Modulating Neural Networks With Transcranial Magnetic Stimulation Applied Over the Dorsal Premotor and Primary Motor Cortices

Abstract

. Modulating neural networks with transcranial magnetic stimulation applied over the dorsal premotor and primary motor cortices. J Neurophysiol 90: 1071-1083, 2003; 10.1152/jn.01105.2002. Our study uses the combined transcranial magnetic stimulation/positron emission tomography (TMS/PET) method for elucidating neural connectivity of the human motor system. We first altered motor excitability by applying low-frequency repetitive TMS over two cortical motor regions in separate experiments: the dorsal premotor and primary motor cortices. We then assessed the consequences of modulating motor excitability by applying single-pulse TMS over the primary motor cortex and measuring: 1) muscle responses with electromyography and 2) cerebral blood flow with PET. Low-frequency repetitive stimulation reduced muscle responses to a similar degree in both experiments. To map networks of brain regions in which activity changes reflected modulation of motor excitability, we generated t-statistical maps of correlations between reductions in muscle response and differences in cerebral blood flow. Low-frequency repetitive stimulation altered neural activity differently in both experiments. Neural modulation occurred in multiple brain regions after dorsal premotor cortex stimulation; these included motor regions in the frontal cortex as well as more associational regions in the parietal and prefrontal cortices. In contrast, neural modulation occurred in a smaller number of brain regions after primary motor cortex stimulation, many of these confined to the motor system. These findings are consistent with the known differences between the dorsal premotor and primary motor cortices in the extent of cortico-cortical anatomical connectivity in the monkey. I N T R O D U C T I O NThe cortical motor system can be separated into the primary motor and the nonprimary motor areas. The nonprimary motor areas are defined as all regions in the frontal lobe that have the potential to influence motor output at the level of both the primary motor cortex and the spinal cord (Dum and Strick 1991); these include the premotor, supplementary motor, and cingulate motor areas. Transcranial magnetic stimulation (TMS) applied in trains of pulses can modulate the motor system in a temporary fashion, lasting beyond the duration of stimulation. Studies that have examined these effects generally applied repetitive stimulation over the primary motor cortex and measured the modulation of motor-evoked potentials (MEPs) recorded in the contralateral hand muscles. Typically, low-stimulation frequencies of 1 to 2 Hz induce inhibitory effects (e. Recent studies demonstrate that low-frequency repetitive TMS applied over the premotor cortex can also induce changes in motor excitability as reflected by: 1) decreases in the amplitude of MEPs elicited by single-pulse stimuli (Gerschlager et al. 2001); 2) increases in intracortical facilitation to pairedpulse stimuli (Münchau et al. 2002); and 3) reductions in the duration of the silent period (Münchau et al. 2002). These resu...

Time-course of “off-line” prefrontal rTMS effects — a PET study

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“…Compensatory mechanisms involving the recruitment of other parts of the motor system could, however, have masked interference with response execution (Lee et al, 2003). Indeed, coactivation and/or inhibition of topographically and functionally related regions, such as the dorsal premotor cortex (PMd), supplementary motor area (SMA), cingulate motor cortex, thalamus, putamen, and contralateral M1 (Chouinard et al, 2003;Bestmann et al, 2004), are possible because TMS induces effects proximal and distal to the site of stimulation (Hallett, 2000).…”

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