Enhancement of long-range EEG coherence by synchronous bifocal transcranial magnetic stimulation

Interregional coupling of distant brain regions can be measured by electroencephalographic (EEG) coherence reflecting the spatial-temporal correlation between two oscillatory signals. It has been suggested that this coherence in activity is a signature of functional integration of multimodal neuronal networks. Repetitive transcranial magnetic stimulation (rTMS) is a well-established technique for non-invasive cortical stimulation. Its modulating effects outlast the train of stimulation and affect behavior. In the present study, we tested the hypothesis that cortico-cortical coherence between distant brain areas can be selectively enhanced by synchronous bifocal rTMS. Cortico-cortical coherence was assessed in 16 healthy human subjects before and after three trains of synchronous high-frequency (10 Hz) rTMS to the left primary motor cortex and the visual cortex at the occipital pole simultaneously. Stimulation of the left M1 alone served as the control condition. Coherence and spectral power were measured between these areas on the stimulated and the homologue contralateral side. Synchronous bifocal rTMS induced an increase of interregional coupling in the alpha and lower beta band on the stimulated side without effects on spectral power. These data indicate that synchronous bifocal rTMS is a feasible technique for selective modulation of interregional EEG coherence. Furthermore, they raise the hypothesis that interventional enhancement of long-range coherence may effectively modulate interregional integration with behavioral consequences.

Spontaneous locally restricted EEG alpha activity determines cortical excitability in the motor cortex

Friedhelm Christoph Hummel

There is growing interest in the functional meaning of rhythmical brain activity. For oscillatory brain activity around 10 Hz in the human electroencephalogram (EEG) it is discussed whether it is associated with the level of cortical excitation. However, it is not clear whether the relation between 10 Hz EEG oscillatory activity and cortical excitability is a global, locally very unspecific phenomenon or whether focal 10 Hz oscillations in the human brain are a highly specific correlate of the cortical excitation level. To determine this open question, multichannel EEG was combined with transcranial magnetic stimulation (TMS) applied to the primary motor cortex in this study. The present data showed that a motor evoked potential was elicited more easily when alpha power immediately preceding the magnetic pulse was low, and vice versa. Interestingly, this effect was only found for very local EEG alpha activity at sites overlying the cortical motor areas to which the TMS pulses were applied. This was verified using source localization in 3D space. These data provide evidence that the magnitude of motor cortical excitability is determined by the amount of topographically specific alpha oscillations in the sensorimotor cortex.

Elsevier2009

Inhibitory control of acquired motor programmes in the human brain

Friedhelm Christoph Hummel

An important basis of skilled human behaviour is the appropriate retrieval of acquired and memorized motor programmes ('motor memory traces'). Appropriate retrieval is warranted if motor programmes are only activated if necessary and are, probably more often, inhibited if required by the context of a given situation. It is unknown how this type of inhibition is accomplished in the brain. We studied context-dependent modulation of motor memory traces in 18 volunteers and six patients with focal dystonia. Cortical function was assessed with transcranial magnetic stimulation over the primary motor cortex (M1) and with task-related analysis of oscillatory EEG activity. An activation (ACT) and inhibition (INH) condition were compared. In both, visual cues were presented at 1/s. In ACT, subjects had to respond to these cues with individual finger movements as learned in a preceding training session. In INH, subjects had to observe the cues without retrieval of motor responses. During INH, inhibitory control of the motor memory trace was confirmed by significant amplitude reduction of motor evoked potentials (MEPs) compared with baseline. This was accompanied by a significant increase of 11-13 Hz oscillatory activity over the sensorimotor areas during INH. During active retrieval of the motor memory traces, the reverse was true (increased MEP amplitudes, decreased oscillatory 11-13 Hz activity). In a small sample of dystonic patients (n = 6), the increase of 11-13 Hz oscillatory activity during INH was consistently absent. The present data demonstrate for the first time cortical correlates of appropriate, context-dependent inhibition of motor memory traces. We propose that focal increases of oscillatory activity are instrumental for inhibitory control at the cortical level. This concept is supported by the preliminary observations in dystonic patients who are known to have deficits of inhibitory motor control and in whom these context-dependent focal increases of oscillatory activity were absent.

Oxford University Press2002

Walking or talking? Behavioral and neurophysiological correlates of action verb processing

Friedhelm Christoph Hummel

Brain activity elicited by visually presented words was investigated using behavioral measures and current source densities calculated from high-resolution EEG recordings. Verbs referring to actions usually performed with different body parts were compared. Behavioral data indicated faster processing of verbs referring to actions performed with the face muscles and articulators (face-related words) compared to verbs referring to movements involving the lower half of the body (leg-related words). Significant topographical differences in brain activity elicited by verb types were found starting approximately 250 ms after word onset. Differences were seen at recording sites located over the motor strip and adjacent frontal cortex. At the vertex, close to the cortical representation of the leg, leg-related verbs (for example, to walk) produced strongest in-going currents, whereas for face-related verbs (for example, to talk) the most in-going activity was seen at more lateral electrodes placed over the left Sylvian fissure, close to the representation of the articulators. Thus, action words caused differential activation along the motor strip, with strongest in-going activity occurring close to the cortical representation of the body parts primarily used for carrying out the actions the verbs refer to. Topographically specific physiological signs of word processing started earlier for face-related words and lasted longer for verbs referring to leg movements. We conclude that verb types can differ in their processing speed and can elicit neurophysiological activity with different cortical topographies. These behavioral and physiological differences can be related to cognitive processes, in particular to lexical semantic access. Our results are consistent with associative theories postulating that words are organized in the brain as distributed cell assemblies whose cortical distributions reflect the words' meanings.

Elsevier2001

Inhibitory control of acquired motor programmes in the human brain

Friedhelm Christoph Hummel

Oxford University Press2002

Walking or talking? Behavioral and neurophysiological correlates of action verb processing

Friedhelm Christoph Hummel

Elsevier2001