Motor and cognitive interference effects on unimanual tapping rates.

Experiments conducted on both normal and disordered populations have led to the hypothesis that the left hemisphere's specialization for language results from its control over motor activities. This control is reflected in the lateralized disruption of manual activity during cerebral time-sharing tasks. Recent studies have challenged this hypothesis, stating that the interference effects reflect both cognitive and motor mechanisms in the left hemisphere. This experiment investigates whether the left hemisphere's control over speech involves both of these components or is purely motor. The question was examined by measuring the effect of concurrent hemispheric activity on single-finger tapping rates. Forty subjects tapped under two conditions: speaking and listening. The data show there may be both motor and cognitive mechanisms involved in left-hemisphere control.