Localization of a pontine vocalization-controlling area

To find out whether there exist additional regions in the pontine brainstem, apart from the phonatory motoneuron pools involved in vocal motor control, the effects of a localized blockade of excitatory neurotransmission in the pons were studied on squirrel monkey vocalization. Vocalization was elicited by electrical stimulation of the periaqueductal gray of the midbrain. Blockade was carried out by stereotaxic injections of kynurenic acid, a nonspecific glutamate antagonist. It was found that injections made into the ventrolateral pons around the ventral nucleus of the lateral lemniscus and superior olive could block periaqueductally elicited vocalization. Injections were only effective ipsilaterally, not contralaterally to the stimulation site. The blockade was limited to one particular class of calls, all of which had in common a characteristic stereotyped frequency modulation over several kHz. It is concluded that critical processing steps of vocal motor control take place in the periolivary region.