title: Pure phase-locking of beta/gamma oscillation contributes to the N30 frontal component of somatosensory evoked potentials.
abstract: BACKGROUND
  Evoked potentials have been proposed to result from phase-locking of electroencephalographic (EEG) activities within specific frequency bands. However, the respective contribution of phasic activity and phase resetting of ongoing EEG oscillation remains largely debated. We here applied the EEGlab procedure in order to quantify the contribution of electroencephalographic oscillation in the generation of the frontal N30 component of the somatosensory evoked potentials (SEP) triggered by median nerve electrical stimulation at the wrist. Power spectrum and intertrial coherence analysis were performed on EEG recordings in relation to median nerve stimulation.
  
  
  RESULTS
  The frontal N30 component was accompanied by a significant phase-locking of beta/gamma oscillation (25-35 Hz) and to a lesser extent of 80 Hz oscillation. After the selection in each subject of the trials for which the power spectrum amplitude remained unchanged, we found pure phase-locking of beta/gamma oscillation (25-35 Hz) peaking about 30 ms after the stimulation. Transition across trials from uniform to normal phase distribution revealed temporal phase reorganization of ongoing 30 Hz EEG oscillations in relation to stimulation. In a proportion of trials, this phase-locking was accompanied by a spectral power increase peaking in the 30 Hz frequency band. This corresponds to the complex situation of 'phase-locking with enhancement' in which the distinction between the contribution of phasic neural event versus EEG phase resetting is hazardous.
  
  
  CONCLUSIONS
  The identification of a pure phase-locking in a large proportion of the SEP trials reinforces the contribution of the oscillatory model for the physiological correlates of the frontal N30. This may imply that ongoing EEG rhythms, such as beta/gamma oscillation, are involved in somatosensory information processing.
authors: Laboratory of Neurophysiology and Movement Biomechanics, Université Libre de Bruxelles (ULB), CP 168, 50 Av F Roosevelt, Brussels, Belgium. gcheron@ulb.ac.be
authors: Guy
authors: G
authors: Cheron
authors: Ana Maria
authors: AM
authors: Cebolla
authors: Caty
authors: C
authors: De Saedeleer
authors: Ana
authors: A
authors: Bengoetxea
authors: Françoise
authors: F
authors: Leurs
authors: Axelle
authors: A
authors: Leroy
authors: Bernard
authors: B
authors: Dan
mesh_terms: D000328:Adult; D001611:Beta Rhythm / Q000379:methods*; D001683:Biological Clocks / Q000502:physiology*; D004558:Electric Stimulation / Q000379:methods; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D005625:Frontal Lobe / Q000502:physiology*; D006801:Humans; D008297:Male; D008475:Median Nerve / Q000502:physiology