Oscillatory activity reflects the excitability of the human somatosensory system Markus Ploner, ⁎ Joachim Gross, Lars Timmermann, Bettina Pollok, and Alfons Schnitzler Department of Neurology, Heinrich-Heine-University, Moorenstr. 5, 40225 Düsseldorf, Germany Received 20 March 2006; revised 16 May 2006; accepted 2 June 2006 Available online 18 July 2006 The neuronal activity of the resting human brain is dominated by spontaneous oscillations in primary sensory and motor areas. These oscillations are thought to reflect the excitability of sensory and motor systems that can be modulated according to the actual behavioral demands. However, so far, evidence for an association between oscillatory activity and excitability has been inconsistent. Here, we used magnetoencephalography to reinvestigate the relationship be- tween oscillatory activity and excitability in the somatosensory system on a single trial basis. Brief painful stimuli were applied to relate pain- induced suppressions of oscillatory activity to pain-induced increases in excitability. The analysis reveals a significant negative correlation between sensorimotor oscillatory activity, particularly in the α-band, and excitability of somatosensory cortices. Oscillatory activity outside the somatosensory system did not correlate with somatosensory excitability. These findings demonstrate that modulations of sensor- imotor oscillatory activity specifically reflect modulations in excitability of the somatosensory system and thus provide direct evidence for the basic tenet of an association between oscillatory activity and cortical excitability. © 2006 Elsevier Inc. All rights reserved. Introduction Spontaneous oscillatory activity represents a basic feature of the neuronal activity of the human brain. Particularly, spontaneous oscillations in the α-band (8–13 Hz) and β-band (14–30 Hz) are consistently observed in primary visual, somatosensory and motor cortices (Berger, 1929; Gastaut, 1952; Hari and Salmelin, 1997; Niedermeyer, 2005). These oscillations have been related to the functional state of sensory and motor systems (Hari and Salmelin, 1997; Pfurtscheller and Lopes da Silva, 2005). A higher amplitude of oscillatory activity has been associated with an idling state whereas a lower amplitude may signal activation of a system (Steriade and Llinas, 1988; Hari and Salmelin, 1997; Niedermeyer, 2005; Pfurtscheller and Lopes da Silva, 2005). Furthermore, oscillatory activity is thought to reflect the excitability of thalamocortical systems that can be modulated by exogenous or endogenous events (Steriade and Llinas, 1988). However, experi- mental evidence for this association between oscillatory activity and cortical excitability is sparse and inconsistent. Some studies showed a positive correlation between oscillations and excitability (Brandt et al., 1991; Arieli et al., 1996; Nikouline et al., 2000; Tamura et al., 2005), whereas others revealed a negative correlation (Brandt and Jansen, 1991; Rossini et al., 1991; Rahn and Basar, 1993a,b; Chen et al., 1999) or did not show any significant relationship (Simoes et al., 2004) between oscillations and excitability. Therefore, we reinvestigated the relationship between excit- ability and oscillatory activity – termed mu-rhythm – in the human somatosensory system on a single trial basis. The mu-rhythm comprises two frequency components in the α- and β-band, which can be suppressed by exogenous or endogenous activation of the sensorimotor system. Particularly, painful stimuli have been shown to suppress the mu-rhythm (Mouraux et al., 2003; Ohara et al., 2004; Raij et al., 2004; Ploner et al., 2006) as well as to increase somatosensory excitability (Ploner et al., 2004). Here, we applied brief painful cutaneous laser stimuli in order to relate pain-induced suppressions of the mu-rhythm to pain-induced increases in cortical excitability. Using a conditioning test stimulus paradigm, cortical responses to tactile test stimuli applied 500 ms after the painful conditioning stimuli were used as a measure of soma- tosensory excitability. Our single trial-based analysis reveals an inverse correlation between oscillatory activity, particularly the α- component of the mu-rhythm, and the excitability of primary (S1) and secondary (S2) somatosensory cortices. Thus, these findings provide direct evidence for the basic tenet of an association between oscillatory activity and cortical excitability in humans. Materials and methods Subjects Eight healthy male subjects with a mean age of 31 years (range, 23–45 years) participated in the experiment. Informed consent was obtained from all subjects before participation. The study was www.elsevier.com/locate/ynimg NeuroImage 32 (2006) 1231 – 1236 ⁎ Corresponding author. Fax: +49 211 811 9033. E-mail address: ploner@neurologie.uni-duesseldorf.de (M. Ploner). Available online on ScienceDirect (www.sciencedirect.com). 1053-8119/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.neuroimage.2006.06.004