Christopher R. L. Cantor

Effects of luminance and saccadic suppression on perisaccadic spatial distortions.

Visual directions of foveal targets flashed just prior to the onset of a saccade are misperceived as shifted in the direction of the eye movement. We examined the effects of luminance level and temporal interactions on the amplitude of these perisaccadic spatial distortions (PSDs). PSDs were larger for both single and sequentially double-flashed stimuli with low than high luminance levels, and there was a reduction of PSDs for low luminance targets flashed immediately before the saccade. Significant temporal interactions were suggested by PSDs for a pair of sequentially presented flashes (ISI = 50 ms) that could not be predicted from the single-flash distortions: PSD increased for the first flash and decreased for the second compared to the single-flash distortions. We also found that when the flash pair was presented near saccade onset, the perceived distortion of the earlier flash overtook that of the later flash, even though the late flash occurred closer in time to the saccade. To explain these effects, we propose that stimulus-dependent nonlinearities (contrast gain control and saccadic suppression) influence the duration of the temporal impulse response of both single- and double-flashed stimuli.

Temporal aspects of spatial interactions affecting stereo-matching solutions

Stereo-matching solutions minimize disparity relative to the horopter (minimum-absolute-disparity or MAD), and differences in disparity between adjacent features (minimum-relative-disparity or MRD). When placed in conflict, spatial proximity promotes MRD over MAD solutions. How does temporal proximity of neighboring features affect strength of these spatial interactions? We quantified the inter-stimulus interval (ISI) over which an unambiguous disparity pattern influenced stereo-matches for patterns with several possible solutions. Likelihood of MRD decreased as ISI increased (48.9 ms time constant) and increased as contrast was reduced for short ISIs, suggesting that monocular persistence (temporal impulse response) underlies the temporal interaction.

The effect of perceptual grouping on perisaccadic spatial distortions.

Perisaccadic spatial distortion (PSD) occurs when a target is flashed immediately before the onset of a saccade and it appears displaced in the direction of the saccade. In previous studies, the magnitude of PSD of a single target was affected by multiple experimental parameters, such as the targets luminance and its position relative to the central fixation target. Here we describe a contextual effect in which the magnitude of the PSD for a target was influenced by the synchronous presentation of another target: PSD for simultaneously presented targets was more uniform than when each was presented individually. Perisaccadic compression was ruled out as a causal factor, and the results suggest that both low- and high-level perceptual grouping mechanisms may account for the change in PSD magnitude. We speculate that perceptual grouping could play a key role in preserving shape constancy during saccadic eye movements.