Abstract
The experiments reported in the present thesis examined the behavioural
processes of Pavlovian fear extinction and latent inhibition. The first series of
experiments studied the reacquisition of extinguished fear responses following
different amounts of extinction training. Rapid reacquisition occurred when rats
were reconditioned after moderate extinction, showing that the original learning
remained intact across this extinction. In contrast, when reconditioning was given
after massive extinction, reconditioned responding was first depressed but then
spontaneously recovered over time. This suggests that massive extinction
produces a relatively permanent loss of the originally learned responding, while
additionally imposes on the extinguished CS a transient latent inhibitory process
that prevented the immediate but not the delayed expression of reconditioning.
The second series of experiments studied the impact of spontaneous
recovery of extinguished fear responses on their additional extinction. These
experiments demonstrated that a CS that had time to show spontaneous recovery
underwent greater response loss across additional extinction than one lacking
recovery. They also showed that an excitor extinguished in compound with a CS
showing recovery suffered greater response loss than an excitor extinguished in
compound with a CS lacking recovery. Further, extinction of a compound
composed of two CSs, one showing recovery and a second lacking recovery,
produced greater extinction to the CS that showed recovery. These results show
that spontaneous recovery of extinguished responses deepens their extinction
through an error-correction mechanism regulated by both common and individual
error terms.
The third series of experiments studied the spontaneous recovery of
latently inhibited and extinguished fear responses in within-subject designs. Using
a compound test procedure, a CS that had received extensive preexposure or
extensive extinction was found to have undergone greater spontaneous recovery
relative to a CS just moderately preexposed or moderately extinguished. A CS
given a mixed history of preexposure and extinction also underwent greater
recovery relative to a CS just preexposed or just extinguished. These results
suggest that both latent inhibition and extinction share a transient depressive
process, and that the resulting recovery of responding is proportional to the
amount of this depression.