title: Somatosensory evoked magnetic fields: relation to pre-stimulus mu rhythm.
abstract: OBJECTIVE
  Brain responses to auditory and visual stimuli have been previously shown to depend on the level of spontaneous brain activity in the 8-13 Hz range. Our aim was to determine whether somatosensory evoked responses are influenced by ongoing rhythmic activity in the 8-13 Hz frequency range originating in the sensorimotor cortex (mu rhythm).
  
  
  METHODS
  We used a whole-head 122 channel magnetoencephalography (MEG) system to record somatosensory evoked fields (SEFs) in response to median nerve stimulation in 11 subjects. Spontaneous oscillations in the 8-13 Hz band over the contralateral sensorimotor cortex were evaluated in 3 different pre-stimulus time intervals using wavelet analysis.
  
  
  RESULTS
  The N20m SEF deflection did not depend on pre-stimulus activity, while the amplitude of the P35m deflection, and to a lesser extent that of the P60m deflection, showed a small positive correlation with the amplitude of the pre-stimulus mu rhythm. Although the amplitude of the mu rhythm varied by a factor of 2.3-5, the maximum variations in P35m and P60m amplitude were only 21 and 12%, respectively. The latencies of the peaks were not affected by the strength of the pre-stimulus mu rhythm.
  
  
  CONCLUSIONS
  It appears that the first excitatory cortical response (N20m) is independent of the oscillatory state (8-13 Hz frequency range) of the sensorimotor cortex. Later parts of the response (P35m and P60m) are also relatively stable compared with the large variations in mu rhythm.
authors.affiliation: BioMag Laboratory, Medical Engineering Centre, Helsinki University Central Hospital, Helsinki, Finland. vadim@biomag.helsinki.fi
authors.forename: V V
authors.initials: VV
authors.lastname: Nikouline
authors.forename: H
authors.initials: H
authors.lastname: Wikström
authors.forename: K
authors.initials: K
authors.lastname: Linkenkaer-Hansen
authors.forename: M
authors.initials: M
authors.lastname: Kesäniemi
authors.forename: R J
authors.initials: RJ
authors.lastname: Ilmoniemi
authors.forename: J
authors.initials: J
authors.lastname: Huttunen
mesh_terms: D000328:Adult; D001683:Biological Clocks / Q000502:physiology; D001931:Brain Mapping; D004558:Electric Stimulation; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D006801:Humans; D015225:Magnetoencephalography; D008297:Male; D008475:Median Nerve / Q000502:physiology; D008875:Middle Aged; D010507:Periodicity; D011930:Reaction Time / Q000502:physiology; D013003:Somatosensory Cortex / Q000502:physiology*