The unilateral field advantage for repetition detection

The visual system is organized such that the right hemisphere processes information in the left visual field, and left hemisphere processes information in the right visual field. Retinoptoic processing is stringently enforced in early visual areas (e.g.. V1, V2), but becomes cruder as information is fed to higher visual areas, which are often integrative over all of visual space. This structure allows the examination of visual processes that are best performed when information is presented unilaterally (to one visual field left or right) versus bilaterally (to both visual fields). Previous research on visual field effects has found that bilateral presentation benefits performance as the number of computations needed to perform a visual task increases. A common element in these tasks is that they cannot be resolved solely on the basis of perceptual processing of stimuli. Implicitly, this view of bilateral presentation advantages suggests that some elements of stimulus processing occur efficiently within a hemifield before interhemispheric integration of visual information takes place. In this thesis I describe evidence of a general unilateral field advantage for detecting repetitions I suggest this advantage arises from a low-level perceptual grouping process that operates efficiently with in a hemifield. I have used a 4-item paradigm to show that subjects are faster and in some cases more accurate in detecting a repeated element when that repetition is presented unilaterally versus bilaterally. This pattern of results holds true (1) for stimuli processed in both the ventral (letters, colors, sizes, orientations) and dorsal (motion trajectories) pathways of the visual system, (2) occurs for both familiar and novel elements, and (3) is critically dependent on the contiguity of the elements that constitute the repetition. Collectively these results indicate the unilateral field advantage reflects a low- level perceptual grouping process based on similarity and proximity. This low-level grouping process treats repeated elements as one unit when the items are adjacent, and is insensitive to familiarity effects. The unilateral field advantage for detecting physical repetitions suggests that this grouping process is more efficient within a hemifield than across hemifields.