A conditioned eyeblink obtained by using electrical stimulation of the facial nerve as the unconditioned stimulus

Cats acquired a conditioned eyeblink (CR) in a situation where the conditioned stimulus (CS) was a click and the unconditioned stimulus (US) was unilateral electrical stimulation of the zygomatic orbital branch of the facial nerve, which is a motor nerve. In a discrimitive training program, cats received both a click paired with a nerve stimulation and an interspersed unpaired hiss delivered randomly; these cats developed a CR only to the click. No CR developed in cats trained with random presentation of both click and nerve stimulation. Of those cases in which a CR was present, as measured electromyographically, this CR was distinguishable from background noise unilaterally approximately 50% of the time and was bilaterally distinguishable the remained of the time. This CR could be extinguished and easily relearned and was specific for blink musculature. Acquisition rates for the CR were somewhat, but not significantly, slower than rates for cats trained with glabella tap US. Cats with a trigeminal rhizotomy on the side for facial stimulation developed a CR, indicating that inadvertent stimulation of trigeminal afferents cannot account for CR development. Experiments on cats with both Vth and VIIth nerves cut unilaterally indicated that other possible afferents were unlikely to be involved. Thus, the sensory excitation ordninarily produced by the US appears to be unnecessary in this case, and firing of facial motoneurons at a suitable interval after the click CS is a sufficient condition for development of this CR.