Probability Distortion Depends on Choice Sequence in Rhesus Monkeys.

Abstract

Humans and other primates share many decision biases, among them our subjective distortion of objective probabilities. When making choices between uncertain rewards, we typically treat probabilities non-linearly: overvaluing low probabilities of reward, and undervaluing high ones. A growing body of evidence, however, points to a more flexible pattern of distortion than the classical inverse-S one, highlighting the effect of experimental conditions in shifting the weight assigned to probabilities, such as task feedback, learning, and attention. Here we investigated the role of sequence structure – the order in which gambles are presented in a choice task – in shaping the probability distortion patterns of rhesus macaques. We presented two male monkeys with binary choice sequences of MIXED or REPEATED gambles against safe rewards. Parametric modeling revealed that choices in each sequence type were guided by significantly different patterns of probability distortion. Whereas we elicited the classical inverse-S shaped probability distortion in pseudorandomly MIXED trial sequences of gamble-safe choices, we found the opposite pattern consisting of S-shaped distortion, with REPEATED sequences. We extended these results to binary choices between two gambles, without a safe option, and confirmed the unique influence of the sequence structure in which the animals make choices. Finally, we showed that the value of past experienced gambles had a significant impact on the subjective value of future ones, shaping probability distortion on a trial-by-trial basis. Taken together, our results suggest that differences in choice sequence are sufficient to reverse the direction of probability distortion.