Detecting a Reward Signal in the Motor Cortex

This study will use transcranial magnetic stimulation (TMS) to determine whether the activity in the brain when someone wins something affects the part of the brain that controls movement. Studies have shown that the brain releases signals to mark rewards for certain behavior, like the activity the brain generates when an animal receives food or drink after performing a certain action. This study will look for a way to detect this kind of signal in humans.

Healthy volunteers between 18 and 60 years of age are eligible for this study. Participants undergo TMS during two experiments slot machine stimulation and key sequence (see below). For TMS, a wire coil is held on the subject s scalp. A brief electrical current is passed through the coil, creating a magnetic pulse that stimulates the brain. The stimulation may cause twitching in muscles of the face, arm or leg, and there may be a pulling sensation on the skin under the coil. The effect of TMS on the muscles is detected with small metal disk electrodes taped onto the skin of the arms or legs.

The stimulation strength needed to activate the hand muscles is determined at the beginning of each experiment. To do this, the subject sits with his or her arms and hand relaxed. Magnetic pulses of varying strengths are applied in order to find the right strength. Also, a series of 45 pairs of magnetic pulses is administered so close to each other that they produce only one movement. Measurements of the movements generated serve as a baseline for comparison with movements generated during the experiments.

Slot Machine Simulation

Subjects play a computer game similar to a slot machine. They press a button to start the game and watch as three barrels of the machine spin into place. Subjects can win $0.25, $1or $5 if all three barrels match when they stop spinning. If all three barrels do not match, subjects do not win any money, except in rare instances, when they are awarded money even when all three barrels do not match. In one trial in this experiment, subjects receive transcranial magnetic stimulation after they see the second barrel stop spinning. In another trial, they receive the stimulation after the third barrel stops spinning.

Key Sequence

Subjects see a letter on a computer screen and press a combination of the three keyboard keys G, H, and J. If they press the keys in the right order and under the time limit, they win $1. At some point, the letter displayed changes, and the subjects must guess a new combination to earn money. Each of the letters corresponds to its own combination of key presses. A few moments after the subjects see whether they pressed the keys in the right order, they receive TMS.