Challenges and Approaches in the Study of Neural Entrainment.

When exposed to rhythmic stimulation, the human brain displays rhythmic activity across sensory modalities and regions. Given the ubiquity of this phenomenon, how sensory rhythms are transformed into neural rhythms remains surprisingly inconclusive. An influential model posits that endogenous oscillations entrain to external rhythms, thereby encoding environmental dynamics and shaping perception.

Opposing neural processing modes alternate rhythmically during sustained auditory attention.

During continuous tasks, humans show spontaneous fluctuations in performance, putatively caused by varying attentional resources allocated to process external information. If neural resources are used to process other, presumably "internal" information, sensory input can be missed and explain an apparent dichotomy of "internal" versus "external" attention. In the current study, we extract presumed neural signatures of these attentional modes in human electroencephalography (EEG): neural entrainment and α-oscillations (~10-Hz), linked to the processing and suppression of sensory information, respectively.

Entrainment echoes in the cerebellum.

Evidence accumulates that the cerebellum's role in the brain is not restricted to motor functions. Rather, cerebellar activity seems to be crucial for a variety of tasks that rely on precise event timing and prediction. Due to its complex structure and importance in communication, human speech requires a particularly precise and predictive coordination of neural processes to be successfully comprehended.

Neural tracking of continuous acoustics: properties, speech-specificity and open questions.

Human speech is a particularly relevant acoustic stimulus for our species, due to its role of information transmission during communication. Speech is inherently a dynamic signal, and a recent line of research focused on neural activity following the temporal structure of speech. We review findings that characterise neural dynamics in the processing of continuous acoustics and that allow us to compare these dynamics with temporal aspects in human speech.

Rhythmic Entrainment Echoes in Auditory Perception.

Rhythmic entrainment echoes-rhythmic brain responses that outlast rhythmic stimulation-can demonstrate endogenous neural oscillations entrained by the stimulus rhythm. Here, we tested for such echoes in auditory perception. Participants detected a pure tone target, presented at a variable delay after another pure tone that was rhythmically modulated in amplitude.

Intelligibility improves perception of timing changes in speech.

Auditory rhythms are ubiquitous in music, speech, and other everyday sounds. Yet, it is unclear how perceived rhythms arise from the repeating structure of sounds. For speech, it is unclear whether rhythm is solely derived from acoustic properties (e.

A checklist for assessing the methodological quality of concurrent tES-fMRI studies (ContES checklist): a consensus study and statement.

Low-intensity transcranial electrical stimulation (tES), including alternating or direct current stimulation, applies weak electrical stimulation to modulate the activity of brain circuits. Integration of tES with concurrent functional MRI (fMRI) allows for the mapping of neural activity during neuromodulation, supporting causal studies of both brain function and tES effects. Methodological aspects of tES-fMRI studies underpin the results, and reporting them in appropriate detail is required for reproducibility and interpretability.

Theta-phase dependent neuronal coding during sequence learning in human single neurons.

The ability to maintain a sequence of items in memory is a fundamental cognitive function. In the rodent hippocampus, the representation of sequentially organized spatial locations is reflected by the phase of action potentials relative to the theta oscillation (phase precession). We investigated whether the timing of neuronal activity relative to the theta brain oscillation also reflects sequence order in the medial temporal lobe of humans.

Human Hippocampal Neurons Track Moments in a Sequence of Events.

An indispensable feature of episodic memory is our ability to temporally piece together different elements of an experience into a coherent memory. Hippocampal time cells-neurons that represent temporal information-may play a critical role in this process. Although these cells have been repeatedly found in rodents, it is still unclear to what extent similar temporal selectivity exists in the human hippocampus.

Visual speech cues recruit neural oscillations to optimise auditory perception: Ways forward for research on human communication.

In pandemic times, when visual speech cues are masked, it becomes particularly evident how much we rely on them to communicate. Recent research points to a key role of neural oscillations for cross-modal predictions during speech perception. This article bridges several fields of research - neural oscillations, cross-modal speech perception and brain stimulation - to propose ways forward for research on human communication.

Sustained neural rhythms reveal endogenous oscillations supporting speech perception.

Rhythmic sensory or electrical stimulation will produce rhythmic brain responses. These rhythmic responses are often interpreted as endogenous neural oscillations aligned (or "entrained") to the stimulus rhythm. However, stimulus-aligned brain responses can also be explained as a sequence of evoked responses, which only appear regular due to the rhythmicity of the stimulus, without necessarily involving underlying neural oscillations.

Auditory Perception: A Rhythm Reflecting the Past.

Previous research has demonstrated that auditory perception fluctuates rhythmically after a cue. New research shows that these 'behavioural oscillations' critically depend on expectations from preceding stimulation.

Perception of Rhythmic Speech Is Modulated by Focal Bilateral Transcranial Alternating Current Stimulation.

Several recent studies have used transcranial alternating current stimulation (tACS) to demonstrate a causal role of neural oscillatory activity in speech processing. In particular, it has been shown that the ability to understand speech in a multi-speaker scenario or background noise depends on the timing of speech presentation relative to simultaneously applied tACS. However, it is possible that tACS did not change actual speech perception but rather auditory stream segregation.

