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Stereoscopic Illusory Contours—Cortical Neuron Responses and Human Perception

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Stereoscopic Illusory Contours—Cortical Neuron Responses and Human Perception Stereoscopic Illusory Contours—Cortical Neuron Responses and Human Perception Barbara Heider1, Lothar Spillmann2, and Esther Peterhans3 Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/14/7/1018/1757651/089892902320474472.pdf by guest on 18 May 2021 Abstract & In human perception, figure–ground segregation sug- signal such illusory contours and can be selective for certain gests that stereoscopic cues are grouped over wide areas of figure–ground directions that human observers perceive at the visual field. For example, two abutting rectangles of these contours. The results suggest that these neurons equal luminance and size are seen as a uniform surface group stereoscopic cues over distances up to 88. In addition, when presented at the same depth, but appear as two we compare these results with human perception and show surfaces separated by an illusory contour and a step in that the mean stimulus parameters required by these depth when presented with different retinal disparities. neurons also induce optimal percepts of illusory contours Here, we describe neurons in the monkey visual cortex that in human observers. & INTRODUCTION representations of such contours have been identified Illusory contours are perceived in visual scenes where in the human visual cortex by means of imaging objects occlude one another and produce overlapping techniques such as positron emission tomography images on the retina. If these objects have equal (PET) and functional magnetic resonance imaging luminance, their retinal images merge because their (fMRI). Representations have been localized early in overlapping borders lack luminance contrast. The visual visual processing, in area V2 (Larsson et al., 1999; system has developed methods to recover the segrega- ffytche & Zeki, 1996; Hirsch et al., 1995), and in higher tion of figure and ground in such situations—it gen- areas such as V3A, V4v, V7, and V8 (Mendola, Dale, erates illusory contours that complete the occluding Fischl, Liu, & Tootell, 1999). At the single-cell level, borders where they lack luminance contrast. Figure 1 representations of contours as shown in Figure 1 have mimics such a situation. It induces the perception of been identified as well as contours between abutting two white rectangles that appear to occlude one line-gratings (thin, widely spaced lines, displaced by another. The upper (occluding) rectangle is perceived half a cycle; see Soriano, Spillmann, & Bach, 1996; as being bounded by an illusory contour. This contour Kanizsa, 1979). These representations were found early is generated from interposition, or occlusion cues, as in visual processing, in the cat (areas 17 and 18) (Sheth, produced by the gray objects (line, two discs) that are Sharma, Rao, & Sur, 1996; Leventhal & Zhou, 1994; perceived as being partially occluded and located Redies, Crook, & Creutzfeldt, 1986) and in the monkey between these rectangles. These cues often have the (mainly area V2) (Grosof, Shapley, & Hawken, 1993; form of line-ends, corners, and different types of junc- Peterhans & von der Heydt, 1989; von der Heydt, tion. As illustrated in Figure 1, they are asymmetrical by Peterhans, & Baumgartner, 1984; von der Heydt & nature and point towards the occluding surface (upper Peterhans, 1989). Further, the results of Nieder and rectangle in this case). They define the location of the Wagner (1999) suggest that birds (owls) also perceive occluding object, relative to a particular contour. With- illusory contours and that their visual system includes out these cues, the two rectangles cannot be segregated; neurons sensitive to such stimuli. the image is perceived as a single, uniform surface. Illusory contours as described above can be perceived The perception of such illusory contours has been monocularly—stereopsis is not required. However, studied extensively in humans using stimulus configu- scenes that include situations of spatial occlusion usu- rations as shown in Figure 1 (Schumann, 1900; Petry & ally also provide binocular cues. These occur in the form Meyer, 1987; Kanizsa, 1979; Varin, 1971). More recently, of binocular disparity, or as unpaired image features that are visible to one eye only and not to the other (von Szily, 1921; for overviews, see Anderson & Nakayama, 1Yale University, 2University of Freiburg, 3University Hospital 1994; Anderson & Julesz, 1995). Occlusion cues as Zurich shown in Figure 1 also interact with stereoscopic cues D 2002 Massachusetts Institute of Technology Journal of Cognitive Neuroscience 14:7, pp. 1018–1029 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/089892902320474472 by guest on 03 October 2021 (Nakayama, 