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Illusory Contour Perception and Amodal Boundary Completion: Evidence of a Dissociation Following Callosotomy

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Illusory Contour Perception and Amodal Boundary Completion: Evidence of a Dissociation Following Callosotomy Illusory Contour Perception and Amodal Boundary Completion: Evidence of a Dissociation Following Callosotomy Paul M. Corballis and Robert Fendrich Dartmouth College Robert M. Shapley New York University Michael S. Gazzaniga Dartmouth College Abstract A fundamental problem in form perception is how the tion of illusory contours and amodal completion. Although visual system can link together spatially separated contour both cerebral hemispheres appear to be equally capable of fragments to form the percept of a unitary shape. Illusory perceiving illusory contours, amodal completion is more read- contours and amodal completion are two phenomena that ily utilized by the right hemisphere. These results suggest that demonstrate this linking process. In the present study we illusory contours may be attributable to low-level visual proc- investigate these phenomena in the divided hemispheres of esses common to both hemispheres, whereas amodal comple- two callosotomy (“split-brain”) patients. The data suggest that tion reects a higher-level, lateralized process. dissociable neural mechanisms are responsible for the genera- INTRODUCTION Figure 1a, no illusory contours are seen. In such cases, the grouping process responsible for the perception of To perceive objects in the environment as unied the rectangle has been termed “amodal completion” wholes, the visual system must often extrapolate from (Michotte, Thines, & Crabbe, 1964).1 incomplete contour or boundary information. Under cer- The relationship between amodal completion and the tain conditions extrapolated contours are perceived in formation of illusory contours has been controversial. It areas of homogeneous retinal stimulation. Because such has been suggested that a common mechanism may be contours are not present in the physical stimulus, they responsible for both of these phenomena (e.g., Kellman are referred to as “illusory” or “subjective.” Illusory con- & Loukides, 1987; Kellman & Shipley, 1991; Ringach & tours are often perceived when the contours or edges Shapley, 1996) or that amodal completion may be a of elements in the visual array are consistent with the prerequisite for the generation of illusory contours (Min- presence of a superimposed surface or object. An exam- guzzi, 1987). However, the fact that amodal completion ple is the “Kanisza rectangle” shown in Figure 1a, which and the perception of illusory contours can dissociate appears to most observers as a white rectangle superim- under certain conditions has caused some investigators posed on four black circles. There is an apparent bright- to argue that these linking processes are mediated by ness discontinuity between the inside and outside of the different neural mechanisms (e.g., Sambin, 1987). rectangle so that the inside appears brighter than the Our initial aim in this study was to investigate the outside. This apparent brightness transition forms the perception of illusory contours by the divided hemi- illusory contour (see Kanisza, 1976; 1979). spheres of two right-handed callosotomy patients, J.W. However, visual grouping processes can act to gener- and V.P. Evidence from functional neuroimaging studies, ate perceived shapes from incomplete boundary infor- investigations of clinical populations with unilateral mation without producing such illusory contours. For brain damage, and psychophysical investigations with example, in Figure 1b most observers perceive a white normal observers has suggested that the mechanism or rectangle mostly occluded by a surface with four holes mechanisms responsible for illusory-contour perception in it so that only the corners are visible. In contrast with may be preferentially lateralized to the right cerebral © 1999 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 11:4, pp. 459–466 Figure 1. (a) A Kanisza rec- tangle. This conguration is typically perceived as a white rectangle superimposed on four black circles. (b) An amo- dally completed rectangle. This conguration is typically perceived as a white rectangle viewed through four aper- tures in an occluding surface. Note that although the shape of the rectangle is still readily visible, no illusory contours are perceived. hemisphere (Atchley & Atchley, 1998; Hirsch et al., 1995; the strength of the binding processes that produce the Wasserstein, Zapulla, Rosen, Gerstman, & Rock, 1987). If perceived shape. In neurological normals, similar im- this is the case, the left hemisphere of a split-brain provements in performance are obtained in conditions patient ought to be signicantly impaired at the percep- in which illusory contours are perceived and when the tion of illusory contours relative to the right hemisphere. pacmen are modied (see Figure 2d) so that perceiving We therefore designed an experiment to directly inves- the illusory rectangle depends upon amodal completion tigate the perception of illusory contours by each of