Paired associative stimulation (PAS)

A non-invasive analogue of spike-triggered stimulation. Classic PAS pairs a peripheral nerve shock with a transcranial magnetic stimulation (TMS) pulse over the corresponding motor cortex, at an inter-stimulus interval chosen so the afferent volley and the cortical pulse coincide in M1 (around 20-25 ms), repeated many times; the timing determines whether corticospinal excitability — measured as the motor-evoked-potential amplitude — is potentiated or depressed, mirroring STDP. Cortico-cortical PAS pairs two TMS coils over connected areas, and spinal PAS pairs cortical and peripheral stimulation to target corticomotoneuronal synapses in spinal-cord injury.

Honesty: PAS is a valuable, mechanistically-grounded human plasticity tool, and is being explored for stroke and spinal-cord-injury recovery, but responses are notoriously variable between and within individuals — sensitive to attention, prior activity, time of day, and genetics — and group effects are often modest. It is better established as a probe of plasticity than as a proven standalone therapy.