Peter Bex Schepens Eye Research Institute Department of Ophthalmology, Harvard Medical School
Where and Why Does Crowding Occur?
Outside the fovea, highly visible objects that are easily identified in isolation can be rendered unrecognizable by nearby features, an effect known as crowding. Given that the vast majority of the visual field is extra-foveal and that natural scenes are packed with features, our vision is primarily limited not by contrast sensitivity or acuity, but by crowding. Competing theories posit that crowding represents the limits of attentional resolution, some form of feature substitution or an averaging of features within large receptive fields. I will describe a series of studies showing that the inputs to crowding have first entered phenomenal awareness and crowding then modifies an object's appearance, but is not a substitution among neighboring features. While attention can modulate the level of crowding, equivalent noise analysis reveals that attention and crowding are dissociable processes. A computational model is developed in which crowded perceptions represent mean image statistics within an eccentricity-dependent area. A reverse correlation paradigm is then used in an attempt to predict when crowding is likely in natural scenes.