Abstract
Question In TMS, the intertrial-interval (ITI) affects the MEPs when stimulation is repeated with the same intensity ( Julkunen et al., 2012 ). Presumably, an ITI longer than 3s has no effects on the MEPs ( Julkunen et al., 2012 , Rossini et al., 1994 ). Thereby, the resting motor threshold (rMT) commonly used to normalize the intensity is calculated by using an ITI of at least 3s. The aim of the present study was to further evaluate, how the applied ITI affects the rMT and MEP amplitudes, and whether the rMT could be determined with short ITIs. Materials & methods The left motor cortices of 11 subjects (7 females, 22-60 years) were mapped to find the optimal representation site for APB muscle at which the rMTguess was calculated by using 3 pulses: rMTguess = ( SI + 8.83 e − 3 ∗ ( A - 50 ) ) / ( 1 + 0.363 e − 3 ∗ ( A - 50 ) ) where SI is the applied intensity and A is the median amplitude of the 3 pulses in microvolts. The formula was based on data of our previous study ( Julkunen et al., 2012 ). Thereafter, 3 single-pulse TMS sequences with varying ITI (1-2s, 2-3s, 3-4s) were applied in a random order with the SIs of 0.75 to 1.25 times rMTguess at 0.05 intervals (total 108 pulses/sequence). The rMTs were calculated from the constructed threshold curves ( Julkunen et al., 2012 ). The effect of ITI on the resulting rMT and on the MEP amplitudes was evaluated with repeated measures ANOVA. Further, the minimum number of trials required for calculating the rMT with each ITI was tested by Monte-Carlo simulations. Results The ITI had no effect on the rMT (F = 0.235, p = 0.683) nor MEP amplitudes (F = 0.90, p = 0.405), but a significant subject by ITI interaction (F = 3.64; p Conclusion At the group-level, the ITI seems to have no clear effect on the rMT or MEP amplitudes. At the individual-level the ITI affects the MEP amplitudes differently between subjects when applying variable stimulation intensity and thus, rMT cannot be reliably estimated with short ITIs. Download : Download high-res image (374KB) Download : Download full-size image
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