Vocalization-correlated single-unit activity in the brain stem of the squirrel monkey

The brain stems of 17 squirrel monkeys (Saimiri sciureus) were systematically explored for vocalization-related single-unit activity during calls electrically elicited from the periaqueductal grey. Of 12,280 cells tested, 1151 fired in relation to vocalization. Of these, 587 reacted to external acoustic stimuli and started firing after vocalization onset. As most of these cells were located in classical auditory relay structures, they probably represent auditory neurones reacting indirectly to self-produced vocalization due to auditory feedback. Seven cells reacted to acoustic stimuli but fired in advance of self-produced vocalization. These cells were located in the pericentral inferior colliculus, dorsal nucleus of the lateral lemniscus, dorsomedial to the ventral nucleus of the lateral lemniscus and immediately lateral to the central grey. They are probably engaged in tuning the auditory system to process self-generated sounds differently from external sounds. 261 neurones reacted to nonphonatory oral movements (chewing, swallowing) and started firing after vocalization onset. These neurones were widely distributed within the brain stem, with the highest density in the spinal trigeminal nucleus and medially adjacent reticular formation. The majority of these cells seem to react to proprioceptive and tactile stimuli generated by phonatory and nonphonatory oral activities. Some of them may exert motor control on muscles that come into play at later stages of phonation. 57 neurones reacted to nonphonatory oral movements but fired in advanced of vocalization onset. These neurones were located mainly in the trigeminal motor nucleus, nucl. ambiguous, reticular formation around these nuclei, parabrachial region and lateral vestibular nucleus. Their role in motor control seems to be related to specific muscles rather than specific functions. 100 of the vocalization-related cells showed a correlation with respiration. Expiration-related cells were found in and around the rostral nucl. ambiguous and in the reticular formation dorsal to the facial nucleus. Inspiration-related cells were located in the rostral and caudal nucl. ambiguous regions, ventrolateral solitary tract nucleus and the lateral reticular formation below the trigeminal motor nucleus. Most of these cells probably represent premotor neurones of respiratory muscles and laryngeal motoneurones of the cricothyroid and posterior cricoarytenoid muscles. Finally, a last group of cells was found that was unresponsive to chewing and swallowing movements, quiet breathing and acoustic stimuli, but changed activity during vocalization. 38 of them became active before vocalization and cricothyroid activity, and 101 afterward.(ABSTRACT TRUNCATED AT 400 WORDS)