Bimanual crossed-uncrossed difference and asynchrony of normal, anterior- and totally-split-brain individuals.

This study investigated interhemispheric transfer in ten normal, four anterior-, and four totally-split-brain individuals, through measures of manual asynchrony and bimanually-recorded crossed-uncrossed difference (CUD). The CUD relied on the difference between crossed and uncrossed responses whereas the asynchrony measure relied on the reaction time difference between the two responding hands. Manipulations of sensory and attentional factors were assessed for both measures. We found a normal CUD (3.8 ms) along with an exacerbated and more variable asynchrony for partial split-brain individuals (40.8 ms) compared to normal individuals (CUD: 0.4 ms, asynchrony: 13.8 ms). In turn, the CUD of total split-brain individuals (20.4 ms) was larger than that of partial split-brain and normal individuals. Also, the asynchrony of total split-brain individuals (57.6 ms) was larger and more variable than that of normal individuals, and more variable than that of partial split-brain individuals. We interpret these results as behavioural evidence of independent mechanisms underlying the CUD and bimanual synchronization, as well as evidence of the joint involvement of both the anterior and the posterior portions of the corpus callosum in bimanual coordination.