Abstract
Surface slant estimates are considered to be facilitated by saccadic gaze shifts between surface edges. Saccadic gaze shifts are assumed to lower slant thresholds by reducing positional noise associated with the retinal eccentricity, and by adding vergence signals to foveal estimates of absolute disparity at target edges where the disparity is largest. Do saccadic gaze shifts lower slant discrimination thresholds?
We measured yaw-slant discrimination thresholds for two adjacent random dot surface patches presented either simultaneously (duration 2000 ms) or sequentially (duration 2000 ms each, inter stimulus interval 100 ms) for various target diameters (4 – 20 deg). When target diameters were large (> 8 deg) horizontal eye movements lowered thresholds for yaw slant discrimination in the simultaneous condition, but elevated thresholds for the sequential condition. We repeated this experiment with a spatial gap in each target that produced a pair of small coplanar test patches that were separated horizontally by 10 degrees. We varied the size of the two coplanar patches from 1 dot to 2 deg. For a given slant, as area increased edge disparities were constant while the visibility of disparity gradients increased. When patch diameters were small (< 1 deg), horizontal saccadic gaze shifts between the two patches improved sequential slant discrimination compared to when foveal fixation was maintained on one patch.
The improvement of simultaneous slant discrimination thresholds with eye movements is attributed to greater sensitivity to large relative edge disparities at the fovea than in the periphery. Sequential slant estimates with large fields appear to rely more on disparity gradients than absolute edge disparities. However, when disparity gradients are noisy (small patches), edge disparities become more useful. When the eyes fixate sequentially between widely separated edges, and thresholds are lowered by combinations of vergence and absolute edge disparities.
EB was supported by a Talent-grant from NWO (The Netherlands Organization for Scientific Research).