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Rewards are often thought of as an after effect to a favorable behavior, but they are physiologically necessary as they provide information from our environment that guides future actions. Many studies have shown that midbrain dopamine projections control the human reward system, but recent studies have shown that motor cortex excitability itself is actually affected by reward, reward magnitude creating an effect on this excitability. However, we do not yet know the affect of cost on this reward induced cortical excitability. To examine this question, we utilized Transcranial Magnetic Stimulation and stimulated the left primary motor cortex in order to induce a muscle response in the pollicus brevis muscle of right hand. We recorded the response of the muscle using electromyography (EMG). Looking at the amplitude of the motor-evoked potentials in the right pollicus brevis after a behavioral task involving variable amounts of effort and reward, we found that the primary motor cortex becomes excited in response to reward, consistent with previous work. Additionally, we found that task-driven effort modulates this reward-induced cortical excitability. These data show that in order to guide future actions favorably, the motor cortex does not only take into account the reward profit of an activity, subtracting the cost from the reward, but rather the summation of the cost and reward cues. The level of effort directly affects the motor cortex in the presence of a reward, becoming most excited in high effort, high reward situations.
Last Modified December 14, 2012