Audio-Vocal Integration Mechanisms and Volitional Control of Vocal Behavior in Marmoset Monkeys (Callithrix jacchus)

Abstract

As a prerequisite for human speech vocal communication has been intensively investigated in various vertebrate species in the last decades. It enables two or more individuals to rapidly transmit information. Although, many vertebrate taxa possess the ability to vocalize, only a few are able to learn their vocal patterns by imitation or invention. A well-known example for learned vocalizations is human speech. However, up to now there is no evidence that our closest relatives, non-human primates, are able to produce learned vocal patterns. Hence, a major question in science is when and how human speech appeared in the primate lineage. Hereby, studying the neural mechanisms underlying vocal behavior in primates might help elucidating these questions. A non-human primate model that has gathered increasing interest in neuroscience in the last decades is the common marmoset monkey (Callithrix jacchus), a highly social and vocal New World primate. In the present thesis I worked with this animal species as a model system to study vocal flexibility, audio-vocal integration mechanisms and cognitive control of vocal behavior combining behavioral, neuroethological, psychophysical and electrophysiological recording techniques. Using acoustic perturbation triggered by the monkeys own vocalizations we found high flexibility in their vocal behavior as well as indications that their vocalizations are built out of small distinct units, overturning decade old thoughts about the structure of primate vocalizations. Furthermore, we showed that marmosets are capable of performing a complex vocalmotor task in a well-controlled environment. Monkeys were trained to vocalize on command in response to a visual cue as well as executing two distinct vocalizations in response to two different visual cues. Finally, we developed a new electrophysiological method enabling the extracellular electrophysiological recording from many single units at the same time in deep brainstem structures. We found vocal-motor and auditory neurons in the ventrolateral pontine brainstem and could show that this method is suitable to investigate neural circuits underlying vocal behavior. The results of this thesis demonstrate that vocal behavior of primates is much more flexible than previously though and, thus, making the marmoset monkey a suitable model to study vocal flexibility, a crucial preadaptation for the evolution of human speech in the primate lineage.