Thresholds for stereo-slant discrimination between spatially separated targets are influenced mainly by visual and memory factors but not oculomotor instability.

Surface-slant variations can be sensed either simultaneously with steady fixation or sequentially with saccadic gaze shifts. Stereo-slant discrimination thresholds are affected by visual, oculomotor, and memory factors. We have investigated the effects of fixation strategy, target separation, and exposure duration on stereo-slant discrimination. With long exposure durations (734 ms), stereo-slant discrimination thresholds measured with simultaneous presentation of test and reference stimuli were lower with gaze shifts than without them when target separations exceeded 4 deg. Above 4-deg target separations, the benefits of improved disparity resolution with foveal gaze shifts outweighed the costs of oculomotor variability associated with saccades. With short exposure durations (167 ms), as target separation increased, stereo-slant discrimination thresholds measured without gaze shifts increased with both sequential and simultaneous stimulus presentations, whereas thresholds with gaze shifts remained constant. This indicates that oculomotor errors are not an important factor in stereo-slant discrimination. In contrast to stereo-slant thresholds, sequential stereo-depth thresholds between two dots, measured with gaze shifts, increased with target separation. Thus, oculomotor error increases with target separation, and it is an important factor in stereo-depth discrimination.