Long-term potentiation of the human blink reflex.

The trigeminal reflex blink is an ideal system to investigate whether stimulus paradigms that produce long-term potentiation (LTP) in vitro modify motor learning in humans. Presentation of 12 trains of low-intensity, high-frequency stimuli (HFS) to the supraorbital branch of the trigeminal nerve (SO) modified subsequent reflex blinks of human subjects. When HFS occurred concurrently with reflex blinks, the procedure potentiated subsequent blinks for >1 hr. Combining HFS with feedback from the lid movement was critical for this facilitation because presenting HFS immediately after the blink did not alter subsequent blinks. When HFS preceded the blink, however, this treatment suppressed subsequent blinks for 30 min. These effects appear to occur within the trigeminal reflex blink circuits rather than at motoneurons, because stimulation of the previously HFS-treated SO evoked altered blinks in both eyelids, whereas stimulation of the untreated SO elicited unaltered blinks in both eyelids. The modified blink amplitude resulted from altering the response to A-fiber inputs to the trigeminal nerve because all stimuli were too weak to activate C-fibers. The data suggest that HFS produce LTP- and long-term depression (LTD)-like effects on wide dynamic range neurons in the trigeminal reflex blink circuit. The data also support the hypothesis that LTP and LTD mechanisms play a role in adaptive modification of human reflex blinks.