Unilateral and bilateral electrotactile stimuli were delivered to both hands of 11 right-brain-damaged (RBD) patients with left tactile extinction and 20 healthy subjects. Bimanual stimuli could be presented simultaneously or with varying stimulus onset asynchronies (SOAs). Subjects indicated their detection of unilateral or bilateral stimuli, their judgements of whether stimuli were simultaneous or successive, and, in the latter case, which side came first. In RBD patients, extinction was maximal with simultaneous presentations and decreased as SOA increased. With short SOAs, omissions of left-sided stimuli occurred with both right-side and left-side stimulus precedence, suggesting a forward and backward interference of the right stimulus on the processing of the left stimulus within a time window of at least 100 msec. In contrast, there was no interference of the left stimulus on the detection of the right stimulus. Unlike controls, extinction patients rarely expressed simultaneity judgements, but those that were produced tended to be veridical or nearly so, like in normal controls. Whereas controls expressed generally accurate judgements of right or left precedence, patients showed a bias toward a right precedence and a maximal uncertainty between left-first and right-first choices when the left stimulus had a lead between 100 and 200 msec. The results are consistent with the hypothesis that, as in other sensory modalities, tactile extinction is associated with an abnormal persistent bias of attention toward the ipsilesional side that delays the processing of contralesional stimuli. However, the finding that both extinction and explicit judgements of simultaneity tended to occur with simultaneous bilateral stimuli suggest the presence of some residual neural capacity to detect precise temporal coincidence.

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