Evidence for metaplasticity in the human visual cortex

Abstract

The threshold and direction of excitability changes induced by low- and high-frequency repetitive transcranial magnetic stimulation (rTMS) in the primary motor cortex can be effectively reverted by a preceding session of transcranial direct current stimulation (tDCS), a phenomenon referred to as "metaplasticity". Here, we used a combined tDCS-rTMS protocol and visual evoked potentials (VEPs) in healthy subjects to provide direct electrophysiological evidence for metaplasticity in the human visual cortex. Specifically, we evaluated changes in VEPs at two different contrasts (90 and 20 %) before and at different time points after the application of anodal or cathodal tDCS to occipital cortex (i.e., priming), followed by an additional conditioning with low- or high-frequency rTMS. Anodal tDCS increased the amplitude of VEPs and this effect was paradoxically reverted by applying high-frequency (5 Hz), conventionally excitatory rTMS (p < 0.0001). Similarly, cathodal tDCS led to a decrease in VEPs amplitude, which was reverted by a subsequent application of conventionally inhibitory, 1 Hz rTMS (p < 0.0001). Similar changes were observed for both the N1 and P1 component of the VEP. There were no significant changes in resting motor threshold values (p > 0.5), confirming the spatial selectivity of our conditioning protocol. Our findings show that preconditioning primary visual area excitability with tDCS can modulate the direction and strength of plasticity induced by subsequent application of 1 or 5 Hz rTMS. These data indicate the presence of mechanisms of metaplasticity that keep synaptic strengths within a functional dynamic range in the human visual cortex.

Controlling brain state prior to stimulation of parietal cortex prevents deterioration of sustained attention

“…Whilst sustained attention decreases with time in the control condition, we expect functional compensation following multi-method stimulation to maintain sustained attention. We conjecture multi-method stimulation would result in sustained inhibition of the IPS, similar to the findings in the motor and visual cortex (Iyer et al, 2003;Bocci et al, 2014) and functional compensation from other nodes of the attention network in response (Paus et al, 1997;Grefkes et al, 2009;Lee & D'Esposito, 2012;Plow et al, 2014;Battelli et al, 2017). We expected HF-tRNS alone to elicit cortical excitation (Herpich et al, 2018), resulting in an increase in sustained attention, while LF-rTMS was expected to only initially decrease contralateral attention (Battelli et al, 2009).…”

“…The influence of neuronal activation history on subsequent neuronal activity has been termed "metaplasticity" (Abraham & Bear, 1996). To date, multi-method stimulation for enduring cortical inhibition have been tested physiologically in the motor (Iyer et al, 2003;Siebner et al, 2004;Lang et al, 2004;Fricke et al, 2010), and visual cortex (Bocci et al, 2014), but never in the parietal cortex using a cognitive task as the outcome measure. In order to harness the predicted behavioral benefit to sustained attention from functional compensation of the dorsal attention network, we aimed to boost the impact of LF-rTMS over the IPS.…”

Consensus Paper: Probing Homeostatic Plasticity of Human Cortex With Non-invasive Transcranial Brain Stimulation

“…Both studies used an NTBS prime preceding a highfrequency tactile stimulation (HFS) protocol of the contralateral hand in order to demonstrate a homeostatic response in the somatosensory cortex. In primary visual cortex, VEP) revealed a homeostatic reaction to a combined TDCS-rTMS protocol [87]. Identifying additional neurophysiological markers of brain plasticity such as recordings of TMS-evoked cortical potentials with combined TMS-EEG [88] might facilitate investigations into homeostatic effects expressed in other cortical areas.…”

Evidence that transcranial direct current stimulation (tDCS) generates little-to-no reliable neurophysiologic effect beyond MEP amplitude modulation in healthy human subjects: A systematic review

“…A ¼ anode; C¼ cathode; S ¼sham. VEP notes: Bocci et al (2013) reported conflicting information regarding stimulation strength and method of variability reporting. For pooling, we assumed and 1.5 mA current strength and variability reported as SEM.…”