Extinction of conditioned fear is better learned and recalled in the morning than in the evening

Abstract

Sleep helps emotional memories consolidate and may promote generalization of fear extinction memory. We examined whether extinction learning and memory might differ in the morning and evening due, potentially, to circadian and/or sleep-homeostatic factors. Healthy men (N = 109) in 6 groups completed a 2-session protocol. In Session 1, fear conditioning was followed by extinction learning. Partial reinforcement with mild electric shock produced conditioned skin conductance responses (SCRs) to 2 differently colored lamps (CS+), but not a third color (CS-), within the computer image of a room (conditioning context). One CS+ (CS + E) but not the other (CS + U) was immediately extinguished by un-reinforced presentations in a different room (extinction context). Delay durations of 3 h (within AM or PM), 12 h (morning-to-evening or evening-to-morning) or 24 h (morning-to-morning or evening-to-evening) followed. In Session 2, extinction recall and contextual fear renewal were tested. We observed no significant effects of the delay interval on extinction memory but did observe an effect of time-of-day. Fear extinction was significantly better if learned in the morning (p = .002). Collapsing across CS + type, there was smaller morning differential SCR at both extinction recall (p = .003) and fear renewal (p = .005). Morning extinction recall showed better generalization from the CS + E to CS + U with the response to the CS + U significantly larger than to the CS + E only in the evening (p = .028). Thus, extinction is learned faster and its memory is better generalized in the morning. Cortisol and testosterone showed the expected greater salivary levels in the morning when higher testosterone/cortisol ratio also predicted better extinction learning. Circadian factors may promote morning extinction. Alternatively, evening homeostatic sleep pressure may impede extinction and favor recall of conditioned fear.

Keywords: Circadian rhythm; Cortisol; Extinction; Fear conditioning; Sleep; Sleep homeostasis; Testosterone.

Copyright © 2013 Elsevier Ltd. All rights reserved.

Figures

Figure 1

The experimental protocol consisted of 6 groups 3 of which received Fear Conditioning and Extinction Learning in the evening (black) and 3 in the morning (grey). During Fear Conditioning, conditioned fear responses were established to 2 different colors (CS+) by a mild electric shock. Immediately afterward, during Extinction Learning, conditioned responses to one CS+ (CS+E) but not the other (CS+U) were extinguished. Within the 3 groups trained in the morning and the 3 trained in the evening, one in each then underwent a 3, 12 or 24-hr delay during which they retained fear and extinction memories. After this delay, they were then tested for Extinction Recall and, immediately afterward, for contextual Fear Renewal. The 3 and 24-hr delays following morning and evening training allowed both training and testing to occur at the same approximate time of day (“time-congruent” participant grouping) and these are indicated in the figure by bars of the same shading as their training session placed above the phases of Session 2.

Figure 2

The duration of the delay following Fear Conditioning and Extinction Learning had no significant effect on the magnitude of subsequent Extinction Recall or Fear Renewal. A. Mean differential skin conductance response across 8 trials of Extinction Recall following each delay. B. Mean differential skin conductance response across 8 trials of Fear Renewal following each delay. CS+E: conditioned response extinguished at Extinction Learning, CS+U: conditioned response not extinguished, SCR1/2d: differential SCR computed using square-root transformed SCRs (= SCR to CS+ minus SCR to temporally closest CS−). Error bars depict standard error of the mean.

Figure 3

Fear conditioning did not differ between morning and evening whereas extinction was significantly better learned in the morning. The starting point of Extinction Learning was equated by examining the upper half of a median split based upon the mean differential SCR of first two trials of Extinction Learning (N=33 evening, 21 morning). Each data point represents the mean differential SCR to two successive CS+. During Fear Conditioning, these were trial-by-trial averages of the 2 different CS+ (i.e., mean of first to-be CS+E and first to-be CS+U, mean of second to-be CS+E and second to-be CS+U, etc.). During Extinction Learning, data points are means for successive pairs of CS+E. Significance indicated for the Time-of-Day main effect (large asterisks) and trial-by-trial (small asterisks). SCR1/2d: differential SCR, ** p

Figure 4

Extinction Recall in the morning showed generalization of extinction memory whereas recall in the evening preserved the differentiation of the extinguished and unextinguished CS+ and favored expression of a greater overall degree of conditioned fear. A. Differential SCR to the CS+E and CS+U at Extinction Recall and Fear Renewal in the morning. B. Differential SCR to the CS+E and CS+U at Extinction Recall and Fear Renewal in the evening. Significance indicated for the CS+E vs. CS+U main effect (large asterisk) and trial-by-trial (small asterisk). C. Comparison of differential SCR in the morning and evening for the average of all 8 trials during Extinction Recall and during Fear Renewal phases collapsing across CS+E and CS+U. SCR1/2d: differential SCR, * p < .05, ** p < .01. Error bars depict standard error of the mean.

Figure 5

Retrospective differential shock expectancy ratings obtained from participants at the end of their second session. Only major learning and memory processes are illustrated. Learning processes include acquisition of differential fear conditioning (comparison of ratings for first and last 2 presentations at Fear Conditioning) and extinction learning (comparison of ratings for first and last 2 presentations of CS+E during Extinction Learning). Memory processes include differentiation of the first 2 presentations of the CS+E in comparison to the CS+U during Extinction Recall and greater shock expectancy for the first 2 presentations of the CS+E at Fear Renewal in comparison to their initial presentations at Extinction Recall (contextual fear renewal). A. Entire sample divided by Delay Duration for which there were no main effects or interactions with processes depicted. B. Time-congruent subsample subjects divided by Time-of-Day for which there were no main effects or interactions with processes depicted. Differential VAS rating is equal to retrospective shock expectancy for the 2 CS+ minus that for the 2 CS− from the same point in time. ** p

Figure 6

Association of endogenous testosterone/cortisol ratio levels, obtained during the morning, with fear and extinction learning. Participants with preconditioning high salivary testosterone to cortisol (T/C) ratio (based upon median split of 39 participants with testosterone data who had Session 1 in the morning) show better extinction learning. Significance indicated for the T/C ratio main effect (large asterisk) and trial-by-trial (small asterisk). SCR1/2d: differential SCR, * p < .05 trial, ** p < .01. Error bars depict standard error of the mean.