Presence and acquired origin of reduced recall for fear extinction in PTSD: results of a twin study

Abstract

Recall of fear extinction, which is thought to aid in recovery from a psychologically traumatic event, is hypothesized to be deficient in post-traumatic stress disorder (PTSD), but this has not yet been demonstrated in the laboratory, nor has its origin been investigated. To address these two issues, 14 pairs of monozygotic twins discordant for combat exposure, in 7 of which the combat-exposed twin had PTSD, underwent a two-day fear conditioning and extinction procedure. On Day 1, subjects viewed colored light conditioned stimuli, some of which were paired with mild electric shock, followed by extinction of the conditioned responses. On Day 2, recall of Day 1 extinction learning (i.e., extinction retention) was assessed. Skin conductance response (SCR) was the dependent measure. There were no group differences during acquisition or extinction learning. However, a significant PTSD Diagnosis (in the exposed twin) x combat Exposure interaction emerged during extinction recall, with the PTSD combat veterans having larger SCRs than their own co-twins, and than the non-PTSD combat veterans and their co-twins. These results indicate that retention of extinction of conditioned fear is deficient in PTSD. Furthermore, they support the conclusion that this deficit is acquired as a result of combat trauma leading to PTSD, rather than being a predisposing factor to developing PTSD upon the stress of combat.

Figures

Figure 1

Intact fear acquisition and extinction learning but impaired extinction recall (retention) in PTSD. A. Largest skin conductance response (SCR) to the conditioned stimulus paired with the shock (CS+) and to the conditioned stimulus not paired with the shock (CS-) exhibited during the Acquisition phase. B. SCR to the CS+ and CS- averaged across the last two trials of the late Extinction Learning phase. C. SCR to the CS+ and CS- averaged across the first two trials of the Extinction Recall phase. *p<0.05 ExP+: PTSD twins, UxP+: their combat-unexposed co-twins; ExP-: combat-exposed non-PTSD twins, UxP-: their combat-unexposed co-twins.

Figure 2

Extinction Learning and Extinction Retention Indices (see text for definition) demonstrate intact extinction learning across all groups but impaired extinction retention in PTSD twins only. *p