Encoding of Three-Dimensional Surface Slant in Cat Visual Areas 17 and 18

Abstract

Sanada, Takahisa M. and Izumi Ohzawa. Encoding of threedimensional surface slant in cat visual areas 17 and 18. J Neurophysiol 95: 2768 -2786, 2006. First published January 4, 2006 doi:10.1152/jn.00955.2005. How are surface orientations of threedimensional objects and scenes represented in the visual system? We have examined an idea that these surface orientations are encoded by neurons with a variety of tilts in their binocular receptive field (RF) structure. To examine whether neurons in the early visual areas are capable of encoding surface orientations, we have recorded from single neurons extracellularly in areas 17 and 18 of the cat using standard electrophysiological methods. Binocular RF structures are obtained using a binocular version of the reverse correlation technique. About 30% of binocularly responsive neurons have RFs with statistically significant tilts from the frontoparallel plane. The degree of tilts is sufficient for representing the range of surface slants found in typical visual environments. For a subset of neurons having significant RF tilts, the degrees of tilt are correlated with the preferred spatial frequency difference between the two eyes, indicating that a modified disparity energy model can account for the selectivity, at least partially. However, not all cases could be explained by this model, suggesting that multiple mechanisms may be responsible. Therefore an alternative hypothesis is also examined, where the tilt is generated by pooling of multiple disparity detectors whose preferred disparities progressively shift over space. Although there is evidence for extensive spatial pooling, this hypothesis was not satisfactory either, in that the neurons with extensive pooling tended to prefer an untilted surface. Our results suggest that encoding of surface orientations may begin with the binocular neurons in the early visual cortex. I N T R O D U C T I O NOne of the fundamental roles of the visual system is to reconstruct a three-dimensional (3D) model of the external world from a pair of two-dimensional images on the two retinae. Horizontal displacement of the eyes causes small differences between the retinal images. This difference of the retinal images is called binocular disparity and stereopsis is the process of determining depth from binocular disparity. Visual information processing for stereopsis begins in the primary visual cortex and neurons found in this area are known to encode binocular disparities of stimuli for a small area of visual field (Barlow et al. 1967;Ferster 1981; Wiesel 1962, 1968;LeVay and Voigt 1988;Nikara et al. 1968; Ohzawa and Freeman 1986a,b;Ohzawa et al. 1990Ohzawa et al. , 1996Ohzawa et al. , 1997.How does the processing of stereoscopic information proceed once binocular disparity for small localized areas is available? Is a possible next stage of processing that of detecting the rate of change of binocular disparity, i.e., detecting 3D orientations of surfaces in depth? Some recent studies have examined these possibilities and report that a sub...

Complex Cells in the Cat Striate Cortex Have Multiple Disparity Detectors in the Three-Dimensional Binocular Receptive Fields

“…The binocular interaction profiles in the XL-XR domain are elongated along the diagonal, indicating that the neuron is sharply tuned for binocular disparity and its preferred disparity is constant for a relatively large range of X positions. Such inseparable interaction profiles were common in most complex cells we analyzed (26 of 34 complex cells; 7 of 28 simple cells), as expected from previous studies using 1D dichoptic stimuli (Ohzawa et al, 1990;Anzai et al, 1999b;Sanada and Ohzawa, 2006). The interaction strength map produced from the matrix of the XL-XR interaction maps exhibits an elliptic profile elongated along a diagonal (Fig.…”

“…The separable interaction profile was a characteristic of most simple cells we analyzed (21 of 28 simple cells, 7 of 34 complex cells). This is expected from previous studies where binocular interaction profiles were examined by using one-dimensional (1D) noise stimuli elongated along the Y-axes (Ohzawa et al, 1990;Anzai et al, 1999a;Sanada and Ohzawa, 2006). The strength of binocular interaction decreases isotropically in the matrix of the XL-XR maps with increasing distance from the center.…”

“…For some maps, lines are drawn at the bottom (solid for the left eye) and left (dashed for the right eye) to indicate the Y positions in the same color as in Figure 4 B, where three Y positions are shown for each eye by rectangles superimposed on the schematized RF and random dot pattern (the first frame of actual stimuli used but with reduced contrast). The preferred orientation of this neuron differed by 15°between the two eyes probably because of cyclorotation caused by anesthesia and paralysis (Blakemore et al, 1972;Nelson et al, 1977;Ohzawa and Freeman, 1986;Sanada and Ohzawa, 2006). The XL-XR maps of this simple cell show checkered profiles, which are separable in the XL-XR domain, i.e., they are expressed as the product of two functions: one dependent only on XL and the other dependent only on XR.…”

“…Visual stimuli were generated by computer and displayed on a cathode ray tube display (a resolution of 1600 ϫ 1024 pixels, refreshed at 76 Hz; GDM-FW900, Sony) using only the green channel to avoid color misconvergence across channels. The animal viewed the display through a haploscope, which allowed visual stimuli to be presented separately to each eye (Sanada and Ohzawa, 2006). The visual fields subtended 23°ϫ 30°for each eye (800 ϫ 1024 pixels) at a viewing distance of 57 cm.…”

Disparity processing in primary visual cortex

“…As in an earlier study [30], some cells responded best when the SF presented to the two eyes was different. In these cells, Baba et al showed that the interocular difference in preferred SF was preserved across subunits.…”