Large depth differences between target and flankers can increase crowding: Evidence from a multi-depth plane display

Abstract

Crowding occurs when highly visible objects are rendered unrecognizable in the presence of nearby features. Several studies have demonstrated that crowding can be reduced by depth differences between target and flanking features, which challenges the assumption of real-world significance because real objects typically occur at differing depths. However, most previous studies tested only small differences in depth and/or used standard stereo displays in which disparity, accommodation, and defocus blur are inconsistent with the real world. For this study we developed a multi-depth plane display consisting of three screens at different distances from the observer to test how crowding is affected by large, real differences in target-flanker depth. Overall, perceptual error decreased with increasing target-flanker spacing, but increased when the target or flankers were displayed at a different depth from fixation and one another. Our findings demonstrate that depth can increase crowding when depth differences are large. Analysis of the perceived appearance of the stimuli suggest that outside Panum's Fusional Area, crowding is increased by suppression to maintain single vision rather than by an increase in clutter because of diplopia. These findings suggest that crowding may be a more significant problem in real scenes than is estimated with 2D displays.