The human cerebellum and associative learning: dissociation between the acquisition, retention and extinction of conditioned eyeblinks.

The present paper is part of a systematic exploration of the neural substrates of conditioned eyeblink responses in humans. Normal subjects and patients with lesions restricted to the cerebellum were examined for their ability to acquire new classically conditioned eyeblinks to an auditory conditioned stimulus and whether they were able to perform and extinguish a previously learned natural anticipatory eyeblink response - the kinesthetic threat eyeblink response (KTER). In classical conditioning to an auditory conditioned stimulus, cerebellar patients failed to acquire new conditioned responses. In contrast to this impairment, in the KTER task both cerebellar patients and control subjects exhibited a high incidence of anticipatory eyeblinks which were initiated before the forehead tap. These results indicate that the cerebellar circuits, which are critical for the acquisition of new conditioned responses, are not essential for the storage and expression of naturally acquired conditioned responses. In the extinction experiment, cerebellar patients failed to extinguish their KTERs. This finding suggests that in humans, the acquisition of new and the extinction of previously learned conditioned responses depends on a similar set of cerebellar circuits.