Activity-dependent / spike-triggered stimulation

A closed-loop protocol in which recorded activity at one site triggers electrical stimulation at another with a fixed short delay, so the two are repeatedly paired to induce a Hebbian change in the connection between them. The canonical demonstration used a small autonomous, free-behaving implant (the 'Neurochip') in monkeys: stimulating one motor-cortical site a few milliseconds after each spike of a neuron at another site, continuously over a day or more, produced lasting shifts in the motor-output map consistent with strengthened connectivity that outlasted the conditioning. Related brain-machine-brain interfaces have bridged a cortical lesion in rodents to restore reaching.

Honesty: these are among the most compelling causal demonstrations that a device can direct plasticity, but they are largely in animals, over a limited connection, and the induced changes — durable for hours to days — are not yet a controllable clinical dose. Translation to human recovery requires stable chronic recording of the trigger unit, safe repeated stimulation, and evidence that the strengthened pathway yields function, not just a shifted evoked response.