Breaking camouflage: binocular disparity reduces contrast masking in natural images.

Visual overlay masking is typically studied with a mask and target located at the same depth plane. Masking is reduced when binocular disparity separates the target from the mask (G. Moraglia & B. Schneider, 1990). We replicate this finding for a broadband target masked by natural images and find the greatest masking (threshold elevation) when target and mask occupy the same depth plane. Masking was reduced equally whether the target appeared at a crossed or an uncrossed disparity. We measure the tuning of masking and determine the extent of the benefit afforded by disparity. Threshold elevation decreases monotonically with increasing disparity until ±8 arcmin. Two underlying components to the masking are evident; one accounts for around two-thirds of the masking and is independent of disparity. The second component is disparity-dependent and results in additional masking when there is zero disparity. Importantly, the reduction in masking with disparity cannot be explained by interocular decorrelation; we use a single-interval orientation discrimination task to exclude this possibility. We conclude that when the target and mask are presented at different depths they activate distinct populations of disparity-tuned neurons, resulting in less masking of the target.