, 2008 and Doi et al., 2011). Neurophysiological studies revealed that cells in V1 exhibit similar responsiveness to RDS and aRDS stimuli ( Ohzawa et al., 1990, Qian and Zhu, 1997 and Cumming and Parker, 1997). In contrast, V4 cells substantially reduce their selectivity to disparities in aRDSs, suggesting that the false matching responses elicited in V1 are largely rejected by the stage of V4 ( Tanabe et al., 2004 and Kumano et al., 2008). Although there is no evidence from single unit studies of any difference in binocular correspondence between V1 and V2 (Okazaki and I.F., unpublished data), Chen et al.
(2008) reported that in V2 thick stripes, near-to-far maps are imaged in response to RDSs, but not aRDSs, suggesting that V2 also plays an important role in rejecting false matches. Conversion of Absolute Disparity to Relative this website Disparity. Disparity cues can be used to calculate absolute distance from the observer. However, a more important function is the determination of distance relative to a background
or another object ( Westheimer, 1979, Erkelens and Collewijn, 1985 and Regan et al., 1986). This requires calculation of relative disparity. Whereas cells in V1 encode local absolute disparity within their receptive field ( Cumming and Parker, 2000), the computation of relative disparity begins in V2 ( Cumming and Parker, 1999 and Thomas et al., 2002). Some cells in V2 exhibit shifts in disparity tuning with MAPK inhibitor shifts in plane of the background, thereby signaling depth relative to the background depth plane ( Thomas et al., 2002). In V4, a much higher proportion of cells display
such shifts in disparity tuning, and, furthermore, the magnitudes of these shifts are greater than those for V2 ( Umeda et al., 2007). Thus, V4 is a stage central to the calculation of relative disparity between spatially adjacent visual planes, a function highly important for fine depth perception and figure-ground segregation. Roles in Size Constancy. Size constancy refers to our ability to perceive the size of an object despite different viewing distances aminophylline ( Figure 5C, right). To achieve this, information regarding the differences in retinal image size at different viewing distances must be incorporated with information about object distance. Where and how does this computation occur? The first electrophysiological study to address this question found that neurons in V4 vary their responses relative to size and distance of the viewing plane ( Dobbins et al., 1998). More recently, Fujita and Tanaka hypothesized that V4 compensates for change in retinal image size by using visual cues for depth, and then calibrates for the perceived size. In the majority of V4 neurons studied, when stimuli were presented with larger crossed disparities (nearer), the size tuning curves of these cells shifted toward larger size preferences.