Cortex can entrain to predictable sequences even in the absence of periodicity
Geoffrey Brookshire1,2, Daniel Casasanto1,2; 1Cornell University, 2University of Chicago
When people perceive quasi-rhythmic stimuli such as language or music, oscillations in the brain lock onto pulses in the stimulus. This phenomenon is called cortical entrainment. According to a standard view, cortical entrainment occurs when endogenous cortical oscillations resonate in response to periodic exogenous stimuli (similar to a wine glass vibrating when someone sings). Here we test an alternative account: Perhaps entrainment does not rely on periodic input, at all, but instead occurs whenever the brain can make temporally precise predictions about upcoming events. To test this proposal, we recorded electroencephalography (EEG) while human participants (N=11) watched sequences of images that varied in their periodicity and temporal predictability. Participants saw sequences that were (1) Predictable by virtue of being periodic [Predictable-Periodic], (2) Predictable but not periodic at the frequency of individual items [Predictable-Aperiodic], and (3) unpredictable and non-periodic [Unpredictable-Aperiodic]. We measured entrainment by computing cross-correlations between stimulus onsets and EEG activity in each electrode. The magnitude of the cross-correlation indicates how strongly the EEG signal predicts a stimulus onset at a given lag. In every participant, we found that pre-stimulus cross-correlations were strongest to Predictable-Periodic sequences, and importantly, that cross-correlations were stronger to Predictable-Aperiodic sequences than to Unpredictable-Aperiodic sequences. This result suggests that cortical entrainment does not rely exclusively on oscillatory resonance to rhythmic stimuli. Instead, cerebral cortex may entrain whenever sequences are predictable, even in the absence of periodicity.
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