Neural mechanisms underlying simple and complex reinforcement learning

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DESCRIPTION (provided by applicant): In order to guide motivated behavior, humans and animals need to be able to flexibly evaluate the affective values - positive or negative - of stimuli in the environment. This process may go awry in psychiatric disorders; for example, in post-traumatic stress disorder (PTSD), an overly negative, inflexible value may be assigned to certain stimuli associated a trauma, even in a context where they should be non- threatening. The focus of this proposal is on how the values of stimuli are represented in the brain, and how this representation changes in response to changing environmental context. To investigate this, neural activity will be recorded simultaneously in two brain areas - the amygdala and orbitofrontal cortex (OFC) - while monkeys perform one of two tasks: a simple conditioning task, in which visual stimuli become linked with reward or punishment, and a context-dependent task, in which the meaning of visual stimuli changes from moment to moment depending upon contextual cues. The simpler task will reveal whether and how stimulus value is represented by individual neurons in OFC and amygdala, and will provide information about the relative timing of these "value signals" that may clarify how these two areas interact. The more complex task will reveal whether these signals can change flexibly enough to reflect the value of a stimulus when it can change rapidly depending on context. It may also reveal whether and how contextual information - i.e., the "rule" for interpreting the value of a stimulus - is encoded in OFC and/or amygdala. Together, these experiments may elucidate how value assignments are formed and then modulated by a neural circuit involving limbic areas (such as the amygdala) and areas that may be involved in the executive control of emotional responses (such as OFC). Learning how this circuit functions normally is an important first step in understanding how it may dysfunction in psychiatric disorders like PTSD, clinical depression, and schizophrenia. Relevance: In order to make everyday decisions, people (and animals) need to be able to assign values (such as "good" or "bad") to objects in the environment, and sometimes to change these values depending on the context in which an object occurs. This process is disrupted in many psychiatric disorders, such as depression and post-traumatic stress disorder. The focus of this proposal is on understanding how emotional and cognitive brain areas interact to learn and modify the values of objects - knowledge that may aid our understanding, and eventually treatment, of the causes of psychiatric disorders.