Task-irrelevant perceptual priors are represented in decision making

Perceptual decision making is a fundamentally hierarchical inference task consisting of two steps. In the first perceptual step, stimulus identity needs to be inferred. In the second step, given the inferred stimulus identity, the correct decision needs to be inferred. At which of these levels are prior expectations used to inform inferences? Previous approaches studying the role of priors in perceptual decision making did not distinguish between these two levels, and focused almost exclusively on the decision level. Here, combining approaches from non-human primate and human infant research, we developed a novel experimental paradigm that allowed us to identify behavioral signatures of perceptual versus decision priors. We trained two macaque monkeys in a modified variant of a delayed match-to-sample task in which we used three stimuli with carefully controlled statistics. The prior over match vs. non-match decisions was uniform, but (irrelevant to the match/non-match decision) each sample stimulus could be followed by one of two different non-match target stimuli drawn from a highly skewed prior. Thus, the prior over target stimuli changed dynamically, trial-by-trial, conditioned on the sample stimulus. In standard decision trials, monkeys were required to maintain central fixation while stimuli were presented peripherally and had to make a match/non-match decision by controlling a lever. Occasional free-viewing trials used an identical design, except that no decision was required and monkeys were allowed to break fixation on the presentation of the target, while we tracked their eye movements. Both accuracy in decision trials, and conditional familiarity as expressed by differential looking times in free-viewing trials showed significant differences between common and rare non-match targets. These results provide converging evidence from two different response modalities for the internal representation of dynamically changing task-irrelevant perceptual priors, opening the way to the study of their neural underpinning.