How to test for phasic modulation of neural and behavioural responses.

Research on whether perception or other processes depend on the phase of neural oscillations is rapidly gaining popularity. However, it is unknown which methods are optimally suited to evaluate the hypothesized phase effect. Using a simulation approach, we here test the ability of different methods to detect such an effect on dichotomous (e.

Speech Entrainment: Rhythmic Predictions Carried by Neural Oscillations.

It has been hypothesized that stimulus-aligned brain rhythms reflect predictions about upcoming input. New research shows that these rhythms bias subsequent speech perception, in line with a mechanism of prediction.

The Involvement of Endogenous Neural Oscillations in the Processing of Rhythmic Input: More Than a Regular Repetition of Evoked Neural Responses.

It is undisputed that presenting a rhythmic stimulus leads to a measurable brain response that follows the rhythmic structure of this stimulus. What is still debated, however, is the question whether this brain response exclusively reflects a regular repetition of evoked responses, or whether it also includes entrained oscillatory activity. Here we systematically present evidence in favor of an involvement of entrained neural oscillations in the processing of rhythmic input while critically pointing out which questions still need to be addressed before this evidence could be considered conclusive.

Phase Entrainment of Brain Oscillations Causally Modulates Neural Responses to Intelligible Speech.

Due to their periodic nature, neural oscillations might represent an optimal "tool" for the processing of rhythmic stimulus input [1-3]. Indeed, the alignment of neural oscillations to a rhythmic stimulus, often termed phase entrainment, has been repeatedly demonstrated [4-7]. Phase entrainment is central to current theories of speech processing [8-10] and has been associated with successful speech comprehension [11-17].

Transcranial electric stimulation for the investigation of speech perception and comprehension.

Transcranial electric stimulation (tES), comprising transcranial direct current stimulation (tDCS) and transcranial alternating current stimulation (tACS), involves applying weak electrical current to the scalp, which can be used to modulate membrane potentials and thereby modify neural activity. Critically, behavioural or perceptual consequences of this modulation provide evidence for a role of neural activity in the stimulated brain region for the observed outcome. We present tES as a tool for the investigation of which neural responses are necessary for successful speech perception and comprehension.

Oscillatory Mechanisms of Stimulus Processing and Selection in the Visual and Auditory Systems: State-of-the-Art, Speculations and Suggestions.

All sensory systems need to continuously prioritize and select incoming stimuli in order to avoid overflow or interference, and provide a structure to the brain's input. However, the characteristics of this input differ across sensory systems; therefore, and as a direct consequence, each sensory system might have developed specialized strategies to cope with the continuous stream of incoming information. Neural oscillations are intimately connected with this selection process, as they can be used by the brain to rhythmically amplify or attenuate input and therefore represent an optimal tool for stimulus selection.

Characterization of neural entrainment to speech with and without slow spectral energy fluctuations in laminar recordings in monkey A1.

Neural entrainment, the alignment between neural oscillations and rhythmic stimulation, is omnipresent in current theories of speech processing - nevertheless, the underlying neural mechanisms are still largely unknown. Here, we hypothesized that laminar recordings in non-human primates provide us with important insight into these mechanisms, in particular with respect to processing in cortical layers. We presented one monkey with human everyday speech sounds and recorded neural (as current-source density, CSD) oscillations in primary auditory cortex (A1).

The Role of High-Level Processes for Oscillatory Phase Entrainment to Speech Sound.

Constantly bombarded with input, the brain has the need to filter out relevant information while ignoring the irrelevant rest. A powerful tool may be represented by neural oscillations which entrain their high-excitability phase to important input while their low-excitability phase attenuates irrelevant information. Indeed, the alignment between brain oscillations and speech improves intelligibility and helps dissociating speakers during a "cocktail party".

EEG oscillations entrain their phase to high-level features of speech sound.

Phase entrainment of neural oscillations, the brain's adjustment to rhythmic stimulation, is a central component in recent theories of speech comprehension: the alignment between brain oscillations and speech sound improves speech intelligibility. However, phase entrainment to everyday speech sound could also be explained by oscillations passively following the low-level periodicities (e.g.

The ability of the auditory system to cope with temporal subsampling depends on the hierarchical level of processing.

Evidence for rhythmic or 'discrete' sensory processing is abundant for the visual system, but sparse and inconsistent for the auditory system. Fundamental differences in the nature of visual and auditory inputs might account for this discrepancy: whereas the visual system mainly relies on spatial information, time might be the most important factor for the auditory system. In contrast to vision, temporal subsampling (i.

Selective perceptual phase entrainment to speech rhythm in the absence of spectral energy fluctuations.

Perceptual phase entrainment improves speech intelligibility by phase-locking the brain's high-excitability and low-excitability phases to relevant or irrelevant events in the speech input. However, it remains unclear whether phase entrainment to speech can be explained by a passive "following" of rhythmic changes in sound amplitude and spectral content or whether entrainment entails an active tracking of higher-level cues: in everyday speech, rhythmic fluctuations in low-level and high-level features always covary. Here, we resolve this issue by constructing novel speech/noise stimuli with intelligible speech but without systematic changes in sound amplitude and spectral content.