title: Intensity modulation of TMS-induced cortical excitation: primary motor cortex.
abstract: 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 (M1(hand)). 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 M1(hand) was an increase in BF. Hemodynamic variables quantified for M1(hand) included value-normalized counts (VNC), intensity (z-score), and extent (mm(3)). 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.
authors.affiliation: Research Imaging Center, University of Texas Health Science Center at San Antonio, 78284, USA. HBMPTF@uthscsa.edu
authors.forename: Peter T
authors.initials: PT
authors.lastname: Fox
authors.forename: Shalini
authors.initials: S
authors.lastname: Narayana
authors.forename: Nitin
authors.initials: N
authors.lastname: Tandon
authors.forename: Sarabeth P
authors.initials: SP
authors.lastname: Fox
authors.forename: Hugo
authors.initials: H
authors.lastname: Sandoval
authors.forename: Peter
authors.initials: P
authors.lastname: Kochunov
authors.forename: Charles
authors.initials: C
authors.lastname: Capaday
authors.forename: Jack L
authors.initials: JL
authors.lastname: Lancaster
mesh_terms: D000328:Adult; D001931:Brain Mapping / Q000379:methods*; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009044:Motor Cortex / Q000098:blood supply / Q000000981:diagnostic imaging / Q000502:physiology*; D049268:Positron-Emission Tomography; D050781:Transcranial Magnetic Stimulation