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The effect of interstimulus intervals on the outputs of nTMS motor mapping
Magnetic resonance imaging navigated transcranial magnetic stimulation (nTMS) is a method of non-invasive brain stimulation that allows precise cortical motor mapping. One of the parameters that can affect the results of nTMS stimulation is the interval between the two consecutive stimuli (the inter-stimulus interval, ISI). However, in nTMS mapping studies, researchers quite rarely report the ISIs values. The study aimed to test the influence of ISIs on the output of nTMS motor mapping.
We analysed data from the ongoing nTMS study, during which the motor cortex was mapped before and after finger dexterity training. The study involved 26 healthy right-handed male volunteers (18-35 y.o., M(SD) = 23.32(4.30)). Two nТMS mapping sessions were separated by two weeks. Each session included two sub-sessions in which the same cortical targets were stimulated. Motor evoked potentials (MEPs) were obtained from upper limb muscles: three hand muscles and three forearm muscles. Since the nTMS mapping was performed by a human operator, the ISIs were calculated offline after the mapping.
Statistical analysis was performed using linear regression models in R. We found no significant interaction between the ISIs values and the amplitudes of MEPs recorded from the abductor pollicis brevis (APB) muscle, nor between the ISIs and the motor cortical representations (MCRs) of all the studied muscles. However, analysis of the individual sub-sessions showed a positive relationship between the ISIs and the MCRs areas of the two hand muscles during the first sub-session (t = 2.32, p<.05; t = 2.08, p<.05) and five muscles in the fourth sub-session (intrinsic hand muscles: t = 2.3, p<.05; t = 3.41, p<.01; forearm muscles: t = 3.34, p<.01; t = 2.87, p<.01; t = 2.3, p<.05). There was also a significant relationship between the number of stimuli per session and the median ISIs for the fourth sub-session (t = 3.39, p<.01).
According to these results, ISIs did not significantly affect the amplitude of the MEPs. The interrelation between the ISIs and MCRs areas during the fourth sub-session was caused by the positive correlation of the ISIs and the number of stimuli. We suggest TMS researchers additionally control differences in the procedures of repeated sessions because the resulting significant differences between sessions may be related to technical mapping parameters. Although ISIs do not directly affect MCRs areas and MEPs, they can be a marker for differences in mapping procedures across sessions.
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