Trends in Cognitive Sciences
ReviewNew insights into rhythmic brain activity from TMS–EEG studies
Section snippets
Basic organizing principles of brain rhythms: a brief introduction
Rhythmic activity is a fundamental property of neural elements and is organized in complex patterns depending on the state of the brain (e.g. sleep or awake) and on the task that is currently being executed. It can be recorded non-invasively from the scalp of human participants by electroencephalography (EEG) or magnetoencephalography (MEG) and many of the oscillatory components of EEG and MEG signals have been attributed a role in a variety of brain operations, including aspects of perception,
Analogies between EEG-, MEG- and TMS-research on rhythmic brain activity: added value of combining these techniques
There is renewed interest in ongoing oscillatory brain activity in the absence of any stimulus input or motor output as an index of the internal state of the brain, and linked to it, the extent to which features of this ongoing activity have predictive power for subsequent sensory experience or cognitive processes (e.g. see Ref. [13]). This interest in how the internal state of the brain shapes forthcoming perception and cognition transpires many fields in neuroscience (for a recent review see
Do specific frequencies reflect specific functions? α-oscillations and the regulation of cortical excitability versus inhibition
Recent EEG and MEG research has identified posterior α-oscillations, recorded before a visual event, as a predictor of the perceptual fate of the stimulus 8, 15, 16, 17. More specifically, the amplitude of α-oscillations over occipito-parietal sites is inversely related to perception of the forthcoming visual event 8, 15, 16, 17 (Figure 1a) with enhancement of α-power being observed under conditions requiring suppression of task-irrelevant visual information 32, 33, 34, 35. Based on these
Topography of brain-rhythms: are α-oscillations intrinsic to specific brain regions?
If specific frequencies have specific functions, they should show some degree of topographic specificity. Over the scalp, the cerebral α-rhythm is predominant over posterior and rolandic sites (over rolandic sites alongside β-oscillations). It is modulated by sensory input or motor output and therefore thought to reflect the spontaneous rhythm of sensory and sensory-motor areas. In line with this view, it has been found to originate in calcarine, occipito-parietal and somatosensory cortex 45, 46
Embedding in the cortical network: cortico-cortical interactions in the modulation of α-activity
If the α-rhythm reflects oscillations of neuronal elements close to signal input- and output-stages as reviewed earlier, it should be adjustable through top-down control from higher-order areas involved in attention and movement control and thus depend on the integrity of these areas. Here, the combined TMS–EEG approach has provided new information by virtue of TMS over higher-order areas and the study of EEG-changes at remote, anatomically connected sites (akin to analogous TMS-studies on
Rhythmic activity: epiphenomenal or causal manifestations of brain function?
Do brain rhythms have a causal functional role or merely represent epiphenomenal manifestations of the processes underlying perception, cognition and action? Historically, the α-rhythm was considered to reflect a cortical idling state, and its prominent suppression during visual and motor tasks [45] a valuable (but epiphenomenal) manifestation of cortical activation. Today, there is a growing body of evidence for α-enhancement beyond baseline (idling) levels, speaking against this
Rhythmic entrainment: the interest of biasing function in desired directions
Could the previously mentioned findings lead to new implementations of rhythmic TMS as a tool in therapy and neurorehabilitation? Several investigations indeed indicate that interacting with cortical activity, by means of rhythmic transcranial stimulation, can positively influence cognitive performance of patients affected by disorders such as aphasia, unilateral neglect or dementia [72]. The modification of cortical activity for an adequate period of time through the use of rhythmic
Concluding remarks
We have reviewed the evolving field of TMS–EEG research from which new insights into brain rhythms emerge. We have shown that the TMS–EEG combination is particularly promising and well suited to address several open questions on the generation and functional role of brain oscillations. This field has provided so far novel information mostly on the α-rhythm, namely on its role in perceptually relevant tuning of occipito-parietal areas, on its origin of generation in sensory-motor regions and
Acknowledgements
We thank Joachim Gross, Jan-Mathijs Schoffelen and three anonymous reviewers for comments, and Stefano Bonezzi for preparing Figure I in Box 1.
Glossary
- Electroencephalography (EEG) and magnetoencephalography (MEG)
- non-invasive electrophysiological recording techniques that sample ongoing electrical brain activity, which oscillates at various frequencies, through electrodes or sensors placed over the whole scalp.
- Frequency bands used to classify brain oscillations
- δ: delta (0.5–4Hz), θ: theta (4–8Hz), α: alpha (8–14Hz), β: beta (14–30Hz), γ: gamma (30–100Hz), fast (100–200Hz) and ultra fast (200–600Hz).
- Transcranial magnetic stimulation (TMS) or
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