Entrainment to an auditory signal: Is attention involved?

Behaviorally relevant environmental stimuli are often characterized by some degree of temporal regularity. Dynamic attending theory provides a framework for explaining how perception of stimulus events is affected by the temporal context within which they occur. At its core, dynamic attending theory (as the name suggests) is a theory about attention – the key insight of dynamic attending theory is that attention is not constant over time, but waxes and wanes with time’s passing, and can become coupled to the temporal structure of environmental stimuli. A wealth of empirical data supports this basic proposition, demonstrating that the detection and discrimination of, as well as response times to, target stimulus events differ based on how the target event was related to its temporal context. For example, the timing or pitch of an event is judged more accurately when that event happens “on time” with respect to a context rhythm, compared to when that same event happens at an unexpected time.

Extrapolating these psychophysical data, a new paper by Kunert and Jongman tests whether effects of temporal context might be observed for memory.

Very briefly, the paradigm involved listening to an 8-tone sequence, where one tone is higher than the others. Dutch pseudowords are presented at different phases of the 8-tone sequence (either at tone 4 or tone 8), and participants either give a speeded lexical decision (Exp. 1a), are asked to recall the words for a later recall test (Exp. 2a), or both (Exp. 2b). Although participants’ lexical decisions were significantly faster for tone-8 words than for tone-4 words, memory for words presented at those positions was not different.

There are a number of confusing aspects of this manuscript, but I’d like to focus on one. The authors ask the following:

“How does the brain react to rhythmic auditory input? Does it increase general attention at moments of rhythmic salience as predicted by the most-widely adopted interpretation of dynamic attending theory (DAT)?”;

…and they suggest that their results “raise[s] the question of what the “attentional energy” postulated by the DAT actually represents in terms of neurophysiology and/or psychology”.

The reason this is confusing to me is that there’s a growing neuropsychological literature on entrainment and its effects on perception. And a number of authors, including Mari Jones and Ed Large, for example, have proposed that neural oscillations are the correlates of “attentional energy” (see here for a shameless self promotion of my own views on the matter). If this is true, this means that fluctuations in “attentional energy” are fluctuations in neuronal excitability. What this means, I think, is easiest to understand from the perspective of a single neuron, who is more likely to fire during a period of high excitability and less likely to fire during a period of low excitability. When fluctuations in neuronal excitability become entrained by a stimulus rhythm, high excitability periods align with future events, improving neuronal responsiveness and perception of that event (or decreasing responsiveness between events as the case may be).

It seems to me that what the authors are then getting at is a question of how widely neural oscillations might be entrained by a modality-specific rhythm. That is, will an auditory rhythm entrain oscillations in brain regions responsible for pseudoword encoding? And this question has certainly not been definitively answered. There is evidence that neural oscillations in sensory cortices are entrained by rhythms presented in their favored  and sometimes nonfavored modality (audition, vision, somatosensation), and there is evidence that auditory rhythms can entrain specific populations of tonotopically tuned cells. So I think we have some idea about how specific entrainment can be, but it seems like we know less well how general entrainment can be. Do auditory rhythms entrain neural oscillations in motor regions, as many of us interested in rhythm and beat perception believe? What about brain regions responsible for memory encoding? It’s really the answer to this question that determines whether entrainment effects on memory are going to be observable, and paradigms designed to answer this question may be more informative when they are developed in the context of what we know about the brain. For example, entrainment of neural oscillations in relevant brain regions in the right relationship by noninvasive brain stimulation does improve memory encoding – it’s a relevant question why we would expect an auditory rhythm to do the same.

– source article: Kunert & Jongman. Entrainment to an auditory signal: Is attention involved? JEP: General.