Cognitive Brain Research 15 (2003) 324–327
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Short communication

Integration process of contours defined by different attributes
Hiromi Morita a,b , *, Masahiko Morita c , Takatsune Kumada aaVisual Cognition Group, Human Science and Biomedical Engineering, National Institute of Advanced Industrial Science and Technology,Tsukuba Central 6, Tsukuba, Ibaraki 305 -8566, Japanb
New Energy and Industrial Technology Development Organization, 3 -1 -1, Higashi-Ikebukuro, Toshima-ku, Tokyo 170 -6028, Japanc
Institute of Engineering Mechanics and Systems, University of Tsukuba, 1 -1 -1, Ten-nodai, Tsukuba, Ibaraki 305 -8573, Japan
Accepted 23 July 2002

Abstract
We examined how contours defined by different attributes are integrated into a shape using the visual search paradigm and found thatthe search for the bar orientation is particularly difficult when each bar is composed of motion-defined and luminance- or color-definedcontours. This does not support the view that the shape integration process is based on the unified contour representation entirely common
to all attributes, but suggests that motion-defined contours are integrated through a different process.
 2002 Elsevier Science B.V. All rights reserved.
Theme: Sensory systems
Topic: Visual psychophysics and behavior
Keywords: Contour; Attribute; Visual search; Orientation; Integration

We can recognize the same shape whether it is depictedby a difference in luminance, color, or by motion, eventhough the visual system processes these attributes separately using their respective modules [4,5]. This leads to thequestion of how such attribute-independence is secured in
the shape processing. A simple and plausible hypothesis isthat contours of an input image are represented attributeindependently, as suggested by evidence such as the interattribute tilt effect [9], and then integrated into shape.
According to this hypothesis, basic shape features should
be extracted irrespective of the attribute defining thecontours. This appears to be supported by Cavanagh et al.
[1], who found that the efficiency of visual search for bar
orientation was not varied even if the attribute defining thebars was changed. Their result, however, does not meanthat the shape integration process is independent of thecontour attributes because in their experiment, each bar
was defined by a single attribute. Here we report that wheneach bar is composed of two kinds of contours, the searchefficiency depend strongly on the combination of contour
attributes, which does not fully support the above view.
*Corresponding author. Tel.: 181-298-61-6732; fax: 181-298-616649.

Subjects were instructed to search for a bar oriented 458
to the right among bars oriented 458 to the left, or viceversa, and to respond to the presence or absence of thetarget by pressing one of two keys. (The orientations of atarget and distractors were counterbalanced between subjects.) A target was present in one-half of the trials. Four,8, 12, or 16 bars were presented on a square background(153158) filled with a black-and-white random-dot pattern; each dot (0.05430.0548) was randomly assigned
black (0.56 cd / m 2 ) or white (30 cd / m 2 ). Each bar
comprised a pair of squares (0.7630.768) adjacent to eachother (Fig. 1).
A luminance-color display contained equal numbers ofluminance-defined and color-defined squares. One-half ofthe luminance-defined squares were filled with a randomdot pattern of black and gray (15 cd / m 2 ), and the otherhalf with a random-dot pattern of gray and white. Similarly, the color-defined squares were filled with a random-dot
pattern of red and black or green and black, where the red
and green stimuli were set to be equiluminant with white
stimuli using the minimum flicker technique for eachsubject. The luminance-motion display was the same as theluminance-color display but the color-defined squares werereplaced by motion-defined ones. One-half of the motion-

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H. Morita et al. / Cognitive Brain Research 15 (2003) 324–327

325

Fig. 1. Schematic representation of luminance-color displays under (a) inter-attribute and (b) intra-attribute conditions.

