Pupil dilation signals recognition salience.

Stimuli that are recognized from a prior encounter elicit larger pupil dilations than those that are not. This study tests an account of this recognition dilation response (RDR) that assumes newly encountered recognition signals, like new percepts, elicit attentional orienting responses. Because orienting is moderated by motivational significance and expectation, the RDR was tested for these properties; manipulating incentives for "old" versus "new" judgments, and isolating the effects of runs of "old" versus "new" decisions on the subsequent RDR, in two experiments. Whereas incentivizing "new" decisions largely eliminated the RDR, incentivizing "old" decisions amplified it. Moreover, the RDR was prominent following runs of "new" decisions, yet minimal following runs of "old" decisions. Thus, the pupil dilates more as recognition memory becomes more valuable and/or unexpected. This recognition-orienting response was functionally separate from an additional, late dilation linked to feedback expectancy. Thus, the pupil separately signals the salience of recognition evidence, and the expectation of post-decision feedback.