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Using transcranial magnetic stimulation to assess reward circuits in humans


2011 Exceptional Summer Student Award Winner Eric Emmons Eric Emmons

Human reward circuits are integral to the processes of learning, yet are still relatively poorly understood. Previous studies indicate that dopamine is the primary neurotransmitter of the reward system, in that dopamine neurons in the midbrain fire when reward is anticipated. We do not know, however, whether these neurons are sensitive to the uncertainty, or predictability, of the reward. In other words, do they respond when you are uncertain of whether you’ll be rewarded? We used transcranial magnetic stimulation (TMS) to indirectly assess the reward response. During TMS administration, a strong electromagnet is held to the scalp, creating a neuronal response in the adjacent brain tissue (here, M1). The TMS-driven response is further modulated by activity of midbrain dopamine neurons; thus, we can assay dopamine neuron activity by measuring changes in TMS response. In this study, we coupled TMS with a behavioral task in which we manipulated uncertainty, anticipating that M1 would vary with uncertainty. We administered TMS at one of three possible time points on each trial. Preliminary results in this pilot study indicate that this was indeed the case. Specifically, a significant relationship was found between the uncertainty of reward and motor cortex response when TMS was delivered 250 ms following the stimulus (500 ms and 750 ms latencies were included but did not produce significant results). These findings emphasize that reward is not a binary variable; rather, uncertainty modulates its effect. Additionally, there appears to be a critical window (250 ms) for observing this effect.

Last updated December 23, 2013