defined squares were filled with random dots of black andwhite moving coherently to the left at a speed of 2.28 / s;the other half were filled with the same dots moving to theright at the same speed. The color-motion display wascomposed of the color-defined and the motion-definedsquares.
For each of these three types of displays, two conditions,inter- and intra-attribute conditions, were used. Under theintra-attribute condition, for a pair of squares forming abar, each had the same attribute but different values (e.g.,
red-and-black and green-and-black random dots), whereas
they were defined by different attributes under the interattribute condition. Note that the components are the samebut the configurations are different for these two conditions.
After a 1-s presentation of a white fixation circle
subtending 0.168 (120 cd / m 2 ) at the center of the display,bars were presented in square areas selected randomly
within a 434 matrix. The display remained until thesubjects responded. Each type of display was observed by
12 paid volunteers who did not know the purpose of thestudy. They participated in one practice and two experimental blocks of 160 trials for each condition. The
experiments were carried out with the understanding andwritten consent of each subject.
The response time (the time measured from the presentation of the display to the subjects’ response) as afunction of the number of bars (search function) wascompared between the inter- and intra-attribute conditions.
We analyzed the data focusing on the slope of the searchfunction (search rate), which corresponds to the difficultyin the search, for the target-present trials (Fig. 2); we
omitted the data in the target-absent trials where subjects
often searched the display more than once and the responsetime varied considerably widely. The search rate is considered to reflect the time required to integrate two squares
and to extract the orientation of the bars. There was no
significant difference in search rate between the inter- andintra-attribute conditions for the luminance-color displays(F(1,10)52.484, P.0.05). The search rates were 31 and21 ms / item for the inter- and intra-attribute conditions,respectively. In contrast, for the luminance-motion displays, the search rate for the inter-attribute condition wassignificantly higher than that for the intra-attribute condition (75 and 35 ms / item, respectively) (F(1,10)516.984,P,0.005). The difference was even greater for the colormotion displays, that is, the search rates for the inter- andintra-attribute conditions were 83 and 18 ms / item, respectively (F(1,11)577.343, P,0.0001).
This difference might be caused by the boundary line
between two squares which can be a key for determining
the orientation of the bars because its saliency is possibly
different for the inter- and intra-attribute conditions. To
test this possibility, we eliminated the boundary line byinserting a small gap of 0.198 between each pair ofsquares. The result was, however, almost the same as theprevious one (Fig. 2), indicating that the boundary line
does not affect the search rate; the difference in search rate
between inter- and intra-attribute conditions was not
significant for the luminance-color display (32 and 25326

H. Morita et al. / Cognitive Brain Research 15 (2003) 324–327

Fig. 2. Response time as a function of the number of items in a display under the inter-attribute (black circles) and intra-attribute (white triangles)
conditions for the luminance-color, luminance-motion and color-motion displays: the results of the experiments (a) using bars composed of adjacentsquares and (b) using bars with a small gap inserted between the adjacent squares. Each point represents the mean for 8 to 12 subjects. Error bars indicate
61 S.E.

ms / item, respectively; F(1,8)52.007, P.0.05), but was
highly significant for the luminance-motion display (75and 24 ms / item, respectively; F(1,11)545.490, P,0.0001), and for the color-motion display (85 and 23
ms / item, respectively; F(1,7)5101.684, P,0.0001). It istherefore concluded that integrating motion-defined contours with luminance- or color-defined contours into acomplete shape requires a much longer time than integrating those defined by the same attribute.
These results are not in accordance with the view thatthe shape integration process is based on a completely
unified representation of contours which is entirely common to all attributes, since although this view predicts thatthe search rate will be constant for all displays andconditions, the contour attributes affected the process oforientation extraction of shape or integrated contours. The
results also suggest that luminance- and color-defined

contours may possibly have unified representation onwhich the shape integration process is based, but motiondefined contours may be processed separately from it.
With regard to this, it is widely known that the ventral
pathway of the visual cortex processes shape and color
perception and the dorsal pathway deals with spatial andmotion processing [3,8,12]. Physiological evidence, however, indicates that motion-defined shape is processed in
the ventral pathway. For example, neurons in the inferior
temporal (IT) cortex respond to their preferred shapeirrespective of whether it is defined by luminance ormotion contrast [10], whereas neurons in area MT do notexhibit an orientation-selective response to motion-defined
contours [6]. Also, orientation discrimination of motiondefined shape is impaired by lesions of area V4 [2,11]. In
addition, neurons with selectivity in orientation of motiondefined contours are found in area V2 [7], and most of

H. Morita et al. / Cognitive Brain Research 15 (2003) 324–327

them respond to luminance-defined contours of the sameorientation. This evidence appears to support a model inwhich information on motion and luminance boundaries
converges on single cells in V2 to form unified contour
representation, and the integration processes in V4 and IT
are based on such representation. Nevertheless, as discussed above, this model does not agree with the results ofour experiment.
A clue to solve this problem might be the finding that
the V2 neurons responded about 40 ms slower to motiondefined boundaries than to luminance-defined ones [7],although the latencies in IT do not differ [10]. This
indicates that contour representation in V2 is attributeindependent but not common to all attributes in thetemporal aspect. Together with our experimental results,
this suggests that the attribute-independence is not securedby the simple bottom-up stream of processing but isinvolved with some top-down process, as assumed in themodel presented by Marcar et al. [7] in which motion
information is injected into the ventral pathway, possiblyat V4 from V3, and sent backward to V2. It is interestingthat the difference in the search rate between inter- andintra-conditions for luminance-motion display (40 or 51ms) is close to the above difference in response onset ofV2 neurons; this may not be a mere coincidence but may
reflect a process of merging two representations by adjusting the time difference.

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