•Review of methods for studying brain oscillations with MEG/EEG.•Covers experimental approaches and analytical methods.•Focus on novel experimental approaches such as entrainment.•Describes methods ...for identifying relation between brain oscillations and behaviour.
Brain oscillations are increasingly the subject of electrophysiological studies probing their role in the functioning and dysfunction of the human brain. In recent years this research area has seen rapid and significant changes in the experimental approaches and analysis methods. This article reviews these developments and provides a structured overview of experimental approaches, spectral analysis techniques and methods to establish relationships between brain oscillations and behaviour.
Rhythms are a fundamental and defining feature of neuronal activity in animals including humans. This rhythmic brain activity interacts in complex ways with rhythms in the internal and external ...environment through the phenomenon of ‘neuronal entrainment’, which is attracting increasing attention due to its suggested role in a multitude of sensory and cognitive processes. Some senses, such as touch and vision, sample the environment rhythmically, while others, like audition, are faced with mostly rhythmic inputs. Entrainment couples rhythmic brain activity to external and internal rhythmic events, serving fine-grained routing and modulation of external and internal signals across multiple spatial and temporal hierarchies. This interaction between a brain and its environment can be experimentally investigated and even modified by rhythmic sensory stimuli or invasive and non-invasive neuromodulation techniques. We provide a comprehensive overview of the topic and propose a theoretical framework of how neuronal entrainment dynamically structures information from incoming neuronal, bodily and environmental sources. We discuss the different types of neuronal entrainment, the conceptual advances in the field, and converging evidence for general principles.
Lakatos, Gross, and Thut review the evidence for neuronal entrainment by environmental, self-produced, and neuromodulatory rhythms, which leads them to propose a new, unifying account of the role of neuronal entrainment in the selection and structuring of information - taking into account the brain in a wider context.
The human brain can be parcellated into diverse anatomical areas. We investigated whether rhythmic brain activity in these areas is characteristic and can be used for automatic classification. To ...this end, resting-state MEG data of 22 healthy adults was analysed. Power spectra of 1-s long data segments for atlas-defined brain areas were clustered into spectral profiles ("fingerprints"), using k-means and Gaussian mixture (GM) modelling. We demonstrate that individual areas can be identified from these spectral profiles with high accuracy. Our results suggest that each brain area engages in different spectral modes that are characteristic for individual areas. Clustering of brain areas according to similarity of spectral profiles reveals well-known brain networks. Furthermore, we demonstrate task-specific modulations of auditory spectral profiles during auditory processing. These findings have important implications for the classification of regional spectral activity and allow for novel approaches in neuroimaging and neurostimulation in health and disease.
During online speech processing, our brain tracks the acoustic fluctuations in speech at different timescales. Previous research has focused on generic timescales (for example, delta or theta bands) ...that are assumed to map onto linguistic features such as prosody or syllables. However, given the high intersubject variability in speaking patterns, such a generic association between the timescales of brain activity and speech properties can be ambiguous. Here, we analyse speech tracking in source-localised magnetoencephalographic data by directly focusing on timescales extracted from statistical regularities in our speech material. This revealed widespread significant tracking at the timescales of phrases (0.6-1.3 Hz), words (1.8-3 Hz), syllables (2.8-4.8 Hz), and phonemes (8-12.4 Hz). Importantly, when examining its perceptual relevance, we found stronger tracking for correctly comprehended trials in the left premotor (PM) cortex at the phrasal scale as well as in left middle temporal cortex at the word scale. Control analyses using generic bands confirmed that these effects were specific to the speech regularities in our stimuli. Furthermore, we found that the phase at the phrasal timescale coupled to power at beta frequency (13-30 Hz) in motor areas. This cross-frequency coupling presumably reflects top-down temporal prediction in ongoing speech perception. Together, our results reveal specific functional and perceptually relevant roles of distinct tracking and cross-frequency processes along the auditory-motor pathway.
Entropy scaling is an intriguingly simple approach for correlating and predicting transport properties of real substances and mixtures. It is convincingly documented in the literature that entropy ...scaling is indeed a firm concept for the shear viscosity of real substances, including hydrogen bonding species and strongly nonspherical species. We investigate whether entropy scaling is applicable for thermal conductivity. It is shown that the dimensionless thermal conductivity (thermal conductivity divided by a reference thermal conductivity) does not show a single-variable dependence on residual entropy, for obvious choices of a reference thermal conductivity. We perform a detailed analysis of experimental data and propose a reference thermal conductivity that is itself a simple function of the residual entropy. We then obtain good scaling behavior for the entire fluid region for water and 147 organic substances from various chemical families: linear and branched alkanes, alkenes, aldehydes, aromatics, ethers, esters, ketones, alcohols, and acids. The residual entropy is calculated from the Perturbed Chain Polar Statistical Associating Fluid Theory equation of state. The correlation of experimental data requires two parameters for pure substances with scarce experimental data and up to five parameters for experimentally well-characterized species. The correlation results for all substances lead to average relative deviations of 4.2% to experimental data. To further assess the approach, we analyze extrapolations to states not covered by experimental data and find very satisfying results.
