Dissociation of explicit and implicit memories in Posttraumatic Stress disorder
Project Leader: Professor Herta Flor, Ph.D., Michèle Wessa, Ph.D.
Posttraumatic stress disorder (PTSD) is characterized by the experience of an event “that involved actual or threatened death or serious injury or a threat to the physical integrity of self or others” (American Psychiatric Association, 1994, p. 424). Characteristic symptoms include re-experiencing of the trauma, avoidance of trauma-related stimuli and heightened emotional arousal. Psychobiological models of PTSD have focused on a number of features of PTSD. One of the most prominent models is that of stress and trauma leading to a high release of glucocorticoids that impairs hippocampal activity and leads to a dissociation of explicit and implicit memory (e.g., Elzinga & Bremner, 2002). Thus, it has been hypothesized that PTSD is characterized by a lack of detailed trauma knowledge.
The goal of the first study was to examine if PTSD-patients exhibit decreased explicit trauma memory and to determine its cerebral correlates. Fifteen subjects who attended the Ramstein airshow disaster with PTSD, fifteen subjects without PTSD (NPTSD) and fifteen healthy controls were presented with questions about specific details of their trauma, other trauma- and non-trauma-related materials and – consecutively - a set of possibly correct answers. EEG from 32 channels and the subjects’ answers were recorded. Analysis of the subjects’ answers revealed no significant differences between PTSD- and NPTSD subjects in the knowledge about the traumatic event, whereas healthy controls knew significantly less about the disaster. On the electrophysiological level, both, PTSD- and NPTSD subjects exhibited a decreased tCNV-amplitude compared to healthy controls, when exposed to trauma-specific questions. Additionally, PTSD patients revealed generally decreased tCNV amplitudes and an elevated baseline heart rate.
Together these data show no deficit in explicit trauma memories in PTSD patients compared to traumatized subjects without PTSD. The result of a decreased tCNV amplitude to trauma-specific question suggests that the continued processing of the presented material and the preparation to respond, differ not with respect to PTSD, but with having experienced a traumatic event. In comparison to healthy controls PTSD patients showed reduced overall tCNV amplitudes. The PTSD patients’ hyperarousal, indicated by an elevated baseline heart rate, and symptoms of emotional numbing might have resulted in a generally reduced cortical activity.
The second study investigated implicit memory processes related to PTSD, i.e. classical conditioning. Although these learning processes are assumed to play a crucial role in the development and maintenance of posttraumatic stress disorder (PTSD), only few studies investigated conditioning processes in PTSD patients. The findings of the given studies suggest that PTSD patients show an altered conditionability either in terms of a stronger acquisition or prolonged extinction of the conditioned response. In the present study, a differential conditioning procedure with a trauma-specific unconditioned stimulus was applied to investigate alterations in the conditionability of PTSD patients with psychophysiological (SCR, heart rate, startle response, ERP) and self-report measures (valence, arousal, US expectancy). PTSD patients (N=14) were compared to trauma-exposed subjects without PTSD (N=15) and healthy controls (N=15). A stronger acquisition of the conditioned response in PTSD patients was shown for the self-reported valence and arousal of the reinforced conditioned stimulus (CS+), but not for the startle response, autonomic and central measures. However, all subjects showed a successful CS+/CS- differentiation with regard to the psychophysiological measures. In contrast to the lack of stronger acquisition, PTSD patients exhibited a prolonged extinction compared to both other groups. This effect was observed for the self-report measures and electrodermal response as well as the US expectancy ratings. The findings of the present study did not fully support the hypothesis of a stronger acquisition of conditioned responses in PTSD patients, but rather suggests that a lack of extinction is crucial for the development or maintenance of PTSD. However, the stronger subjective acquisition of the conditioned response in PTSD patients is in line with cognitive theories of PTSD, proposing that they interpret trauma-related cues as current threat.
UP |
