Research Article EEG Oscillatory Phase-Dependent Markers of Corticospinal Excitability in the Resting Brain Barbara Berger, 1 Tamas Minarik, 1 Gianpiero Liuzzi, 2 Friedhelm C. Hummel, 3 and Paul Sauseng 1 1 Brain and Behaviour Research Group, School of Psychology, University of Surrey, Guildford, Surrey, GU2 7XH, UK 2 Clinic for Neurology, Universit¨ atsspital Z¨ urich, Raemistr. 100, 8091 Z¨ urich, Switzerland 3 Brain Imaging and Neurostimulation Lab, Department of Neurology, University Medical Center Hamburg-Eppendorf, Martinistr. 52, 20246 Hamburg, Germany Correspondence should be addressed to Paul Sauseng; p.sauseng@surrey.ac.uk Received 28 February 2014; Revised 12 May 2014; Accepted 14 May 2014; Published 11 June 2014 Academic Editor: Vincenzo Romei Copyright © 2014 Barbara Berger et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Functional meaning of oscillatory brain activity in various frequency bands in the human electroencephalogram (EEG) is increasingly researched. While most research focuses on event-related changes of brain activity in response to external events there is also increasing interest in internal brain states inluencing information processing. Several studies suggest amplitude changes of EEG oscillatory activity selectively inluencing cortical excitability, and more recently it was shown that phase of EEG activity (instantaneous phase) conveys additional meaning. Here we review this ield with many conlicting indings and further investigate whether corticospinal excitability in the resting brain is dependent on a speciic spontaneously occurring brain state relected by amplitude and instantaneous phase of EEG oscillations. We applied single pulse transcranial magnetic stimulation (TMS) over the let sensorimotor cortex, while simultaneously recording ongoing oscillatory activity with EEG. Results indicate that brain oscillations relect rapid, spontaneous luctuations of cortical excitability. Instantaneous phase but not amplitude of oscillations at various frequency bands at stimulation site at the time of TMS-pulse is indicative for brain states associated with diferent levels of excitability (deined by size of the elicited motor evoked potential). hese results are further evidence that ongoing brain oscillations directly inluence neural excitability which puts further emphasis on their role in orchestrating neuronal iring in the brain. 1. Introduction Spontaneously occurring oscillatory electrical activity in the central and the peripheral nervous system relects rhythmic changes in membrane potential. hus, it was suggested that the phase of these oscillatory luctuations of membrane potential is related to neural spiking [1]. Furthermore, the phase of a dominant rhythmical electroencephalographic (EEG) activity recorded in healthy humans is shown to inluence response times to visual stimuli [2, 3]. his leads to the assumption that brain oscillations relect fast cycles of cortical excitability changes and highlights the importance of spontaneous cortical oscillatory activity for stimulus percep- tion and signal transmission. Much of the past research in the area of human cortical oscillations looked at event-related amplitude decrease and increase. It has been shown that cortical deactivation and activation in various cognitive tasks involving diferent sen- sory modalities are selectively indicated by such amplitude changes (for an overview see [4]). hese parameters have only a temporal precision of a few hundred milliseconds though, but as cortical neuronal communication happens in the range of only a few milliseconds EEG amplitude seems to rather relect sustained activation and deactivation patterns [5]. To overcome this temporal restriction researchers have started to analyse instantaneous phase of brain oscillations in association with neural activity (e.g., [6]). Using EEG, the relation of ongoing oscillatory activity in the human brain in Hindawi Publishing Corporation BioMed Research International Volume 2014, Article ID 936096, 8 pages http://dx.doi.org/10.1155/2014/936096