Modulation of Plasticity in Human Motor Cortex after Forearm Ischemic Nerve Block

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Abstract
Deafferentation leads to cortical reorganization that may be functionally beneficial or maladaptive. Therefore, we were interested in learning whether it is possible to purposely modulate deafferentation-induced reorganization. Transient forearm deafferentation was induced by ischemic nerve block (INB) in healthy volunteers. The following five interventions were tested: INB alone; INB plus low-frequency (0.1 Hz) repetitive transcranial magnetic stimulation of the motor cortex ipsilateral to INB (INB+rTMSi); rTMSi alone; INB plus rTMS of the motor cortex contralateral to INB (INB+rTMSc); and rTMSc alone. Plastic changes in the motor cortex contralateral to deafferentation were probed with TMS, measuring motor threshold (MT), motor evoked-potential (MEP) size, and intracortical inhibition (ICI) and facilitation (ICF) to the biceps brachii muscle proximal to the level of deafferentation. INB alone induced a moderate increase in MEP size, which was significantly enhanced by INB+rTMSc but blocked by INB+rTMSi. INB alone had no effect on ICI or ICF, whereas INB+rTMSc reduced ICI and increased ICF, and conversely, INB+rTMSi deepened ICI and suppressed ICF. rTMSi and rTMSc alone were ineffective in changing any of these parameters. These findings indicate that the deafferented motor cortex becomes modifiable by inputs that are normally subthreshold for inducing changes in excitability. The deafferentation-induced plastic changes can be up-regulated by direct stimulation of the “plastic” cortex and likely via inhibitory projections down-regulated by stimulation of the opposite cortex. This modulation of cortical plasticity by noninvasive means might be used to facilitate plasticity when it is primarily beneficial or to suppress it when it is predominately maladaptive.