Variance differences in asymmetry scores on bilateral versus unilateral tasks

Abstract

Kim and Levine (1991a) report that the correlations between subjects' asymmetry scores on left- and right-hemisphere specialised tasks are more positive when stimuli are presented bilaterally than when they are presented unilaterally (asymmetry scores computed as R-L for both tasks). This larger positive correlation may reflect the greater sensitivity of bilateral presentation to individual differences in non-stimulus-specific perceptual asymmetry, referred to in this paper as “characteristic perceptual asymmetry.” If this is the case, variance in subjects' asymmetry scores on bilaterally presented tasks should be greater than on unilaterally presented tasks, reflecting the contribution of individual differences in characteristic perceptual asymmetry as an additional source of variance. The findings reported in this paper are consistent with this hypothesis. That is, a re-analysis of relevant data reported in the literature shows that for both visual and auditory laterality tasks, variance in asymmetry scores is significantly greater under conditions of bilateral than unilateral stimulation. Similarities between the differential effect of bilateral vs. unilateral stimulation in normal subjects and the clinical finding that “hemi-inattention” following unilateral brain damage is more readily observed with bilateral than unilateral stimulation are discussed.