Abstract
Neurologically normal observers demonstrate a small but significant leftward shift in the perceived midpoint of visual stimuli such as lines, a phenomenon called pseudoneglect (Jewell & McCourt, 2000). This leftward bias is attributed to a surplus of spatial attention deployed into left hemispace by the right hemisphere, which is dominant with respect to the control and allocation of spatial attention. It is presently unknown to what extent a similar asymmetry might exist for spatial stimuli presented via the auditory modality. If asymmetries arise at a site where space is represented amodally, then leftward bias should be independent of modality. On the other hand, if spatial attention is modality-specific, bisection error for visual and auditory objects might be dissociable. Moreover, visual attention is deployed across the visual field (200°), whereas auditory spatial attention can be deployed panoramically (360°). Healthy dextral subjects (N=21) performed an auditory interval bisection task. In the auditory bisection task the spatial interval to be bisected was defined by a sequence of two complex tones (200 and 400 Hz squarewaves; 300 ms duration) separated by 22° of angle. Subjects judged whether the location of a third, sequentially presented, complex tone (300 Hz) was leftward or rightward of the center of the spatial interval defined by the first two tones. Subjects performed the auditory bisection task when facing toward and away from the speaker array, allowing an index of attentional asymmetry in both front and back space. Subjects also performed a standard tachistoscopic visual line bisection task (McCourt & Olafson, 1997; McCourt & Jewell, 1999). A significant leftward bias was found in the visual bisection task, whereas significant rightward biases were found in the auditory bisection task, in both front- and back-space conditions. The results suggest that spatial attention is modality specific.
Supported by P20 RR020151.