Interaction between handedness and the attentional bias during tests of dichotic listening performance.

Perceptual advantages are typically interpreted as direct expressions of underlying hemispheric functional asymmetries. However, many other confounding factors including the asymmetric distribution of attention may also contribute to either the magnitude or direction of any of these advantages. In a series of experiments, Mondor and Bryden (1991, 1992a, 1992b) found that right-handed subjects bias their attention toward the right ear when faced with a difficult dichotic listening task. The present study was undertaken to determine whether the direction of the attentional bias is associated with handedness. The focus of auditory spatial attention was manipulated in a dichotic listening paradigm by presenting a pretrial tone cue to the ear from which the subject was required to report. The time period between the onset of the cue and the onset of the dichotic trial (Stimulus Onset Asynchrony--SOA) was varied in order to control the time available to orient attention to the cued ear. Whereas performance for the right ear improved substantially with SOA, that for the left ear improved only marginally. In addition, because of this differential effect of the cue on right ear and left ear performance, the magnitude of the right ear advantage was inflated at 450 ms SOA from that apparent at 150 ms and 750 ms SOA. These findings are interpreted as evidence that, in contrast to right-handers, left-handers bias their attention toward the left ear. This relation between handedness and the direction of the attentional bias is shown to have important implications for the interpretation of perceptual asymmetries.