When a stimulus such as a light, which engenders little behavioral effect before pairing, is paired with an aversive stimulus such as a foot shock, the light (conditioned stimulus) can elicit seemingly fearful responses in animals: autonomic changes, freezing, and an increase in the amplitude of the startle reflex elicited by an auditory stimulus in the presence of the light. The last is called the fear-potentiated startle effect and can occur with an auditory, visual, tactile, or olfactory conditioned stimulus under conditions where startle is elicited by either a loud sound or an air puff.

Fear-potentiated startle is a valid measure of classical conditioning because it occurs only following paired rather than unpaired or "random" presentations of the conditioned stimulus. Potentiated startle shows considerable temporal specificity because its magnitude in testing is greatest at the interval after light onset that matches the light-shock interval in training...