Shailesh S. Kantak, Beth E. Fisher, Katherine J. Sullivan and Carolee J. Winstein
Low frequency (1Hz) repetitive transcranial magnetic simulation (TMS) is known to reduce motor corticospinal excitability. The purpose of this study was to systematically investigate the effects of different intensities and durations of LF-rTMS on measures of motor cortical excitability and inhibition, while trying to minimize sources of variability that affect corticospinal excitability. 9 non-disabled young adults were recruited and screened for contraindications to TMS. We employed a repeated measures design to investigate the effect of the following four intensity-duration combinations (doses) on motor corticospinal excitability: 1) subthreshold intensity (90%Resting Motor Threshold (RMT)) for 10 minutes, 2) subthreshold intensity (90%RMT) for 20 minutes, 3) suprathreshold intensity (110%RMT) for 10 minutes and 4) suprathreshold intensity (110%RMT) for 20 minutes. Each rTMS dose was administered at 4 different sessions separated by at least 7 days. Changes in the motor corticospinal excitability and inhibition were measured using 1) MEP amplitude evoked by 120% RMT at rest and during active contraction and 2) cortical silent period. 1Hz rTMS applied at suprathreshold intensity (110% RMT) reduced corticospinal excitability at rest, irrespective of the duration of stimulation. In contrast, subthresold 1Hz rTMS signi fi cantly decreased corticospinal excitability at rest only when applied for a longer duration (20 min compared to 10 min).Subthreshold rTMS when applied for 10 min induced cortical inhibition as evidenced by a signi fi cant lengthening of the silent period. Down regulation of corticospinal excitability is dose-dependent with supra-threshold 1Hz rTMS more effective, even with a shorter duration compared with a longer duration of stimulation. Further, our study while not con fi rmatory, suggests that different doses of 1Hz rTMS may affect excitatory and inhibitory circuits differently within the motor cortex.
