Within-hand variability was reduced on a repetitive tapping task when individuals tapped with 2 hands in comparison to single-handed tapping. When the total variability was decomposed into central timing and peripheral implementation components (A.M. Wing & A.B. Kristofferson, 1973), the bimanual advantage was attributed to decreased central variability. The improved consistency does not require that the movements involve homologous muscles. However, unlike phase coupling, the bimanual advantage is not found when the 2 movements are produced by different individuals, but rather requires that the 2 movements be produced by 1 individual. It is proposed that separate timing mechanisms are associated with each effector. During bimanual movements, the outputs from these timing mechanisms are integrated prior to movement execution, and it is this integration that results in the bimanual advantage.

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Recent theories suggest that the human cerebellum may contribute to the performance of cognitive tasks. We tested a group of adult patients with cerebellar damage attributable to stroke, tumor, or atrophy on four experiments involving verbal learning or attention shifting. In experiment 1, a verb generation task, participants produced semantically related verbs when presented with a list of nouns. With successive blocks of practice responding to the same set of stimuli, both groups, including a subset of cerebellar patients with unilateral right hemisphere lesions, improved their response times. In experiment 2, a verbal discrimination task, participants learned by trial and error to pick the target words from a set of word pairs. When age was taken into account, there were no performance differences between cerebellar patients and control subjects. In experiment 3, measures of spatial attention shifting were obtained under both exogenous and endogenous cueing conditions. Cerebellar patients and control subjects showed similar costs and benefits in both cueing conditions and at all SOAs. In experiment 4, intra-and interdimensional shifts of nonspatial attention were elicited by presenting word cues before the appearance of a target. Performance was substantially similar for cerebellar patients and control subjects. These results are presented as a cautionary note. The experiments failed to provide 1Corresponding author. support for current hypotheses regarding the role of the cerebellum in verbal learning or attention. Alternative interpretations of previous results are discussed.

Timing variability on a repetitive tapping task was studied in subjects with unilateral cerebellar lesions. During unimanual tapping, within-hand variability was larger when tapping with the ipsilesional hand in comparison to tapping with the contralesional hand. However, variability in the impaired hand was greatly reduced when subjects tapped with two hands together. The improvement in within-hand variability during bimanual tapping was associated with a reduction in central variability rather than response implementation variability according to the two-process model of Wing and Kristofferson (1973). It is proposed that (1) each half of the cerebellum independently regulates the temporal aspects of movements on the ipsilateral side and (2) temporal coupling constraints require these separate signals to be integrated prior to response implementation for bimanual movements.

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