Preconditioning of Low-Frequency Repetitive Transcranial Magnetic Stimulation with Transcranial Direct Current Stimulation: Evidence for Homeostatic Plasticity in the Human Motor Cortex

TLDR

Homeostatic plasticity stabilizes neuronal circuit properties, as shown in animal studies. The study reports evidence of homeostatic cortical plasticity in healthy humans. The authors combined anodal or cathodal transcranial direct current stimulation with 1 Hz repetitive transcranial magnetic stimulation over the left primary motor cortex to modulate corticospinal excitability. Anodal TDCS preconditioning lowered, while cathodal TDCS preconditioning raised, the subsequent 1 Hz rTMS‑induced changes in corticospinal excitability, indicating a homeostatic mechanism that stabilizes motor cortex output.

Abstract

Recent experimental work in animals has emphasized the importance of homeostatic plasticity as a means of stabilizing the properties of neuronal circuits. Here, we report a phenomenon that indicates a homeostatic pattern of cortical plasticity in healthy human subjects. The experiments combined two techniques that can produce long-term effects on the excitability of corticospinal output neurons: transcranial direct current stimulation (TDCS) and repetitive transcranial magnetic stimulation (rTMS) of the left primary motor cortex. “Facilitatory preconditioning” with anodal TDCS caused a subsequent period of 1 Hz rTMS to reduce corticospinal excitability to below baseline levels for >20 min. Conversely, “inhibitory preconditioning” with cathodal TDCS resulted in 1 Hz rTMS increasing corticospinal excitability for at least 20 min. No changes in excitability occurred when 1 Hz rTMS was preceded by sham TDCS. Thus, changing the initial state of the motor cortex by a period of DC polarization reversed the conditioning effects of 1 Hz rTMS. These preconditioning effects of TDCS suggest the existence of a homeostatic mechanism in the human motor cortex that stabilizes corticospinal excitability within a physiologically useful range.