Habituation and sensitization of components of the human eyeblink reflex.

The supraorbital branch of the right trigeminal nerve was stimulated with trains of electric shocks, 10 in each train, with interpulse intervals of 1, 2, or 5 sec. Electromyographic reflex activity in the palpebral musculature was measured to each pulse in the train. Each response consisted of two components, a fast brief ipsilateral burst (R1) and a slower prolonged bilateral burst (R2). Over the 10-pulse series, the amplitudes of R1 increased in strength (sensitization), whereas the allied R2 amplitudes declined (habituation). Both of these effects were enhanced as the interpulse interval was reduced. A consideration of the anatomical substrate for each reflex component, together with some other data in the literature, suggests that reflex sensitization occurred in the efferent limb of this brain stem reflex while simultaneously habituation occurred in its central link.