Pupil dilation tracks divergent learning processes in aware versus unaware Pavlovian conditioning

Evidence regarding unaware differential fear conditioning in humans is mixed and even less is known about the effects of contingency awareness on appetitive conditioning. Phasic pupil dilation responses (PDR) might be more sensitive for capturing implicit learning than other measures, such as skin conductance responses (SCR). Here, we report data from two delay conditioning experiments utilizing PDR (alongside SCR and subjective assessments) to investigate the role of contingency awareness in aversive and appetitive conditioning. In both experiments, valence of unconditioned stimuli (UCS) was varied within participants by administering aversive (mild electric shocks) and appetitive UCSs (monetary rewards). Preceding visual stimuli (CSs) predicted either the reward, the shock (65% reinforcement), or neither UCS. In Exp. 1, participants were fully instructed about CS-UCS contingencies, whereas in Exp. 2, no such information was given. PDR and SCR demonstrated successful differential conditioning in Exp. 1 and in (learned) aware participants in Exp. 2. In non-instructed participants who remained fully unaware of contingencies (Exp. 2), differential modulation of early PDR (immediately after CS onset) by appetitive cues emerged. Associations with model-derived learning parameters further suggest that early PDR in unaware participants mainly reflect implicit learning of expected outcome value, whereas early PDR in aware (instructed/learned-aware) participants presumably index attentional processes (related to uncertainty/prediction error processing). Similar, but less clear results emerged for later PDR (preceding UCS onset). Our data argue in favor of a dual-process account of associative learning, suggesting that value-related processing can take place irrespective of mechanisms involved in conscious memory formation.