Introduction: The ventral striatum (VS) is a deep brain structure involved in the processing of reward. As abnormalities of reward valuation are involved in addiction, a trial of VS deep brain stimulation for the treatment of alcoholism has been recently approved at our institution. We are studying the role of the VS in reward valuation by recording neuronal activity during an intertemporal choice task, in which reward size and delay vary.
Methods: Two rhesus macaques were trained to associate visual cues with various combinations of water amount and delay during a bar touch and release task. A visual cue indicates the reward size-delay combination for each trial, which can be accepted or skipped. The monkeys accepted the trials in a predictable and reliable manner—large rewards with short waiting times were most preferred. We recorded VS neuronal activity from 121 neurons between the two monkeys engaged in this task.
Results: Two patterns of neuronal firing were apparent, corresponding to putative phasically active neurons (PANs) and tonically active neurons (TANs). We identified 88 PANs and 43 TANs. Neuronal firing rate following cue presentation was compared between the conditions using a two factor ANOVA. Overall 66/121 (55%) neurons demonstrated an effect of reward size or delay on the firing rate following cue presentation. Reward size alone impacted the neuronal firing rate in 24/88 (27%) PANs and 8/43 (19%) TANs. Reward delay alone impacted the neuronal firing rate in 3/88 (3%) PANs and 6/43 (14%) TANs. Both size and delay impacted the neuronal firing rate in 18/88 (20%) PANs and 7/43 (16%) TANs.
Conclusions: Reward size and delay are encoded by both phasically and tonically active neurons in the ventral striatum. We find evidence for both separate and combined encoding of the two components in these neurons.
Patient Care: A better understanding of the normal electrophysiology of the ventral striatum in non-human primates is essential for studies involving deep brain stimulation of this structure.
Learning Objectives: Understand the activity of different types of neurons in response to reward related stimuli