Figure 1. Risk, choice task and basic behavior.

(A) Relationship between risk measured as reward variance and reward probability. (B) Choice task. The animal made a saccade-choice between two visual stimuli (fractals, ‘objects’) associated with specific base reward probabilities. Object reward probabilities varied predictably trial-by-trial according to a typical schedule for eliciting matching behavior and unpredictably block-wise due to base-probability changes. Probabilities were uncued, requiring animals to derive reward risk internally from the variance of recently experienced rewards. Left-right object positions varied pseudorandomly. (C) Matching behavior shown in log ratios of rewards and choices. Relationship between log-transformed choice and reward ratio averaged across sessions and animals (N = 16,346 trials; linear regression; equally populated bins of reward ratios; standard errors of the mean (s.e.m.) were smaller than symbols). (D) Cumulative object choices in an example session. The choice ratio in each trial block (given by the slope of the dark blue line) matched the corresponding reward ratio (light blue). (E) Adaptation to block-wise reward-probability changes. Matching coefficient (correlation between choice and reward ratio) calculated using seven-trial sliding window around base probability changes (data across sessions, asterisks indicate significant correlation, p<0.05).