Conditioning- and time-dependent increases in context fear and generalization

Abstract

A prominent feature of fear memories and anxiety disorders is that they endure across extended periods of time. Here, we examine how the severity of the initial fear experience influences incubation, generalization, and sensitization of contextual fear memories across time. Adult rats were presented with either five, two, one, or zero shocks (1.2 mA, 2 sec) during contextual fear conditioning. Following a recent (1 d) or remote (28 d) retention interval all subjects were returned to the original training context to measure fear memory and/or to a novel context to measure the specificity of fear conditioning. Our results indicate rats that received two or five shocks show an "incubation"-like enhancement of fear between recent and remote retention intervals, while single-shocked animals show stable levels of context fear memory. Moreover, when fear was tested in a novel context, 1 and 2 shocked groups failed to freeze, whereas five shocked rats showed a time-dependent generalization of context memory. Stress enhancement of fear learning to a second round of conditioning was evident in all previously shocked animals. Based on these results, we conclude that the severity or number of foot shocks determines not only the level of fear memory, but also the time-dependent incubation of fear and its generalization across distinct contexts.

Figure 1.

Study and research design for experiments 1 and 2. In experiment 1 (A) subjects were conditioned with zero, one, two, or five foot shocks then tested for retention and generalization following a 1 or 28 d-retention interval. Experiment 2 (B) expanded upon this by counterbalancing the order of the memory retention and generalization tests, as well as observing extinction and sensitization to novel contexts daily following the initial retrieval tests.

Figure 2.

Experiment 1, contextual fear memory (context A) measured at either 1 or 28 d following initial fear conditioning by either zero, one, two, or five foot shocks. Total mean percentage of time spent freezing (± SEM) during a 4-min context fear test. Asterisks (*) represents significance at the P < 0.05 criterion.

Figure 3.

Experiment 1, context generalization (context B) measured at either 1 or 28 d following initial fear conditioning by either zero, one, two, or five foot shocks. Total mean percentage of time spent freezing (± SEM) during a 4-min context generalization test.

Figure 4.

Experiment 2, contextual fear memory (context A) measured at either 1 or 28 d following initial fear conditioning by either one, two, or five foot shocks. Total mean percentage of time spent freezing (± SEM) during a 4-min context fear test. Asterisks (*) represents significance at the P < 0.05 criterion.

Figure 5.

Experiment 2, context generalization (context B) measured at either 1 or 28 d following initial fear conditioning by either one, two, or five foot shocks. Total mean percentage of time spent freezing (± SEM) during a 4-min context generalization test. Asterisks (*) represents significance at the P < 0.05 criterion.

Figure 6.

Experiment 2, context discrimination ratio (context A freezing/(context A + context B freezing)) (± SEM) computed from animals conditioned by either one, two, or five foot shocks measured either 1 or 28 d following the initial conditioning. Asterisks (*) represents significance at the P < 0.05 criterion.

Figure 7.

Experiment 2, extinction of context fear (context A) measured at either 1 or 28 d following initial fear conditioning by one, two, or five foot shocks. Twenty minute extinction session were conducted 1 d following the retrieval and generalization tests. Each block represents a 2-min interval of the mean percent time spent freezing (± SEM) during the extinction session.

Figure 8.

Experiment 2, stress enhancement of fear (context C) measured at 2 d following context extinction training in subjects initially tested at either 1 or 28 d following initial fear conditioning by either zero, one, two, or five foot shocks. Total mean percentage of time spent freezing (± SEM) during a 4-min context fear test. Asterisks (*) represents significance at the P < 0.05 criterion.