Disparity selectivity

Whichever of these two forms of selectivity is present (or a mixture of them), a system of binocular disparity based on retinal noncorresponding points is firmly locked to the coordinate frames for each retina. As Figure 49.1 illustrates, when the eyes converge or diverge to inspect a new target, mechanisms locked to noncorresponding points register a new disparity value for an object O that remains at a fixed distance from the observer. The geometric projection lines for object O alter as the binocular fixation point moves closer or farther in depth. By fixating O, the visual system brings the object to corresponding points on the left and right retinas, hence a disparity of zero (Fig. 49.1A). Converging closer than O gives O a far disparity, and converging farther than O gives O a near disparity. Thus, when the disparity of O is measured in terms of retinal noncorresponding points, its disparity is subject to fluctuation as the eyes move, despite the fact that it is actually at a constant distance from the observer. Most physiological studies of disparity selectivity have explored within this framework, which may be termed absolute disparity (Cumming and Parker, 1999).

Another way of describing binocular disparity has been current in the literature on the psychology of depth perception. Here the emphasis has been not primarily on noncorresponding points in retinal coordinates but on the difference in the retinal projections of two visible objects at different depths from the observer, for example, points O and N in Figure 49.1D. The angles that are subtended by the gap between O and N are, of course, different in the left and right eyes. Figure 49.1E shows that this difference is not substantially affected by the vergence angle of the eyes (at least to the extent that the eyes may be assumed to rotate around their true optical centers, which is reasonable for small rotation angles). This measure of disparity has been termed relative disparity and is in fact equal to the difference in the absolute disparities of the two visible points under consideration.

An estimate of relative disparity can only be achieved by the nervous system if there are at least two features that can be seen in the visual field. By comparison, the nervous system can estimate the absolute disparity of a single point in an otherwise empty visual field. Absolute and relative disparity are associated with different visual and oculomotor functions. Absolute disparity is an important parameter in controlling disparity-driven vergence movements (Rashbass and Westheimer, 1961), whereas the finest stereoacuities are obtained with stimulus arrays that include relative disparity (Andrews et al., 2001; Westheimer, 1979). Most compelling is a demonstration by Erkelens and Collewijn (1985) and Regan et al. (1986). Observers view a display with two depth planes defined by a relative disparity between them. A change in the absolute disparity of the entire display causes a human observer to track the change with vergence movements of the eyes, but no perceptible change of depth is generated. On the other hand, movement of one of the planes in depth relative to the other gives rise to a vivid sense of motion in depth.

Disparity-selective neurons have been identified not just within the striate cortex but also throughout the visual areas of the extrastriate cortex (Cumming and DeAngelis, 2001). This makes it clear that the discovery of a specialized set of neurons responsible for stereoscopic depth perception has to go beyond the identification of disparity selectivity in single neurons. We need a specific comparison between the perceptual properties of stereoscopic vision and the properties of single neurons studied in neurophysiological experiments. Considerable progress toward this goal has been made in recent years.