title: 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.
authors.affiliation: Unit of Neurology, Department of Clinical and Experimental Medicine, Pisa University Medical School, Pisa, Italy.
authors.forename: Tommaso
authors.initials: T
authors.lastname: Bocci
authors.forename: Matteo
authors.initials: M
authors.lastname: Caleo
authors.forename: Silvia
authors.initials: S
authors.lastname: Tognazzi
authors.forename: Nikita
authors.initials: N
authors.lastname: Francini
authors.forename: Lucia
authors.initials: L
authors.lastname: Briscese
authors.forename: Lamberto
authors.initials: L
authors.lastname: Maffei
authors.forename: Simone
authors.initials: S
authors.lastname: Rossi
authors.forename: Alberto
authors.initials: A
authors.lastname: Priori
authors.forename: Ferdinando
authors.initials: F
authors.lastname: Sartucci
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D001703:Biophysics; D004558:Electric Stimulation; D004569:Electroencephalography; D005074:Evoked Potentials, Visual / Q000502:physiology*; D005260:Female; D007839:Functional Laterality; D006801:Humans; D008297:Male; D009473:Neuronal Plasticity / Q000502:physiology* / Q000528:radiation effects; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation; D014793:Visual Cortex / Q000502:physiology* / Q000528:radiation effects; D055815:Young Adult