title: Subcortical substrates of TMS induced modulation of the cortico-cortical connectivity.
abstract: BACKGROUND
  Transcranial magnetic stimulation (TMS) can modulate transiently the physiological brain oscillations, e.g. the alpha rhythm. It has been hypothesized that this effect is not limited to the stimulated region but involves subcortical and distant cortical areas.
  
  
  METHODS
  We applied single pulse TMS to the primary motor cortex (M1) of healthy subjects to interfere the cortical oscillatory activity recorded by simultaneous EEG and calculated the cortico-cortical coherence and power in the alpha and beta band. To study the structural substrate of the functional connectivity we performed diffusion tensor imaging and fractional anisotropy analysis (FA). To capture the pathways involved we applied probabilistic tractography to reconstruct the entire network.
  
  
  RESULTS
  Suprathreshold TMS of M1 induced a consistent enhancement of interhemispheric cortico-cortical alpha band coherence that lasted ca. 175 ms. after the pulse has been applied. The changes were confined to the interhemispheric central EEG electrodes (i.e. C3-C4). There were no consistent changes in the beta band. Power analysis revealed a longer lasting increase in the beta band after TMS pulses. A cluster in the contralateral thalamus showed a linear relationship between regional FA and TMS induced change in alpha band coherence. Probabilistic tractography presents the transcallosal and the contralateral thalamocortical pathways as essential for the observed oscillatory synchronisation.
  
  
  CONCLUSIONS
  TMS induces an enhancement of oscillatory interaction between corresponding central regions of both hemispheres in the alpha band. The contralateral thalamus, transcallosal fibres and the contralateral thalamocortical pathways may constitute critical brain structures mediating the TMS induced change in oscillatory coupling.
authors.affiliation: Department of Neurology, Christian-Albrechts University, Schittenhelmstr 10, Kiel 24105, Germany. s.groppa@neurologie.uni-kiel.de
authors.forename: Sergiu
authors.initials: S
authors.lastname: Groppa
authors.forename: Muthuraman
authors.initials: M
authors.lastname: Muthuraman
authors.forename: Birte
authors.initials: B
authors.lastname: Otto
authors.forename: Günther
authors.initials: G
authors.lastname: Deuschl
authors.forename: Hartwig R
authors.initials: HR
authors.lastname: Siebner
authors.forename: Jan
authors.initials: J
authors.lastname: Raethjen
mesh_terms: D000328:Adult; D000513:Alpha Rhythm / Q000502:physiology; D001611:Beta Rhythm / Q000502:physiology; D001931:Brain Mapping; D003344:Cortical Synchronization / Q000502:physiology; D056324:Diffusion Tensor Imaging; D004569:Electroencephalography; D019054:Evoked Potentials, Motor / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D009415:Nerve Net / Q000502:physiology*; D013788:Thalamus / Q000502:physiology*; D050781:Transcranial Magnetic Stimulation