A common cortical basis for variations in visual crowding
Peripheral vision is limited by crowding, the disruptive effect of clutter on object recognition. Crowding varies markedly around the periphery, with e.g. stronger performance decrements with increasing eccentricity and in the upper vs. lower visual field. Although a number of neural substrates have been proposed for crowding, none to date can explain the full pattern of these variations. Here we examine the effects of crowding on object appearance. These effects are central to many models of crowding, and also vary markedly, causing target objects to appear more similar to flanker objects (assimilation) in some instances and dissimilar (repulsion) in others. We took 3 manipulations known to vary crowded performance (flankers in the same vs. different hemifield, the upper-lower visual field anisotropy, and the radial-tangential flanker anisotropy) and examined whether the effects on appearance vary similarly. In all cases, manipulations that increased performance impairments also increased assimilative errors, e.g. flankers on the radial axis around fixation gave high threshold elevation and assimilation, with reduced elevation and repulsion errors for tangential flankers. These linked variations in performance and appearance are well described by a population-coding model of crowding that varies the weighted combination of target vs. flanker population responses. We further demonstrate that this pattern is inconsistent with crowding being driven by either the cortical distance between elements or receptive-field size variations on their own. Instead, using a series of models we show that crowding could be driven by receptive field overlap - the intermixing of the spatial distribution of target/flanker population responses. Crowding is strong (with high performance decrements and assimilative biases) when the degree of spatial overlap in population responses is high and reduced (with low threshold elevation reduced assimilation or repulsion) when these responses are separable.