Modulatory effects of low‐ and high‐frequency repetitive transcranial magnetic stimulation on visual cortex of healthy subjects undergoing light deprivation

Abstract

The aim of the present study was to explore further the effects of light deprivation (LD) on visual cortex excitability. Healthy subjects reporting reliable induction of phosphenes by occipital transcranial magnetic stimulation (TMS) underwent 60 min of complete LD. Phosphene threshold (PT) was measured before (T 0 ), after 45 min (T 1 ) and 60 min (T 2 ) of LD, and then every 10 min after light re-exposure until recovery to T 0 values. Repetitive TMS (rTMS) (at 1 or 10 Hz) was applied in separate sessions during the last 15 min of LD. PTs significantly decreased after 45 min of LD. rTMS differentially modified the effects of 60 min LD on PTs depending on stimulation frequency. One hertz rTMS did not change the decreasing of PT values as observed in baseline condition, but significantly prolonged the time to recover T 0 PT values after light re-exposure. By contrast, 10 Hz rTMS significantly increased PT and the time to recover T 0 PT values after light re-exposure was shortened. The results of this study show that the modulatory effects of different rTMS frequencies on visual cortex critically depend on the pre-existing excitability state of inhibitory and facilitatory circuits, and provide novel insights into the neurophysiological changes that take place in the visual cortex following functional visual deafferentation.

Voltage-sensitive dye imaging of transcranial magnetic stimulation-induced intracortical dynamics

“…Hence, the excitatory levels found after high-frequency stimulation might be underestimated, whereas notch strength before TMS might be overestimated. Future studies may provide further mechanistic insights through systematic variation in TMS intensity (35,67,68) or by testing the dependence of the TMS effect on cortical states (22,39,49,69). However, our results suggest reduced inhibitory drive as one major origin of rTMSinduced increased excitability of the visual cortex.…”

“…In cat V1, 1 Hz rTMS was shown to reduce visually evoked potentials whereas 10 Hz rTMS led to increased amplitudes (35). Likewise in human visual cortex, 1 Hz vs. 10 Hz TMS regimes showed opposing effects on excitability, as measured by changes of phosphene thresholds (38,39). In particular, effectiveness of 10 Hz rTMS over human V1 was demonstrated by improved contrast sensitivity of amblyopic patients (40), as well as by modulation of performance in visual detection and feature discrimination tasks (41).…”

Visual Phosphene Perception Modulated by Subthreshold Crossmodal Sensory Stimulation

“…Specifically, after 45 min of complete visual deprivation (blindfolded), normally sighted subjects show a significant decrease in PT (that is, an increase in cortical excitability), as well as complete recovery to baseline levels after re-exposure to light (Boroojerdi et al, 2000;Fierro et al, 2005). Therefore, it is possible that the ability of our subjects to perceive phosphenes would result from an enhancement in overall visual cortex excitability over time as a consequence of visual deprivation.…”

A double dissociation between striate and extrastriate visual cortex for pattern motion perception revealed using rTMS

“…By applying TMS pulses at a frequency of 1 Hz it is possible to induce a temporary reduction of excitability in the stimulated region of cortex as measured by the relative strength of TMS stimulation required to elicit a certain cortical response pre-and post-rTMS stimulation [Fitzgerald et al, 2006]. This effect has been demonstrated specifically in visual cortex [Brighina et al, 2002;Fierro et al, 2005]. Offline 1 Hz rTMS has also been used to demonstrate that striate cortex is involved in the dichoptic (separate stimuli to each eye) combination of two components into a plaid percept [Saint-Amour et al, 2005].…”