BME PhD Preliminary Exam Announcement for YunWen (Darren) Chu (S. Pluta and K. Jayant, co-advisors) Everyone is invited to attend the public presentation beginning at 10:00am. Title: The neural representation of stimulus priority in the superior colliculus Exam date: 12th Dec 2023 Time: 10:00am Venue: MJIS 2001 Advisory Committee: Scott R. Pluta, Co-Chair; Krishna Jayant, Co-Chair; Maria C. Dadarlat Makin; Michael G. Heinz Abstract: Decision making relies heavily on the ability to distinguish a highly valued stimulus from less valued objects in the environment, such as sensing food amongst rocks. Along the process of perceiving and interpreting the sensory information, a value is assigned to each perceived stimulus such that this newly learned information can assist future goal-directed tasks. How value-modulated sensory processing evolves across cortical and subcortical regions remains poorly understood. Moreover, it is unknown how the neural representations of stimulus value changes with engagement. To answer these questions, we investigate the mouse primary somatosensory cortex (S1) and the superior colliculus (SC) while the animal is performing an active spatial discrimination task. The mice are trained to actively touch and associate a whisker-dependent stimulus with reward (GO whisker) while ignoring tactile input to the adjacent whisker (NoGo whisker). Extracellular neuronal spikes were recorded with high density extracellular silicon probes, and whisker dynamics were recorded with a high-speed camera. Periodically, task engagement was modified by temporarily removing the reward lickport so that the animal lost motivation to respond. We hypothesize that sensory information in the S1 neocortex is encoded in the form of physical features such as location, which is then converted into a map of stimulus value in the SC. We found that both S1 and SC neurons accurately discriminated between adjacent whisker stimuli with the SC displaying a much stronger preference than S1 for the higher-valued stimulus. Unique to SC, many neurons displayed a bidirectional response profile, where spike rate increased during reward-whisker touch, yet it decreased during NoGo whisker touch. Moreover, pre-stimulus baseline activity of SC neurons was strongly modulated by task engagement and in some cases was able to predict upcoming trial performance, suggesting a direct role in initiating goal-directed action. On completion, this study will expand on the model of how value-modulated sensory information is encoded across cortical and subcortical depths, improving our understanding of goal-directed behavior.