The Stokes–Einstein (SE) equation is often applied as an approximation of self-diffusion coefficients D of molecular species based on the shear viscosity of fluids. The SE relation gives rough ...estimates of self-diffusion coefficients in liquid states and (divergently) high deviations for the gaseous phase. Inspired by Rosenfeld’s entropy scaling approach, we find a universal function of residual entropy f(s res) to describe the ratio between experimental and calculated D from SE equation for all investigated substances. The so-obtained modified SE equation, containing 5 universal parameters adjusted to experimental data of 61 substances of 11 different chemical families, can be used for predicting D in the entire fluid region, with averaged absolute deviations of 12.3% compared to experimental data (5407 data points). The application of the proposed model to diffusion coefficients at infinite dilution shows average deviations of 20.8% for widely different systems (412 data points).
Abstract Background Periodic stimulation of occipital areas using transcranial alternating current stimulation (tACS) at alpha ( α ) frequency (8–12 Hz) enhances electroencephalographic (EEG) ...α-oscillation long after tACS-offset. Two mechanisms have been suggested to underlie these changes in oscillatory EEG activity: tACS-induced entrainment of brain oscillations and/or tACS-induced changes in oscillatory circuits by spike-timing dependent plasticity. Objective We tested to what extent plasticity can account for tACS-aftereffects when controlling for entrainment “echoes.” To this end, we used a novel, intermittent tACS protocol and investigated the strength of the aftereffect as a function of phase continuity between successive tACS episodes, as well as the match between stimulation frequency and endogenous α-frequency. Methods 12 healthy participants were stimulated at around individual α-frequency for 11–15 min in four sessions using intermittent tACS or sham. Successive tACS events were either phase-continuous or phase-discontinuous, and either 3 or 8 s long. EEG α-phase and power changes were compared after and between episodes of α-tACS across conditions and against sham. Results α-aftereffects were successfully replicated after intermittent stimulation using 8-s but not 3-s trains. These aftereffects did not reveal any of the characteristics of entrainment echoes in that they were independent of tACS phase-continuity and showed neither prolonged phase alignment nor frequency synchronization to the exact stimulation frequency. Conclusion Our results indicate that plasticity mechanisms are sufficient to explain α-aftereffects in response to α-tACS, and inform models of tACS-induced plasticity in oscillatory circuits. Modifying brain oscillations with tACS holds promise for clinical applications in disorders involving abnormal neural synchrony.
Oscillations in brain activity have long been known, but many fundamental aspects of such brain rhythms, particularly their functional importance, have been unclear. As we review here, new insights ...into these issues are emerging from the application of intervention approaches. In these approaches, the timing of brain oscillations is manipulated by non-invasive brain stimulation, either through sensory input or transcranially, and the behavioural consequence then monitored. Notably, such manipulations have led to rapid, periodic fluctuations in behavioural performance, which co-cycle with underlying brain oscillations. Such findings establish a causal relationship between brain oscillations and behaviour, and are allowing novel tests of longstanding models about the functions of brain oscillations.
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An event in one sensory modality can phase reset brain oscillations concerning another modality 1–5. In principle, this may result in stimulus-locked periodicity in behavioral performance 6. Here we ...considered this possible cross-modal impact of a sound for one of the best-characterized rhythms arising from the visual system, namely occipital alpha-oscillations (8–14 Hz) 7–9. We presented brief sounds and concurrently recorded electroencephalography (EEG) and/or probed visual cortex excitability (phosphene perception) through occipital transcranial magnetic stimulation (TMS). In a first, TMS-only experiment, phosphene perception rate against time postsound showed a periodic pattern cycling at ∼10 Hz phase-aligned to the sound. In a second, combined TMS-EEG experiment, TMS-trials reproduced the cyclical phosphene pattern and revealed a ∼10 Hz pattern also for EEG-derived measures of occipital cortex reactivity to the TMS pulses. Crucially, EEG-data from intermingled trials without TMS established cross-modal phase-locking of occipitoparietal alpha oscillations. These independently recorded variables, i.e., occipital cortex excitability and reactivity and EEG phase dynamics, were significantly correlated. This shows that cross-modal phase locking of oscillatory visual cortex activity can arise in the human brain to affect perceptual and EEG measures of visual processing in a cyclical manner, consistent with occipital alpha oscillations underlying a rapid cycling of neural excitability in visual areas.
► Sound modulates visual perception by phase-locking occipital alpha phase ► Visual perception cocycles with the phase alignment of occipital alpha oscillations ► Alpha rhythm causally shapes perception in a periodic fashion ► Occipital alpha phase indexes excitability of visual areas
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