Data from: Predictions about reward outcomes in rhesus monkeys

Human infants and nonhuman animals respond to surprising events by looking
longer at unexpected-than-expected situations. These looking responses
provide core cognitive evidence that nonverbal minds make predictions
about possible outcomes and detect when these predictions fail to match
reality. We propose that this phenomenon has crucial parallels with the
processes of reward prediction error, indexing the difference between
expected and actual reward outcomes. Most work on reward prediction errors
to date involves neurobiological techniques that cannot be implemented in
many relevant populations, so we developed a novel behavioral task to
assess monkeys’ predictions about reward outcomes using looking time
responses. In Study 1, we tested how semi-free-ranging monkeys (n = 210)
responded to positive error (more rewards than expected), negative error
(less rewards than expected), and number control. We found that monkeys
looked longer at a given reward when it was unexpectedly large or small,
compared to when the same quantity was expected. In Study 2, we compared
responses in the positive error condition in monkeys ranging from infancy
to old age (n = 363), to assess lifespan changes in sensitivity to reward
predictions. We found that adolescent monkeys showed heightened responses
to unexpected rewards, similar to patterns seen in humans, but showed no
changes during aging. These results suggest that monkeys’ looking
responses can be used to track their predictions about rewards and that
monkeys share some developmental signatures of reward sensitivity with
humans, providing a new approach to accessing cognitive processes
underlying reward-based decision-making.