Impairment of Cyclopean Surface Processing by Disparity-Defined Masking Stimuli

Binocular disparity signals allow for the estimation of three-dimensional shape, even in the absence of monocular depth cues. The perception of such disparity-defined form depends, however, on the linkage of multiple disparity measurements over space. Performance limitations in cyclopean tasks thus inform us about errors arising in disparity measurement and difficulties in the linkage of such measurements. We used a cyclopean orientation discrimination task to examine the perception of disparity-defined form. Participants were presented with random-dot sinusoidal modulations in depth, and asked to report whether they were clockwise, or counter-clockwise rotated. To assess the effect of different noise structures on measurement and linkage processes, task performance was measured in the presence of binocular, random-dot masks, structured as either anti-phase depth sinusoids, or as random distributions of dots in depth. For a fixed number of surface dots, the ratio of mask-to-surface dots was varied to obtain thresholds for orientation discrimination. Anti-phase masks were found to be more effective than random depth masks, requiring a lower mask-to-surface dot ratio to inhibit performance. For anti-phase masks, performance improved with decreased cyclopean frequency, increased disparity amplitude and/or an increase in the total number of stimulus dots. While a cross-correlation model of disparity measurement could account for anti-phase mask performance, random depth masking effects were consistent with limitations in relative disparity processing. This suggests that performance is noise-limited for anti-phase masks and complexity-limited for random masks. We propose that use of differing mask types may prove effective in understanding these distinct forms of impairment