1996; Carman & Welch, 1992; Ramachan- dran & Cavanagh, 1985; Harris & Gregory, 1973). Neuro- nal signals of bars defined by stereoscopic cues have been found recently by Bakin, Nakayama, and Gilbert (2000) using stimuli as described by von Szily (1921) (see also Ehrenstein & Gillam, 1998, Figure 6). While these stimuli induced the perception of a narrow sur- face bounded by an illusory contour on either side, we aimed to study single contours. Therefore, we designed Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/14/7/1018/1757651/089892902320474472.pdf by guest on 18 May 2021 a novel stimulus that produced a single illusory contour and step in depth in which the figure–ground direction was defined by stereoscopic cues. This stimulus con- sisted of two abutting rectangles of equal luminance and size that were presented with different retinal disparities—one rectangle usually with zero disparity, Figure 2. Illusory contour defined by stereoscopic cues. The figure the other with either crossed or uncrossed disparity. illustrates the perception of an illusory contour stimulus in Figure 2 shows how human observers perceived this which the upper rectangle was presented with crossed (near) disparity and the lower rectangle with zero disparity. In this stimulus, namely, as two plane surfaces separated by a situation, the visual system uses the stereoscopic information of the step in depth and an illusory contour bounding the contrast borders (‘‘inducing borders,’’ indicated by arrows) to nearer (upper) surface. Since we aimed to determine generate the illusory contour. To study the effects of these cues the effects of the contrast borders that induced this selectively, we introduced disparity orthogonal to these borders contour (‘‘inducing borders,’’ indicated by arrows), (see Figure 3). we introduced disparity orthogonal to these borders (see Figure 3). No segregation occurred when the two rectangles were presented with the same disparity; human observers perceive at these contours. In addi- the stimulus was perceived as a single, uniform surface tion, we studied the perception of these contours in (see inset row 4 of Figure 4A). human observers and show that the mean stimulus In the following, we show that neurons in the monkey visual cortex signal such illusory contours and show similar orientation selectivity for these contours as for contours defined by luminance contrast (bars, edges). Furthermore, we show that some of these neurons are sensitive to the direction of the step in depth that Figure 3. Examples of stimuli. (A) The upper rectangle is presented Figure 1. Illusory contour defined by occlusion cues. The figure with crossed disparity (arrows), the lower rectangle with zero induces the perception of two white rectangles that appear to overlap disparity. If the reader fuses the images of the left and right panels, one another, with the upper (occluding) rectangle bounded by an the upper rectangle appears nearer than the lower rectangle and illusory contour. This perception is induced by the interposition or bounded by an illusory contour. (B) Here, the lower rectangle is occlusion cues represented by the visible parts of the gray objects presented with zero, the upper rectangle with uncrossed disparity (line, two discs) that appear to be located in between these rectangles. (arrows). Upon fusion of the images, the reader now perceives the As shown here, these cues often have the form of line-ends and lower rectangle to be nearer than the upper rectangle and bounded corners that are asymmetrical by nature and indicate the location of by an illusory contour. (The cross marks an average position of the the occluding surface. fixation target of the monkey). Heider, Spillmann, and Peterhans 1019 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/089892902320474472 by guest on 03 October 2021 Figure 4. Neuronal signals of stereoscopic illusory contours. (A) Responses of a neuron of area V2 to monocular stimula- tion (‘‘left’’ and ‘‘right eye,’’ respectively), to an illusory contour stimulus (‘‘binocular’’; lower rectangle zero disparity, upper rectangle 30 min arc uncrossed disparity), and to a binocular control stimulus (both rectangles zero Downloaded from http://mitprc.silverchair.com/jocn/article-pdf/14/7/1018/1757651/089892902320474472.pdf by guest on 18 May 2021 disparity). Ellipses indicate the response field as mapped with a bar stimulus. All stimuli were presented at the neuron’s preferred orientation (128) and were moved parallel to the lateral borders of the rectangles (arrows). Responses recorded in the forth sweeps of stimulus movement are shown in the left half, those recorded in the back sweep in the right half of the dot displays. Each dot represents an action potential; 32 cycles of stimulus movement were recorded for each stimulus. The figures underneath each display
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