Mapping of Extracorporeal Space by Vibrotactile Reaction Times: A Far-Left-Side Disadvantage

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Abstract
Vibrotactile reaction times in normal dextrals were measured for the two hands separately when either hand was located at each of seven possible positions: 90°, 45°, and 15° to the left and right of the chest midline, and at the midline itself (0°). Reaction times for the two hands did not differ and there was no Hand by Position interaction. At 90° left, reaction times were significantly slower than at any other position except 45° right. However, none of the other positions, including 45° right, differed from each other. Performance in this task, therefore, was relatively uniform from 90° right to 45° left, but markedly slower at 90° left. This far-left-side disadvantage may reflect a difficulty (for dextrals) in focussing covert attention in the far-left part of space for a block of trials. Since vibrotactile reaction times are sensitive to attentional factors in normal subjects, the paradigm should allow quantification of the clinical symptoms of the hemineglect syndrome; some preliminary observations of this syndrome with another vibrotactile design are reported.