The chromatic selectivity of visual crowding

Purpose:  Precortical vision is mediated by three opponent mechanisms which linearly combine receptoral outputs to form a luminance channel (L + M) and two chromatic channels, red‐green (L/M) and blue‐yellow (S/L + M). These colour channels are known to undergo considerable cortical reorganisation in order to form multiple, ‘higher‐order’ mechanisms tuned to a variety of axes in colour space. Here we ask the extent to which the basic colour‐opponent mechanisms interact in the phenomenon of visual crowding, where nearby targets interfere with the spatial processing of a central test target. Methods:  The task was to identify the orientation of a central Gabor patch whilst an annular plaid was positioned around the patch. The radius of the annulus was varied in order to produce different amounts of crowding. The chromatic content of the central patch and the annulus could be varied independently along the (L + M), (L/M) and (S/L + M) cardinal axes. Results:  For all targets, when the target and crowding annulus were of the same chromaticity, the crowding effect increased with decreasing separation of the target and annulus. When the test and crowding stimuli had different chromatic properties, very little crowding was observed, even at the minimum separation of test target and crowding annulus. Conclusions:  The crowding effect appears to be critically dependent on whether or not the target and crowding stimuli share a common chromatic axis. These results suggest a method by which performance in crowding‐limited tasks might be enhanced through appropriate selection of chromatic stimulus content.