Dissociable effects of visual crowding on the perception of colour and motion

Abstract

Our ability to recognise objects in peripheral vision is fundamentally limited by crowding, the deleterious effect of clutter that disrupts the recognition of features ranging from orientation and colour to motion and depth. Prior research is equivocal on whether this reflects a singular process that disrupts all features simultaneously or multiple processes that affect each independently. We examined crowding for motion and colour, two features that allow a strong test of feature independence. 'Cowhide' stimuli were presented 15 degrees in peripheral vision, either in isolation or surrounded by flankers to give crowding. Observers reported either the target hue (blue/purple) or its direction (clockwise/counterclockwise from upwards). We first established that both features show systematic crowded errors (predominantly biased towards the flanker identities) and selectivity for target-flanker similarity (with reduced crowding for dissimilar target/flanker elements). The multiplicity of crowding was then tested with observers identifying both features: a singular object-selective mechanism predicts that when crowding is weak for one feature and strong for the other that crowding should be all-or-none for both. In contrast, when crowding was weak for colour and strong for motion, errors were reduced for colour but remained for motion, and vice versa with weak motion and strong colour crowding. This double dissociation follows the predictions of independent crowding processes, suggesting that crowding can disrupt vision in a feature-specific manner. The ability to recognise one aspect of a cluttered scene, like colour, thus offers no guarantees for the correct recognition of other aspects, like motion. Significance statementThe fundamental limitation on our peripheral vision is crowding, the disruptive effect of image clutter on object recognition. Crowding is widely assumed to be a singular process, affecting all of the features (e.g. orientation, motion, and colour) within an object simultaneously. In contrast, we find a double dissociation for these disruptions: observers can make errors in judging the colour of a crowded object whilst correctly judging its movement direction, and vice versa. This dissociation is well described by a population-coding model that independently pools the motion and colour of target and flanker elements. We suggest that crowding may be a generic set of processes distributed throughout the visual system that independently simplify the representation of our visual field.