The Neurophysiology of Social Decision Making
The ultimate goal of the nervous systems of all animals is conceptually simple: Manipulate the external environment to maximize one's own survival and reproduction. The myriad means animals employ in pursuit of this goal are astoundingly complex, but constrained by common factors. For example, to ensure survival, all animals must acquire the necessary nutrients to sustain metabolism. Similarly, social interaction of some form is necessary for mating and reproduction. For some animals, the required social interaction goes far beyond that necessary for mating. Humans and many other primates exist in complex social environments, the navigation of which are essential for adaptive behavior. This dissertation is concerned with processes of transforming sensory stimuli regarding both nutritive and social information into motor commands pursuant to the goals of survival and reproduction. Specifically, this dissertation deals with these processes in the rhesus macaque. Using a task in which monkeys make decisions simultaneously weighing outcomes of fruit juices and images of familiar conspecifics, I have examined the neurophysiology of social and nutritive factors as they contribute to choice behavior; with the ultimate goal of understanding how these disparate factors are weighed against each other and combined to produce coherent motor commands that result in adaptive social interactions and the successful procurement of resources. I began my investigation in the lateral intraparietal cortex, a well-studied area of the primate brain implicated in visual attention, oculomotor planning and control, and reward processing. My findings indicate the lateral intraparietal cortex represents social and nutritive reward information in a common neural currency. That is, the summed value of social and nutritive outcomes is proportional to the firing rates of parietal neurons. I continued my investigation in the striatum, a large and functionally diverse subcortical nuclei implicated in motor processing, reward processing and learning. Here I find a different pattern of results. Striatal neurons generally encoded information about either social outcome or juice rewards, but not both, with a medial or lateral bias in the location of social or juice information encoding neurons, respectively. In further contrast to the lateral intraparietal cortex, the firing rates of striatal neurons coding social and nutritive outcome information is heterogeneous and not directly related to the value of the outcome. This dissertation represents a few incremental steps toward understanding how social information and the drive toward social interaction are incorporated with other motivators to influence behavior. Understanding this process is a necessary step for elucidating, treating, and preventing pathologies
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