The dorsomedial striatum reflects response bias during learning

Previous studies have established that neurons in the dorsomedial striatum track the behavioral significance of external stimuli, are sensitive to contingencies between actions and outcomes, and show rapid flexibility in representing task-related information. Here, we describe how neural activity in the dorsomedial striatum changes during the initial acquisition of a Go/NoGo task and during an initial reversal of stimulus-response contingencies. Rats made nosepoke responses over delay periods and then received one of two acoustic stimuli. Liquid rewards were delivered after one stimulus (S+) if the rats made a Go response (entering a reward port on the opposite wall of the chamber). If a Go response was made to other stimulus (S-), rats experienced a timeout. On 10% of trials, no stimulus was presented. These trials were used to assess response bias, the animals' tendency to collect reward independent of the stimulus. Response bias increased during the reversal, corresponding to the animals' uncertainty about the stimulus-response contingencies. Most task-modulated neurons fired during the response at the end of the delay period. The fraction of response-modulated neurons was correlated with response bias and neural activity was sensitive to the behavioral response made on the previous trial. During initial task acquisition and initial reversal learning, there was a remarkable change in the percentages of neurons that fired in relation to the task events, especially during withdrawal from the nosepoke aperture. These results suggest that changes in task-related activity in the dorsomedial striatum during learning are driven by the animal's bias to collect rewards.