Abstract
Visual features are encoded in a distributed manner spanning numerous cortical areas, with transformations in neuronal selectivity and cortical representation occurring along the visual hierarchy. To understand this distributed sensory code and how it underlies perception, we must identify how feature representations generalize across changes in the environment. Visual crowding, whereby judgments of target features are impaired by adjacent distractor stimuli, provides a powerful paradigm to address these questions, as it involves a range of spatial scales of integration and thus places profound constraints on the relationship between neuronal representations and perception. We asked how crowding by distractor gratings altered the representation of target grating orientation, in simultaneously recorded neuronal populations in V1 and V4 of two fixating macaques. Using a linear population decoding approach, we found that crowding resulted in marked impairments in target orientation discriminability in both cortical areas. Information losses under crowding were more pronounced in V4 populations and varied over a larger range of spatial scales than in V1. Information loss occurred because distractors modulated neuronal responsivity and variability, with both response suppression and facilitation under crowding limiting feature encoding. While both cortical areas exhibited diversity in modulation, the responsivity of V4 neurons was more often facilitated by distractors, in part due to their larger spatial receptive fields. In addition, V4 tuning for targets was altered more strongly by distractors than V1 tuning was. Small changes in the orientation or location of distractor stimuli could strongly perturb tuning in V4, evident both in individual neurons and in neuronal populations. These results reveal that crowding alters the V1 representation of target stimuli, and that these effects are compounded in V4, through greater spatial integration and configuration-dependent tuning. Together these limits on feature discriminability closely approximate those seen in human perception as measured using similar displays.