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After practice, subjects balanced a dowel rod on the right and on the left index finger while speaking and while remaining silent. As compared to control, the verbal condition yielded shorter balancing times for the right hand, but longer ones for the left. A speculative model postulates enhancement of the practised skill by virtue of the distraction effect of the concurrent activity. This is counteracted on the right by interference with right-sided motor control by the left cerebral hemisphere due to verbal activity programmed by the same hemisphere.
Kinsbourne et al. (Sun,) studied this question.