Abstract
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.
Highlights
Low Frequency (1 Hz) repetitive transcranial magnetic stimulation (LFrTMS/ 1Hz repetitive TMS (rTMS)) is a useful method to study brain-behavior relationships by modulating cortical excitability
Out of the four 1Hz rTMS doses, three were shown to significantly downregulate resting motor cortical excitabilty (Figure 2). 1 Hz rTMS at 90%MT for 20 min (t= 4.324, p= 0.003), 110%MT for 10 (t= 6.274, p< 0.001) and 20 minutes (t= 3.352, p= 0.012) led to significant reduction in the resting Motor Evoked Potential (MEP) amplitude compared to baseline
There was no significant difference in mean MEP amplitude reduction between the three doses (p=0.721). 1Hz rTMS at 90%MT applied over M1 for 10 min did not significantly affect MEP amplitude compared to baseline (t= 1.362, p= 0.21)
Summary
Low Frequency (1 Hz) repetitive transcranial magnetic stimulation (LFrTMS/ 1Hz rTMS) is a useful method to study brain-behavior relationships by modulating cortical excitability. The suppressive effect of 1 Hz rTMS shows considerable variability across studies. Many studies have failed to show any modulation in corticospinal excitability with LFrTMS while in other studies, the reported decrease ranges in magnitude from 16% to 30% [9,11,12] and in durations from 10 minutes to 1 hour [13,14]. There are a number of factors that contribute to the observed variability in response to LFrTMS across studies. Maeda et al [11] reported considerable inter-individual variability in the modulation of cortical responses to rTMS, [18]. One group showed a suppression of corticospinal excitability with 1Hz rTMS and an increase in corticospinal excitability after 20 Hz rTMS; while the other group showed the exact opposite pattern of modulation (increase in cortical excitability after 1Hz rTMS and decrease in cortical excitability after 20 Hz rTMS) [19]
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