SELECTIVE REPRESENTATION OF REINFORCEMENT LEARNING VARIABLES IN SUBPOPULATIONS OF THE EXTERNAL GLOBUS PALLIDUS

The basal ganglia are a network of subcortical nuclei that play a central role in regulating motor and cognitive functions. A critical hub in these structures is the external globus pallidus (GPe) as it links all nuclei in this network. This nucleus was long considered a homogenous relay in the motor-suppressing indirect pathway. However, the GPe is anatomically and functionally diverse. How the GPe subpopulations contribute differentially to cognitive and motor function is currently unknown. Here, we find (1) that the GPe encodes motor and value signals in distinct subpopulations, (2) that the GPe encodes contralateral but not ipsilateral action value and these values are only updated by negative action outcomes, and (3) that specific functional GPe subpopulations are genetically distinct and project to particular downstream targets. To investigate the functional encoding of the GPe, we trained mice on a dynamic foraging task and use reinforcement learning models to extract the strategy and cognitive/motor variables that mice use to solve the task. We then link structure and function within the GPe by combining high-density Neuropixel probe recordings with intersectional viral strategies and antidromic optogenetic tagging to determine how specific GPe subpopulations encode the identified task variables. In summary, our results demonstrate that rather than being a homogenous relay, the GPe is a collection of specialized subcircuits that together support its role in cognitive and motor functions.