Effect of darkness on acoustic startle in Vietnam veterans with PTSD

Exaggerated startle is a symptom of posttraumatic stress disorder (PTSD), but empirical studies have not consistently documented elevated baseline startle in PTSD. The authors proposed in a previous study that Vietnam veterans with PTSD exhibit exaggerated startle only under stressful conditions. They reported that darkness facilitated startle in humans, suggesting that the startle reflex is sensitive to the aversive nature of darkness. In the present study they tested the hypothesis that the magnitude of facilitation of startle by darkness would be greater in Vietnam veterans with PTSD than in comparison groups of subjects without PTSD. Prepulse inhibition was also investigated.

The magnitude of startle and prepulse inhibition were assessed in alternating periods of darkness and light in 19 nonmedicated Vietnam veterans with PTSD, 13 Vietnam veterans without PTSD, and 20 civilians without PTSD.

The overall startle level was higher in the veterans with PTSD than in either of the two groups of subjects without PTSD. Startle was facilitated by darkness, and the magnitude of this facilitation was greater in the veterans with PTSD than in the civilians without PTSD, but it was not greater in the veterans without PTSD. Prepulse inhibition was not affected by darkness and did not significantly differ among groups.

Contrary to the hypothesis, elevated sensitivity to darkness was specific to individuals with combat experience, not to individuals with PTSD, perhaps because veterans had become aversively conditioned to darkness during their combat experiences. The more general increase in startle reactivity in the veterans with PTSD is consistent with clinical observations and descriptions of symptoms in DSM-IV.