Abstract
Transcranial magnetic stimulation (TMS) with simultaneous electroencephalography applied to the primary motor cortex provides two parameters for cortical excitability: motor evoked potentials (MEPs) and TMS-evoked potentials (TEPs). This study aimed to evaluate the effects of systematic coil shifts on both the TEP N100 component and MEPs in addition to the relationship between both parameters. In 12 healthy adults, the center of a standardized grid was fixed above the hot spot of the target muscle of the left primary motor cortex. Twelve additional positions were arranged in a quadratic grid with positions between 5 and 10 mm from the hot spot. At each of the 13 positions, TMS single pulses were applied. The topographical maximum of the resulting N100 was located ipsilateral and slightly posterior to the stimulation site. A source analysis revealed an equivalent dipole localized more deeply than standard motor cortex coordinates that could not be explained by a single seeded primary motor cortex dipole. The N100 topography might not only reflect primary motor cortex activation, but also sum activation of the surrounding cortex. N100 amplitude and latency decreased significantly during stimulation anterior-medial to the hot spot although MEP amplitudes were smaller at all other stimulation sites. Therefore, N100 amplitudes might be suitable for detecting differences in local cortical excitability. The N100 topography, with its maximum located posterior to the stimulation site, possibly depends on both anatomical characteristics of the stimulated cortex and differences in local excitability of surrounding cortical areas. The less excitable anterior cortex might contribute to a more posterior maximum. There was no correlation between N100 and MEP amplitudes, but a single-trial analysis revealed a trend toward larger N100 amplitudes in trials with larger MEPs. Thus, functionally efficient cortical excitation might increase the probability of higher N100 amplitudes, but TEPs are also generated in the absence of MEPs.
Highlights
Transcranial magnetic stimulation (TMS) is a noninvasive technique to examine excitability in the brain
The N100 is considered a stable indicator of cortical inhibition, which is modulated through the activity of the inhibitory neurotransmitter GABA-B, causing the characteristic long N100 latency (Nikulin et al, 2003; Premoli et al, 2014)
These findings suggest the N100 is an indicator of local excitability, Du et al (2017) suggest that the N100 instead reflects a global neuronal response because stimulation of various cortical areas leads to similar N100 topographies at the vertex
Summary
Transcranial magnetic stimulation (TMS) is a noninvasive technique to examine excitability in the brain. The N100 has been evoked in various cortical areas (e.g., motor cortex, prefrontal cortex, and parietal cortex; Lioumis et al, 2009; Casarotto et al, 2010; Kerwin et al, 2018), suggesting that it is generated by local stimulated cortex with topographical maxima ipsilateral to the stimulation site (Bender et al, 2005; Bonato et al, 2006). An important step is to, investigate the cortical generators of TEPs and, in particular, how stimulation position affects topography and amplitude of the N100 component These questions can be optimally addressed by applying TMS-EEG within the motor cortex, allowing MEPs to provide additional information on the magnitude of motor cortex stimulation (Julkunen et al, 2009)
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