Intensity modulation of TMS-induced cortical excitation: Primary motor cortex

Peter T. Fox, Shalini Narayana, Nitin Tandon, Sarabeth P. Fox, Hugo Sandoval, Peter Kochunov, Charles Capaday, Jack L. Lancaster

Research output: Contribution to journalArticlepeer-review

44 Scopus citations


The intensity dependence of the local and remote effects of transcranial magnetic stimulation (TMS) on human motor cortex was characterized using positron-emission tomography (PET) measurements of regional blood flow (BF) and concurrent electromyographic (EMG) measurements of the motor-evoked potential (MEP). Twelve normal volunteers were studied by applying 3 Hz TMS to the hand region of primary motor cortex (M1hand). Three stimulation intensities were used: 75%, 100%, and 125% of the motor threshold (MT). MEP amplitude increased nonlinearly with increasing stimulus intensity. The rate of rise in MEP amplitude was greater above MT than below. The hemodynamic response in M1hand was an increase in BF. Hemodynamic variables quantified for M1hand included value-normalized counts (VNC), intensity (z-score), and extent (mm3). All three hemodynamic response variables increased nonlinearly with stimulus intensity, closely mirroring the MEP intensity-response function. VNC was the hemodynamic response variable which showed the most significant effect of TMS intensity. VNC correlated strongly with MEP amplitude, both within and between subjects. Remote regions showed varying patterns of intensity response, which we interpret as reflecting varying levels of neuronal excitability and/or functional coupling in the conditions studied.

Original languageEnglish (US)
Pages (from-to)478-487
Number of pages10
JournalHuman Brain Mapping
Issue number6
StatePublished - Jun 2006


  • BF
  • Blood flow
  • EMG
  • M1
  • MEP
  • PET
  • Positron emission tomography
  • Primary motor cortex
  • TMS
  • Transcranial magnetic stimulation

ASJC Scopus subject areas

  • Anatomy
  • Radiological and Ultrasound Technology
  • Radiology Nuclear Medicine and imaging
  • Neurology
  • Clinical Neurology


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