolutions (e.g., fMRI, PET etc.), this will obviously lead to a better
understanding of perception mechanisms.

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