Associative Motor Cortical Plasticity is Modulated by the Activation History of Motor Cortex

Pairing median nerve stimulation repetitively with transcranial magnetic stimulation over the contralateral motor cortex (paired associative stimulation, PAS) may increase, or decrease, human motor cortical excitability at interstimulus intervals of 25 ms (PAS25), or 10 ms (PAS10). The properties of this plasticity resemble associative long-term potentiation (LTP) and long-term depression (LTD) as established in vitro. We employed these protocols to examine possible physiological correlates of motor memory formation. Subjects performed brisk isometric abductions with their right thumb at a defined force window a…