Encoding of reward expectation by monkey anterior insular neurons.

The insula, a cortical brain region that is known to encode information about autonomic, visceral, and olfactory functions, has recently been shown to encode information during reward-seeking tasks in both single neuronal recording and functional magnetic resonance imaging studies. To examine the reward-related activation, we recorded from 170 single neurons in anterior insula of 2 monkeys during a multitrial reward schedule task, where the monkeys had to complete a schedule of 1, 2, 3, or 4 trials to earn a reward. In one block of trials a visual cue indicated whether a reward would or would not be delivered in the current trial after the monkey successfully detected that a red spot turned green, and in other blocks the visual cue was random with respect to reward delivery. Over one-quarter of 131 responsive neurons were activated when the current trial would (certain or uncertain) be rewarded if performed correctly. These same neurons failed to respond in trials that were certain, as indicated by the cue, to be unrewarded. Another group of neurons responded when the reward was delivered, similar to results reported previously. The dynamics of population activity in anterior insula also showed strong signals related to knowing when a reward is coming. The most parsimonious explanation is that this activity codes for a type of expected outcome, where the expectation encompasses both certain and uncertain rewards.