How does visual crowding interfere with depth discrimination?

The presence of flanking targets can impair depth discrimination, presumably through a form of lateral interaction or visual crowding. This study investigates how stereoscopic crowding interferes with foveal depth discrimination when tests and flanking stimuli of different spatial configuration are located on and off the horopter in normal subjects by using psychophysical means. The magnitude of crowding increased when the flanking bars were in close spatial proximity to the test, between 1 to 2 arc min, and returned to unflanked levels for wider separations of 4 arc min and beyond. The magnitude of crowding depended on the extent to which the test and the flanking bars width matched. When flankers were placed at the optimum crowding distance (OCD) and displaced off the horopter, crowding reduced but the flanker effect was restored at greater flanker disparity. On the contrary, flankers positioned at the least crowding distance (LCD) at the onset generally showed an increase in thresholds from the fixation plane with increasing flanker disparity. Crowding was produced at similar small test- flanker separation for the range of 0.5 to 4 cpd flanker spatial frequency composition used. The magnitude of crowding was greater for test and flanker of similar spatial frequency, though some crowding was produced when their spatial frequency differed. Overall, the results confirm previous reports showing that depth discrimination thresholds increase in the presence of flanking contours, but in addition suggest that disparity integration relative to the fixation demonstrates a dichotomy of fine and coarse mechanisms driven by salience attraction. Additionally, the results show that the crowding effect can be reduced by depth cues related to the width, and disparity of flanking stimuli. The crowding effect may be attributed to the action of local disparity interactions, but suggest the involvement of Gestalt factors (for larger flanker widths) and luminance flux (for thinner flanker widths) factors.