Failure of Extinction of Fear Responses in Posttraumatic Stress Disorder: Evidence From Second-Order Conditioning

Objectives: Posttraumatic stress disorder (PTSD) is characterized by the re-experiencing of a traumatic event, although the trauma itself occurred in the past. The extinction of the traumatic response might be impeded if trauma reminders maintain fear responses by their association with the original trauma through second-order conditioning. Method: A differential conditioning paradigm with a trauma-specific picture, used as an acquired unconditioned stimulus, and graphic representations, used as conditioned stimuli, were employed in 14 PTSD patients, 15 trauma-exposed subjects without PTSD, and 15 healthy comparison subjects. The authors used event-related potentials of electroencephalogram (EEG), self-report measures, skin conductance responses, heart rate, and startle modulation to assess the differential conditioned response among subjects. Results: Trauma-exposed subjects with and without PTSD but not healthy comparison subjects showed successful differential conditioning to the trauma-relevant cue indicative of second-order conditioning. Only PTSD patients exhibited enhanced conditioned responses to the trauma reminder during acquisition and impaired extinction as evident in more negative evaluations of the conditioned stimuli associated with a trauma reminder as well as enhanced peripheral and brain responses. Conclusions: These findings suggest that PTSD may be maintained by second-order conditioning where trauma-relevant cues come to serve as unconditioned stimuli, thus generalizing enhanced emotional responses to many previously neutral cues and impeding extinction. The extinction deficit in PTSD patients observed in this study underlines the need for therapies focusing on the extinction of learned responses, such as behavioral treatment, with or without the addition of pharmacological substances that enhance the extinction of a learned response.