Figure 2.

Responses to anti-correlated random-dot stereograms and to relative disparity. (a) The leftmost two random-dot stereograms are correlated, i.e. corresponding dots in the left and middle image have the same colour. Free-fusing these images led to the percept of a circular patch protruding from a background. The rightmost two random-dot stereograms are anti-correlated: the dots in both images are located at exactly the same positions, and thus have exactly the same disparities with the leftmost dot pattern, but the contrast polarity of the corresponding dots in each figure has been reversed: white dots in the middle figure are black in the right figure, and vice versa. In contrast to correlated random-dot stereograms, free-fusing the leftmost and rightmost dot pattern does not elicit the percept of a circular patch protruding from a background. (b) Average responses of a V1 neuron (left panel) and a V4 neuron (right panel) to correlated (black filled symbols) and anti-correlated (white open symbols) random-dot stereograms with different disparities (x-axis). The stimuli used in these studies were similar to those shown in (a). The V1 neuron is still markedly tuned to the disparity in anti-correlated stereograms. The V4 neuron, however, is only weakly tuned to the disparity of the dots in the anti-correlated random-dot stereogram. (Figure adapted with permission from Parker and Tanabe et al. [40,41].) (c) In experiments wherein the relative disparity tunings of neurons are examined, stimuli with relative disparities are placed at different positions in depth with respect to the fixation plane. For example, a surrounding annulus can be placed in front of, behind or at the fixation plane. One can then manipulate the disparity of the circular centre stimulus, which brings the centre stimulus to the front or the back of the fixation plane. If neurons are tuned to the relative disparity between the centre and surrounding stimulus, their tuning curves should shift by the same amount as that of the applied surround disparity. (d) This V4 neuron preferred a nearby centre (negative/crossed disparities) when the surround was placed at the plane of fixation (filled rectangular symbols). When the surround was moved to a position −0.4° in front of the fixation plane (open circular symbols), the tuning curve to the centre stimulus moved by approximately the same amount to the left. Positioning the surround +0.4° behind the fixation plane, moves the tuning curve to the right by a similar amount. Hence, regardless of the surround disparity, this neuron preferred a centre that was positioned in front of the surround. This tuning behaviour reflects selectivity to a relative disparity between the centre and surround stimuli. (Figure adapted with permission from Umeda et al. [42].)