The midbrain periaqueductal gray: a brainstem structure involved in vocalization.

Experiments were performed on one Macaca fascicularis and three Macaca mulatta monkeys. Electrical stimulation of the midbrain periaqueductal gray (PAG) elicited phonation closely resembling that naturally produced by such animals. Electromyographic (EMG) recordings from laryngeal, thoracic, tongue, and facial muscles showed increased levels of activity following PAG stimulation. Recordings were made from single neurons located within the PAG in one monkey. These neurons increased their firing rates prior to the onset of laryngeal electromyographic activity or vocalization. Spike-triggered averaging and microstimulation indicated that some of the PAG neurons may project to the laryngeal system. The function of the PAG is discussed with regard to vocal initiation, voice intensity, coordination of different muscle systems during vocalization, and for emotional vocalizations.