The pupillary response discriminates between subjective and objective familiarity and novelty

Abstract

The pupil response discriminates between old and new stimuli, with old stimuli characterized by larger pupil dilation patterns than new stimuli. We sought to explore the cause of the pupil old/new effect and discount the effect of targetness, effort, recollection retrieval, and complexity of the recognition decision. Two experiments are reported in which the pupil response and the eye fixation patterns were measured, while participants identified novel and familiar object stimuli, in two separate tasks, emphasizing either novelty or familiarity detection. In Experiment 1, familiarity and novelty decisions were taken using a rating scale, while in Experiment 2 a simpler yes/no decision was used. In both experiments, we found that detection of target familiar stimuli resulted in greater pupil dilation than the detection of target novel stimuli, while the duration of the first fixation discriminated between familiar and novel stimuli as early as within 320 ms after stimulus onset. Importantly, the pupil response distinguished between the objective (during an earlier temporal component) and the subjective (during a later temporal component) status of the stimulus for misses and false alarms. In the light of previous findings, we suggest that the pupil and fixation old/new effects reflect the distinct neural and cognitive mechanisms involved in the familiarity and novelty decisions. The findings also have important implications for the use of pupil dilation and eye movement patterns to explore explicit and implicit memory processes.

Keywords: Familiarity; Novelty; Pupil old/new effect; Pupil response; Recognition memory.

Figure 1

Design of Experiments 1 and 2. In both experiments, a perceptual matching‐to‐sample procedure was used at encoding. At retrieval, participants completed two tasks emphasizing either novelty (NT condition) or familiarity (FT condition) detection. In Experiment 1, familiarity and novelty decisions were taken using a rating scale while in Experiment 2, a simpler yes/no decision was used. Eye tracking data were recorded in both experiments during the FT and NT conditions.

Figure 2

A: Peak pupil dilation (in mm) across the three levels of familiarity and novelty strength in Experiment 1. B: Duration of first fixation (in ms) across the three levels of familiarity and novelty strength in Experiment 1.

Figure 3

A: Pupil responses across time for target familiar (Hit_FT) and target novel (Hit_NT) stimuli in Experiment 1. B: Pupil responses across time for misses in FT and NT (M_FT and M_NT, respectively) in Experiment 1. Shaded areas on the time series show standard errors of the mean.

Figure 4

Comparison of pupil responses in Experiment 2 for different recognition outcomes in the FT and NT conditions. Hit_FT = hits in FT condition (target familiar stimuli); Hit_NT = hits in NT condition (target novel stimuli); M_FT = misses in FT condition (old stimuli deemed new); M_NT = misses in NT condition (new stimuli deemed old); FA_NT = false alarms in NT condition (old stimuli deemed new). Shaded areas on the time series show standard errors of the mean.