Pupil Old/New Effect Is Modulated by Task Instruction—Evidence for an Attentional Saliency Account

Pupil dilation (PD) is more pronounced during the correct recognition of previously encountered target items than during the correct rejection of novel, previously unseen foils. This phenomenon, known as the pupil old/new effect, has been interpreted in several ways-one prominent account suggests that it reflects a bottom-up attentional orienting response triggered by activated mnemonic information. Given that the magnitude of an orienting response to a stimulus is influenced by its task relevance, we tested in two experiments whether task instructions modulate the pupil old/new effect. Participants were presented with previously encoded material intermixed with novel items and were instructed to categorize the items as animate/inanimate (nonmnemonic task) and then to make an old/new recognition judgment on a subset of the items (mnemonic task). When the items had strong memory representations, an old/new effect emerged in the nonmnemonic task condition (i.e., when no explicit memory decision was required). In contrast, this effect could not be consistently demonstrated for weakly encoded items. Moreover, when the encoded stimuli were not overlearned, the old/new effect was more pronounced in the mnemonic than in the nonmnemonic task, suggesting that the effect is driven by attentional orientation modulated by task relevance. The pattern of our results thus suggests that strong memory representations can automatically trigger increased PD (i.e., without explicit memory instruction), and that the pupil old/new effect reflects an attentional orienting response toward activated mnemonic information.