Direction-dependent distortions of retinocentric space in the visuomotor transformation for pointing.

The aim of this study was to: (1) quantify errors in open-loop pointing toward a spatially central (but retinally peripheral) visual target with gaze maintained in various eccentric horizontal, vertical, and oblique directions; and (2) determine the computational source of these errors. Eye and arm orientations were measured with the use of search coils while six head-fixed subjects looked and pointed toward remembered targets in complete darkness. On average, subjects made small exaggerations in both the vertical and horizontal components of retinal displacement (tending to overshoot the target relative to current gaze), but individual subjects showed considerable variations in this pattern. Moreover, pointing errors for oblique retinal targets were only partially predictable from errors for the cardinal directions, suggesting that most of these errors did not arise within independent vertical and horizontal coordinate channels. The remaining variance was related to nonhomogeneous, direction-dependent distortions in reading out the magnitudes and directions of retinal displacement. The largest and most consistent nonhomogeneities occurred as discontinuities between adjacent points across the vertical meridian of retinotopic space, perhaps related to the break between the representations of space in the left and right cortices. These findings are consistent with the hypothesis that at least some of these visuomotor distortions are due to miscalibrations in quasi-independent visuomotor readout mechanisms for "patches" of retinotopic space, with major discontinuities existing between patches at certain anatomic and/or physiological borders.