In our daily life, we often choose one item or action from several alternatives by considering their values and efforts to obtain them. To know the mechanism of such decision-making process, we developed a decision-making schedule task to obtain a reward and recorded single neuronal activity from monkey orbitofrontal cortex (OFC) which has been reported to be one of the important brain area for reward-guided behaviors.
Two monkeys were initially trained to perform a reward schedule task. In this task, the monkey had to complete the schedule composed of 1, 2 or 4 trials of visual discriminations to earn 1, 2 or 4 drops of liquid reward. After the monkey learned this task, the decision-making schedule task was introduced. The decision-making schedule task was consisted of the decision-making part and the reward schedule part. In the decision-making part, two kinds of choice target (CT) were presented sequentially at the center of the computer monitor (these targets called first and second target, respectively). Brightness and length of the CT were proportional to amount of liquid reward (1, 2 or 4 drops) and required number of the visual-discrimination trials (1, 2 or 4 trials) to be performed, respectively. After CTs were presented sequentially, these two CTs simultaneously reappeared on both sides of the fixation point in random order. Then the monkey was required to choose one of the two CTs by touching the corresponding bar in the chair. Following a choice of one target, the chosen reward schedule task was started.
We recorded from 191 neurons in the OFC. Over 85% of the recorded neurons activated in the decision-making part. In the second CT presented period, some neuronal activities were linearly related with difference in value of the two CTs (5.8%). Other 7.3% of neurons showed larger/smaller responses when the two choice target values were close. These results suggest that OFC neurons play an important role in the decision-making by reward value information processing.