Determinants of the enhancement of the right visual field advantage by bilateral vs. unilateral stimuli

Bilateral presentation of two different verbal inputs usually produces a stronger left hemispheric/right visual field (RVF) processing advantage than unilateral presentation. Previous reports of this effect confounded stimulus load with display configuration. Two experiments are presented in which the number of items during unilateral and bilateral trials was equated. The RVF advantage was enhanced during a free recall paradigm (Experiment I) but not during a partial report paradigm in which subjects were cued in advance to report only the top or the bottom of the display (Experiment II). This demonstrates that the effect does not depend upon the number of inputs per display and can not invariably be produced by bilateral/bihemispheric stimulation. It is suggested that subjects may be strongly biased to give priority to items delivered to the RVF/left hemisphere during bilateral trials. However, when this bias is constrained by rendering half of the RVF inputs irrelevant, subjects may be forced to pay equal attention to LVF inputs. Thus, eliminating the attentional bias eliminates the enhancement of the RVF advantage during bilateral trials.