Abstract
Increasing sensitivity of modern evaluation tools allows for the study of weaker electric stimulation effects on neural populations. In the current study we examined the effects of sham continuous theta burst (cTBS) transcranial magnetic stimulation to the left dorsolateral prefrontal cortex (DLPFC) upon somatosensory evoked potentials (SEP) and frontal-parietal phase coupling of alpha and beta bands. Sham TMS results in an induced electric field amplitude roughly 5% that of real TMS with a similar spatial extent in cortex. Both real and sham cTBS reduced the amplitude of the frontal P14-N30 SEP and increased local phase coupling in the alpha-beta frequency bands of left frontal cortex. In addition, both sham and real cTBS increased frontal-parietal phase coupling in the alpha-beta bands concomitant with an increase in amplitude of parietal P50-N70 complex. These data suggest that weak electric fields from sham cTBS can affect both local and downstream neuronal circuits, though in a different manner than high strength TMS.
Highlights
In the current study we examined the effects of sham continuous theta burst transcranial magnetic stimulation to the left dorsolateral prefrontal cortex (DLPFC) upon somatosensory evoked potentials (SEP) and frontal-parietal phase coupling of alpha and beta bands
Non-invasive neuromodulation methods rely on effects produced by a broad range of electric field strengths ranging from 100 mV/mm for transcranial magnetic stimulation (TMS; Miranda et al, 2007; Salvador et al, 2011) to only a fraction of 1 mV/mm for transcranial direct current stimulation or transcranial alternating current stimulation
In the present study we investigated how weak electric fields induced by sham TMS over the left dorsolateral prefrontal cortex (DLPFC) exert effects upon local and functionally connected neuronal circuitry using electroencephalographic (EEG) measures of phase connectivity and amplitude of somatosensory evoked potentials (SEPs)
Summary
Non-invasive neuromodulation methods rely on effects produced by a broad range of electric field strengths ranging from 100 mV/mm for transcranial magnetic stimulation (TMS; Miranda et al, 2007; Salvador et al, 2011) to only a fraction of 1 mV/mm for transcranial direct current stimulation or transcranial alternating current stimulation (tDCS/tACS; Datta et al, 2009; Salvador et al, 2010; Miranda et al, 2013). CTBS typically operates at 70% of individual motor threshold which corresponds to electric field strengths in the range of 50–80 mV/mm. This range of electric field strength is approximately two orders of magnitude higher than what is minimally needed to cause an effect on neurons (Francis et al, 2003). The effects of weak electric fields induced by TMS have not been extensively studied These issues are especially important for TMS studies applying sham protocols. While it is generally assumed that these sham coils have no neuronal effect (Duecker and Sack, 2013) the induced electric fields are on a range that could affect cortical excitability
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