the (Ringach & Shapley, 1996). isolated hemispheres of two such patients. In our lateralized version of this task, illusory-contour A second goal of our investigation was to compare (IC), amodal-completion (AC), or local-orientation (LO) amodal boundary completion to illusory contour per- control stimuli were briey ashed to the left or right of ception in each hemisphere. We reasoned that if amodal a xation point, so that their processing would be completion and illusory-contour perception depend on conned to the hemisphere contralateral to the side of the same mechanisms, any relative hemispheric advan- xation to which they were presented. Two callosotomy tage for one of these effects should also be found for the patients, J.W. and V.P., were asked to judge whether the other. If, however, amodal completion depends on differ- Kanisza rectangle appeared thin or fat (IC and AC tasks) ent mechanisms than illusory-contour perception, these or whether the pacmen tilted up or down (LO control processes might exhibit different patterns of lateraliza- task). tion. We investigated illusory-contour perception and amo- RESULTS AND DISCUSSION dal boundary completion using a lateralized version of the shape-discrimination task introduced by Ringach and Separate analyses were conducted for the comparison Shapley (1996). In this task the observer is asked to between the IC and LO tasks and between the IC and judge whether a deformed illusory rectangle appears to AC tasks. Psychometric functions showing percentage have convex or concave vertical sides (i.e., appears “fat” correct as a function of angular displacement of the or “thin;” see Figure 2a and b). The deformed illusory pacman inducers for both patients are displayed in Fig- rectangles are created by rotating the “pacman” inducers ures 3 (IC versus LO tasks) and 4 (IC versus AC tasks). by a small amount so that the sides of the rectangle appear to bend, creating the appearance of a thin or fat Patient J.W. rectangle. Subjects’ ability to discriminate thin from fat illusory rectangles is compared to performance in a Illusory Contours vs. Local Orientation “local orientation” control task in which the inducers all Inspection of Figure 3 indicates that J.W.’s discrimination face in the same direction and the subjects judge performance was better for the IC task than the LO task whether they are tilted “up” or “down” (see Figure 2c). in both hemields. This suggests that both hemispheres Ringach and Shapley have shown that neurologically are capable of perceiving illusory contours. It is also normal observers can detect smaller pacman rotations apparent from Figure 3 that J.W.’s performance at both in the shape-discrimination task than in the control task. tasks was better in the left visual eld (LVF) than the The magnitude of this difference provides an index of right visual eld (RVF). 460 Journal of Cognitive Neuroscience Volume 11, Number 4 Figure 2. Examples of the stimuli used in these experi- ments. (a) A “fat” Kanisza rec- tangle used in the IC task. The inducers in this gure are each rotated by 4° so that the vertical sides of the rectangle appear to be convex. (b) A “thin” Kanisza rectangle used in the IC task. The inducers in this gure are each rotated by 4° so that the vertical sides of the rectangle appear to be concave. (c) An example of a stimulus from the LO task. The pacmen in this gure are each rotated downward by 4°. (d) A “fat” stimulus from the AC task. The inducers in this gure are each rotated by 4° so that the vertical sides of the rectangle appear to be concave. Multidimensional-c 2 analyses (Winer, Brown, & and response reects J.W.’s superior performance in the Michels, 1991) were conducted to determine the effects IC task over the LO task (c 2(1) = 28.94, p < 0.01; of task (IC versus LO), hemield (LVF versus RVF), angle proportion correct = 0.778 for the IC task, 0.628 for the (1, 2, 3, or 4°) and stimulus condition (thin/up versus LO task). Finally, an interaction between hemield, con- fat/down) on response choice (thin/up versus dition, and response reects his overall superior perfor- fat/down). In this analysis, discrimination accuracy is mance when stimuli were presented in the LVF (c 2(1) = indexed by the interaction between stimulus condition 17.19, p < 0.01; proportion correct = 0.761 for the LVF, and response. The effects of other factors, or combina- 0.645 for the RVF). The four-way interaction between tions of factors, on accuracy are indexed by higher-order task, hemield, condition, and response was not sig- interactions involving both condition and response. The nicant (c 2(1) = 1.01, n.s.). This indicates that the differ- interaction between stimulus condition and response ence in discrimination accuracy between the IC and LO choice was signicant (c 2(1) = 212.25, p < 0.01), indicat- tasks was not signicantly different for LVF and RVF ing that that J.W.’s overall performance was signicantly stimuli. No interactions involving angle reached sig- better than chance (overall proportion correct was nicance. 0.703). A signicant interaction between task, condition, Overall, the results of this analysis suggest that al- Corballis et al.
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