Individual hemispheric asymmetry in vocal fold control of the squirrel monkey.

In ten squirrel monkeys (Saimiri sciureus), electrodes were implanted into the left and right facial motor cortex at sites producing bilateral vocal fold adduction when electrically stimulated. All animals, in addition, had electrodes in the periaqueductal grey of the midbrain (PAG) at sites producing vocalization when electrically stimulated. In eight out of ten animals, motorcortical stimulation during periaqueductally elicited vocalization caused a change in vocal output. This change, in four cases, was more severe with left-sided ipsilateral motor cortex/PAG stimulation than with right-sided ipsilateral stimulation. In the other four cases, right-sided stimulation was more effective than left-sided stimulation. It is concluded that the majority of squirrel monkeys show a hemispheric asymmetry in vocal fold control with left- and right-sided dominance distributed about equally in the population.