The effect of repetition on intersubject synchrony assessed with fMRI.

We investigated how repeated exposure to a stimulus affects intersubject synchrony in the brains of young and older adults. We used functional magnetic resonance imaging (fMRI) to measure brain responses to familiar and novel stimuli. Young adults participated in a familiarization paradigm designed to mimic 'natural' exposure while older adults were presented with stimuli they had known for more than 50 years. Intersubject synchrony was calculated to detect common stimulus-driven brain activity across young and older adults as they listened to the novel and familiar stimuli. Contrary to our hypotheses, synchrony was not related to the amount of stimulus exposure; both young and older adults showed more synchrony to novel than to familiar stimuli regardless of whether the stimuli had been heard once, known for a few weeks, or known for more than 50 years. In young adults these synchrony differences were found across the brain in the bilateral temporal lobes, and in the frontal orbital cortex. In older adults the synchrony differences were found only in the bilateral temporal lobes. This reduction may be related to an increase in idiosyncratic responses after exposure to a stimulus but does not seem to be related to how well the stimuli are learned or to differences in attention. Until the effects of repeated exposure on synchrony are fully understood, future studies using intersubject synchrony, where the novelty of the stimuli cannot be guaranteed, may consider exposing all of their participants to the stimuli once before data are collected to mitigate the effects of any systematic differences in stimulus exposure.