Effects of Stimulus Rate and Periodicity on Auditory Cortical Entrainment to Continuous Sounds

Abstract

The neural mechanisms underlying the exogenous coding and neural entrainment to repetitive auditory stimuli have seen a recent surge of interest. However, few studies have characterized how parametric changes in stimulus presentation alter entrained responses. We examined the degree to which the brain entrains to repeated speech (i.e., /ba/) and non-speech (i.e., click) sounds using phase-locking value (PLV) analysis applied to multichannel human electroencephalogram (EEG) data. Passive cortico-acoustic tracking was investigated in N=24 normal young adults utilizing EEG source analyses that isolated neural activity stemming from both auditory temporal cortices. We parametrically manipulated the rate and periodicity of repetitive, continuous speech and click stimuli to investigate how speed and jitter in ongoing sound streams affect oscillatory entrainment. Neuronal synchronization to speech was enhanced at 4.5 Hz (the putative universal rate of speech) and showed a differential pattern to that of clicks, particularly at higher rates. Phase-locking to speech decreased with increasing jitter but remained superior to clicks. Surprisingly, EEG entrainment to clicks was invariant to periodicity manipulations. Our findings provide evidence that the brain's neural entrainment to complex sounds is enhanced and more sensitized when processing speech-like stimuli, even at the syllable level, relative to non-speech sounds. The fact that this specialization is apparent even under passive listening suggests a priority of the auditory system for synchronizing to behaviorally relevant signals.Significance StatementTo examine the effects of stimulus factors and oscillatory activity mapping of auditory cortical entrainment, we compared cortico-acoustic tracking for speech-like stimuli vs. click stimuli across various speeds (below, within, and above the nominal speech rate) and acoustic periodicity (jitter in 20% steps). Overall, our results demonstrate that brain responses are more sensitive to changes in rhythm and periodicity of speech even during passive listening. The prioritization of speech in the brain might be partially due to the increased neuronal entrainment it elicits.