Preconditioning of low-frequency repetitive transcranial magnetic stimulation with transcranial direct current stimulation: evidence for homeostatic plasticity in the human motor cortex.
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.
Siebner
Hartwig R
HR
Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College of London, London WC1N 3BG, United Kingdom.
Lang
Nicolas
N
Rizzo
Vincenzo
V
Nitsche
Michael A
MA
Paulus
Walter
W
Lemon
Roger N
RN
Rothwell
John C
JC
D000328:Adult; D003213:Conditioning, Psychological / Q000502:physiology / Q000528:radiation effects; D004558:Electric Stimulation / Q000295:instrumentation / Q000379:methods*; D004574:Electromagnetic Fields; D019054:Evoked Potentials, Motor / Q000502:physiology / Q000528:radiation effects; D006706:Homeostasis / Q000502:physiology* / Q000528:radiation effects; D006801:Humans; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology* / Q000528:radiation effects; D009473:Neuronal Plasticity / Q000502:physiology* / Q000528:radiation effects; D011712:Pyramidal Tracts / Q000502:physiology; D011930:Reaction Time / Q000502:physiology / Q000528:radiation effects; D012016:Reference Values; D050781:Transcranial Magnetic Stimulation