Strain Differences in Acquisition and Extinction of Fear Responses in Rats

5 different strains of rats (Roman high and low avoiders, Maudsley reactive and non-reactive and random-bred animals) were subjected repeatedly to extinction trials following Pavlovian fear conditioning. The duration of the extinction trials was varied for different groups of animals. Fear was measured by latency of escape into the “safe” compartment in Exp. I and by step-down latency in Exp. II during a final fear-retention test. Results showed no differences between Roman high and low avoiders; for the Maudsley strains, however, results suggested that the higher the basal fear level the stronger is the acquired fear response and the more time is required for its extinction. Fearfulness in the animal and duration of extinction trials were jointly and severally causal in determining degree of extinction of the conditioned fear response.