Forward Entrainment: Evidence, Controversies, Constraints, and Mechanisms

Abstract

We define forward entrainment as that part of the entrainment process that outlasts the entraining stimulus. In this study, we examine conditions under which one may or may not observe forward entrainment. In part 1, we review and evaluate studies that have observed forward entrainment using a variety of psychophysical methods (detection, discrimination and reaction times), different target stimuli (tones, noise, gaps), different entraining sequences (sinusoidal, rectangular or sawtooth waveforms), a variety of physiological measures (MEG, EEG, ECoG, CSD), in different modalities (auditory and visual), across modalities (audiovisual and auditory- motor), and in different species. In part 2, we review those studies that have failed to observe forward entrainment, with emphasis on evaluating the methodological and stimulus design differences that may clarify the contrasting findings across these two classes of studies. In part 3, we describe those experimental conditions under which we ourselves have failed to observe forward entrainment, and provide new data on use of complex envelope patterns as entraining stimuli, show data on intersubject variability, and provide new findings on psychometric functions that characterize the strength of forward entrainment at different SNRs. In part 4 we theorize on potential mechanisms, describe how neurophysiological and psychophysical studies approach the study of entrainment, and caution against drawing direct causal inferences between the two without compelling evidence beyond correlative measures.

Rhythms in cognition: The evidence revisited

“…After a rhythmic cueing period with amplitude‐modulated noise, the participants responded to target tones embedded in constant noise but at different times relative to the phase of the pre‐target noise. Although target detection fluctuated periodically for some participants, these periodicities were not consistently linked with the rhythmic cueing (but see Saberi & Hickok, 2021).…”

Slow phase-locked modulations support selective attention to sound

“…Under the endogenous attentional entrainment account, we would expect that the width of the phasic attentional modulation would scale inversely with increasing rate regardless of tone duration, with broader modulations for slower rates. We would also potentially expect that phasic attentional modulation would continue into the silent period at the end of each trial (“forward entrainment”, Saberi and Hickok 2021 ). By contrast, on an attentional filter account, we would expect that the attentional modulation would remain limited to the time points associated with the N1, and that the phasic attentional EEG waveform would not continue into the silent period of each trial.…”

“…If attentional modulation of neural entrainment reflects the alignment of endogenous neural oscillators with the temporal structure of attended stimuli, then attention-driven neural modulations could continue for a time even once stimuli have ceased (“forward entrainment”, Saberi and Hickok 2021 ), given that some oscillators are self-sustaining (Doelling & Assaneo, 2021). To test this prediction, we designed the task such that participants attended to a series of short tone sequences so that we could investigate whether the sinusoidal attentional modulation continued through the silence between sequences.…”

“…The endogenous entrainment account also predicts that attentional modulations could continue through the silence between tone sequences (“forward entrainment”; Saberi and Hickok, 2021 ), as some oscillatory systems are self-sustaining. By contrast, the attentional filter account predicts that attentional modulations should die out rapidly after the offset of the final tone of the sequence.…”

Rhythmic Modulation of Visual Discrimination is Dependent on Individuals’ Spontaneous Motor Tempo

“…Several recent studies examining the role of external rhythmic structure in guiding perception show high variability across individuals (Bauer, Jaeger, et al, 2015;Lin et al, 2021;Saberi & Hickok, 2021). Different sources of variability were proposed, such as musical experience (Doelling & Poeppel, 2015) and the strength of neural coupling between frontal and auditory brain areas (Assaneo et al, 2019).…”