Stereo correspondence is optimized for large viewing distances.

Stereo '3D' vision depends on correctly matching up the differing images of objects seen by our two eyes. But vertical disparity between the retinal images changes with binocular eye posture, reflecting for example the different convergence angles required for different viewing distances. Thus, stereo correspondence must either dynamically adapt to take account of changes in viewing distance, or be hard-wired to perform best at one particular viewing distance. Here, using psychophysical experiments, we show for the first time that human stereo correspondence does not adapt to changes in physical viewing distance. We examine performance on a stereo correspondence task at a short viewing distance (30 cm) and show that performance is improved when we simulate the disparity pattern for viewing infinity, even though these disparities are impossible at the physical viewing distance. We estimate the vertical extent of the retinally fixed 'search zones' as < 0.6° at 14° eccentricity, suggesting that most V1 neurons must be tuned to near-zero vertical disparity. We also show that performance on our stereo task at 14° eccentricity is affected by the pattern of vertical disparity beyond 20° eccentricity, even though this is irrelevant to the task. Performance is best when vertical disparities within and beyond 20° eccentricity both indicate the same convergence angle (even if not the physical angle), than when the pattern of vertical disparity across the visual field is globally inconsistent with any single convergence angle. This novel effect of the periphery may indicate cooperative interactions between disparity-selective neurons activated by the same eye postures.