Behavioral and Neurophysiological Studies of Vocal Production in Marmoset Monkeys

Humans are known to have sophisticated control over speech sounds. The neural mechanisms underlying this control were found to be related to the frontal cortex. Non-human primates, although evolutionally close to humans, were traditionally thought to have limited flexibility in vocal production. It remains an open question whether or how frontal cortex was involved in the generation of vocalizations in monkeys. Recently, studies in highly-vocal primate species, such as tamarins and marmosets, have revealed evidence for abilities to modify vocal production, including voluntary control of the initiation time, stop of an ongoing call in real time and changes of spectrotemporal structures. New wireless recording techniques enabled studies in freely roaming and vocalizing marmosets, which led to findings that premotor and prefrontal cortices were involved in self-initiated vocal production during communication. In this thesis, we investigated the capacity of vocal control and its neural correlate in marmoset monkeys using both behavioral and neurophysiological approaches. In the behavioral part, we tested explicitly whether marmosets can voluntarily control the spectral parameters in their vocal structure. Through a perturbation paradigm, we found that marmosets shifted the fundamental frequency of their phee calls during vocal communication, in directions away from the spectra of perturbation signals. Further analysis of the data demonstrated that these shifts occurred with a prediction and learning components. In the neurophysiology part, we developed methods to allow for reliable acoustic and wireless neural recordings in the colony with rich social context. Neurons in premotor cortex were found to be modulated by major call types used in social communications. A subset of these neurons showed distinction in modulations for different call types. We also found the local field potentials were modulated by vocal production and oscillatory activities were found at some sites during the production of twitter calls. These results suggested a higher level of vocal control in non-human primates than previously known and provided evidence for potential functions of premotor cortex in social communications