Pupil Size Reflects Trial-Level Variability in Imagery Vividness During Immersive Storytelling but Not (or Hardly) Individual Differences in Trait Imagery.

Previous research has shown that the eyes' pupils are larger when imagining dark as compared to bright objects or scenes. On the basis of this, it has been claimed that pupil size is a sensitive marker of mental imagery vividness. We investigated this claim in three experiments, conducted in two countries (Norway and The Netherlands; Ntotal = 115), in which participants read, listened, or freely imagined stories that evoked a sense of darkness or brightness. In addition, self-reports of vividness were collected for each story to measure variations in imagery vividness during the experiment; and through questionnaires (VVIQ, SUIS), to measure differences in task-unrelated imagery abilities at the individual level. We found that the effect of larger pupils for darkness-evoking stories than brightness-evoking stories was highly variable. Importantly, we found that this pupil-size difference (dark-bright) was consistently largest for vividly imagined stories. Finally, we did not find any convincing relationship between this pupil-size difference and individual differences in questionnaire-based imagery. We conclude that the strength of pupil-size changes in response to imagined darkness or brightness better reflects trial-by-trial fluctuations in imagery vividness within an individual than individual differences in imagery vividness as a personal trait.