Shift in visual laterality within blocks of trials

Single random polygons were briefly presented for subsequent recognition in either the left or the right visual half-field, randomized within each of three blocks of 40 trials. No overall perceptual asymmetry was found, but the distribution of correct recognitions within a block showed a shift in favor of the right visual field for later as compared to early trials (experiment 1). The shift was due to a systematic improvement of right visual field performance within blocks, left visual field performance remaining unchanged. This effect was replicated in experiment 2 and in the control condition of experiment 3. It was not caused by increasing familiarity of the stimulus material (experiment 2). When an unrelated auditory verbal task was given before each visual trial, an overall right half-field superiority was found, evenly distributed across trials (experiment 3). Thus extraexperimental subverbalization could have primed the left hemisphere in terminal subblocks in the earlier experiments. However, neither of two manipulations designed to preclude subverbalization yielded an overall left half-field superiority (experiment 4). Indeed, precluding subverbalization seemed to improve left hemisphere performance in the present task, in which verbal coding is not useful. Difficulty in demonstrating reliable left visual field superiority for relatively nonverbal stimuli thus does not necessarily reflect weak or unstable specialized processing by the right hemisphere. The same outcome can result from gradual gains in left hemisphere performance within a block of trials. In the present task, for which there is relative right hemisphere specialization, the left hemisphere has the potential to match right hemisphere performance.