Caudate serotonin signaling during social exchange distinguishes essential tremor and Parkinson's disease patients.

Dynamic changes in dopamine, noradrenaline, and serotonin release are believed to causally contribute to the neural computations that support reward-based decision making. Accordingly, changes in signaling by these systems are hypothesized to underwrite multiple cognitive and behavioral symptoms observed in many neurological disorders. Here, we characterize the release of these neurotransmitters measured concurrently in the caudate of patients with Parkinson's disease or essential tremor undergoing deep brain stimulation surgery as they played a social exchange game. We show that violations in the expected value of monetary offers are encoded by opponent patterns of dopamine and serotonin release in essential tremor, but not Parkinson's disease, patients. We also demonstrate that these changes in serotonin signaling comprise a neurochemical boundary that subsegments these two neuromotor diseases. Our combined results point to a neural signature of altered reward processing that can be used to understand the signaling deficiencies that underwrite these diseases.