Learned modification of the nictitating membrane reflex by auditory stimuli in the barn owl.

The barn owl, Tyto alba, extends its nictitating membrane (NM) in response to an air puff to the cornea or a mild para-orbital electrodermal shock. The NM extension habituated rapidly if the stimulus was repeated. Habituation was prevented by pairing the aversive stimulus with a sound. The sound stimulus did not, by itself, induce an NM extension. Repeated pairing of sound with the aversive stimulus caused the subjects to modify the duration of their NM extension, increasing the duration when exposed to longer aversive stimuli and decreasing in response to shorter stimuli. No transference of the response was seen from the aversive stimulus to the sound. The learned change in duration of the NM extension resisted extinction. This modification of the NM extension reflex resembles previous descriptions of primer-produced facilitation.