Wesley (Wes) P. Jordan


If an animal encounters a situation repeatedly in which nothing important occurs, it learns to ignore the stimuli associated with the innocuous event. Habituation, this reduction of responsiveness to repeated stimulation, is a good model system from which to study the brain's role in learning.

The most widely used model of habituation in vertebrates is the acoustic startle response. This simple reflex is reliably elicited by sudden auditory stimuli in all mammals that have been studied. If a rat is presented with an auditory stimulus once per day, the amplitude of the startle response habituates or declines. We know that the midbrain reticular formation (MRF) is necessary for this long-term habituation, because damage to this brain area prevents new habituation (Jordan & Leaton, 1982 and Jordan & Leaton, 1983) and abolishes previously acquired habituation (Jordan, 1989).

Examples of student projects in this area include making lesions in the MRF and injections of a local anesthetic or drugs into the MRF to try and abolish or alter habituation.

We've studied how the environment in which an animal is tested affects habituation. Most theories of habituation assume that during habituation the brain's perceptual mechanisms change that way the animal responds to a stimulus. Other theories, however, argue that habituation is a learned response in which the environmental context present during habituation becomes associated with the diminished response. If this association is broken, for instance by testing a habituated animal in a new context, habituation will disappear.

Work from the lab has shown that habituation of the startle response is insensitive to a change in testing context. However, if animal is thirsty and drinking water when the auditory stimulus is presented, it will interrupt its licking. With repeated presentations of the stimulus, the animal interrupts its drinking for less time. This habituation of lick suppression is dependent upon the testing context. If the animal is habituated to the tone in its home cage and then is tested in another environment, habituation of lick suppression disappears. Likewise, training the animal in one experimental context and testing it in another, produces recovery of habituation. We also have videotaped the animal's orienting response to tones and found that habituation of this response also is context dependent.