Neuronal activity in the periaqueductal gray and bordering structures during vocal communication in the squirrel monkey.

In seven freely moving squirrel monkeys (Saimiri sciureus), the neuronal activity in the periaqueductal gray (PAG) and bordering structures was registered during vocal communication, using a telemetric single-unit recording technique. In 9.3% of the PAG neurons, a vocalization-correlated activity was found. Four reaction types could be distinguished: a) neurons, showing an activity burst immediately before vocalization onset; b) neurons, firing during vocalization, and starting shortly before vocalization onset; c) neurons, firing exclusively during vocalization; d) neurons, firing in the interval between perceived vocalizations (i.e. vocalizations produced by group mates) and self-produced vocal response. All PAG neurons showed a marked vocalization-type specificity. None of the neurons reflected simple acoustic parameters, such as fundamental frequency or amplitude, in its discharge rate. None of the neurons reacted to vocalizations of other animals not responded to by the experimental animal. All four reaction types found in the PAG were also found in the reticular formation bordering the PAG, though in lower density.