The vivid, intrusive visual images that are a hallmark of post-traumatic disorder (PTSD) are based on a separate memory system from intrusive verbal thoughts. That's according to a new study that claims to provide empirical support for psychologist Chris Brewin's dual-representation theory of PTSD.
Brewin's theory posits two memory systems, one that's largely sensation-based, inflexible and automatically accessed and another that's more deliberately accessible, containing material that is contextualised and can be easily put into words. By this account, a traumatic event can end up lodged in the sensation-based memory system, leading to sensory intrusions - 'flashbacks' - of the event being easily triggered by sights, sounds and smells that are reminiscent of the original experience.
The new study involved 79 participants watching traumatic video footage of car crash scenes, including commentary on the accidents and people involved. Crucially, some of the participants were told to keep still while they watched the footage and others were hypnotised so that they couldn't move. Past research has shown that keeping still cranks up the trauma simulation, perhaps because it is reminiscent of being frozen in terror or trapped. A final group were free to move. For a week after watching the car-crash videos the participants kept a diary of intrusive verbal thoughts and visual images associated with the videos. The key finding was that participants who had to keep still while watching the videos had significantly more intrusive visual images than the participants who were allowed to move. By contrast, the number of intrusive verbal thoughts did not differ between the groups.
A second study largely replicated the first, except rather than some participants being allowed to move while others kept still, this time some participants watched a neutral film while others watched the traumatic car-crash film. The traumatic film led to more intrusive visual images, but not more intrusive verbal thoughts, than the neutral film.
In both studies, participants who reported feeling more anxious and horrified after the traumatic videos tended to also experience more intrusive visual imagery. In contrast, intrusive verbal thoughts were not connected to mood effects in this way.
Taken altogether Hagenaars and her team said their findings suggest that intrusive visual imagery is a separate phenomenon from intrusive verbal thoughts and can be manipulated independently. 'Understanding these basic processes is likely to be valuable in developing more effective treatments for PTSD that focus on maximising change in verbal thoughts and intrusive images separately,' they concluded.
Hagenaars, M.A., Brewin, C.R., van Minnen, A., Holmes, E.A., & Hoogduin, K.A.L. (2010). Intrusive images and intrusive thoughts as different phenomena: Two experimental studies. Memory, 18 (1), 76-84 DOI: 10.1080/09658210903476522
Post written by Christian Jarrett (@psych_writer) for the BPS Research Digest.
Image credit: Symphonie