Memory for extinction of conditioned fear is long-lasting and persists following spontaneous recovery

Abstract

Conditioned fear responses to a tone paired with footshock rapidly extinguish when the tone is presented in the absence of the shock. Rather than erase conditioning, extinction is thought to involve the formation of new memory. In support of this, extinguished freezing spontaneously recovers with the passage of time. It is not known, however, how long extinction memory lasts or whether extinction interferes with consolidation of conditioning if given on the same day. To address this, we gave rats 7 trials of auditory fear conditioning followed 1 h later by 20 extinction trials, and tested for spontaneous recovery after a delay of 0, 1, 2, 4, 6, 10, or 14 d. Conditioned freezing to the tone gradually recovered with time to reach 100% by day 10. No-extinction controls indicated that the increase in freezing with time was not owing to incubation of conditioning memory. Complete spontaneous recovery indicates that extinction training given 1 h after conditioning does not interfere with the consolidation of conditioning memory. Despite complete recovery of freezing, rats showed savings in their rate of re-extinction, indicating persistence of extinction memory. These data support the idea that conditioning and extinction of fear are learned by independent systems, each able to retain a long-term memory.

Figure 1

Fear conditioning protocol. On day 0 (d0), all rats were given seven conditioning trials (tone-shock) followed 1 h later by 20 extinction trials (tone alone). The intertrial interval averaged 4 min. After a variable number of days, rats were given additional extinction trials to test for spontaneous recovery of freezing to the tone. Data are shown for group d1, which was tested 1 d after extinction. Trial blocks consist of two trials. In this and subsequent figures, error bars show ±SEM.

Figure 2

Freezing to the tone at three time points: (A) Pre-extinction on day 0 (d0; tones 1–2); (B) post-extinction on d0 (tones 19–20); and (C) spontaneous recovery on the test day (tones 1–2). The x-axis indicates experimental groups, which were tested either 1, 2, 4, 6, 10, or 14 d after extinction. The degree of spontaneous recovery was expressed as percent rebound, which is the percent of conditioned freezing acquired on d0 recovered on the test day. Asterisks indicate significant difference from 100% (p < 0.05), demonstrating significant expression of extinction memory. Extinction was expressed through 6 d, but not at 10 or 14 d. Inset: Individual extinction curves for each group across the 20 extinction trials on d0.

Figure 3

Distribution of individual rebound values for each group on the test day. Rebound of freezing (spontaneous recovery) gradually shifted toward 100% with the passage of time.

Figure 4

Savings in the rate of re-extinction. Extinction curves for original extinction on day 0 (d0; empty circles) and re-extinction on d14 (filled circles) are shown. Despite 100% spontaneous recovery, rats reextinguished significantly faster than they extinguished originally, indicating that some component of extinction memory was preserved. Asterisks indicate significant post-hoc comparisons (p < 0.05) following significant main effects of group and trial in ANOVA (see text).