Incubation of conditioning-specific reflex modification: implications for Post Traumatic Stress Disorder

Abstract

Incubation of fear has been used to account for the delayed manifestation of symptoms of fear and anxiety including the delayed onset of Post Traumatic Stress Disorder (PTSD). We have shown the utility of classical conditioning-specific modification of the rabbit nictitating membrane response (NMR) as a model of PTSD. This modification includes an exaggeration in the size and a change in the timing of the unconditioned NMR after several days of classical conditioning. To assess the effects of incubation on conditioning-specific modification, we measured changes in responding as a function of the time between classical conditioning and NMR testing. After just one day of classical conditioning resulting in modest levels of learning, increases in response size were an inverted-U shaped function of days of incubation with little if any change occurring one and ten days after training but significant change occurring after six days. The incubation effect persisted for a week. An unpaired control group showed no change in the size of the response confirming the incubation effect was associative. The results bear a striking resemblance to symptoms of PTSD that do not always occur immediately after trauma and become exacerbated over time and then persist. They point to a window when incubation can exacerbate symptoms and speak to the vulnerability of re-experiencing trauma too soon. This could be a serious problem for military or emergency personnel recalled to combat or a disaster site without sufficient time to deal with the effects of their initial experiences.

Figure 1

Mean (± SEM) percent conditioned responses (CRs) to the conditioned stimulus as a function of 20-trial blocks during conditioned stimulus-unconditioned stimulus pairings or unpaired stimulus presentations during Acquisition and to conditioned stimulus-alone presentations during Extinction. Extinction took place following one, six or ten days of incubation and two Post Test sessions (Post1 and Post2) that were separated by seven days.

Figure 2

The three panels show sample unconditioned response topographies for individual rabbits as a function of the first 20-trial block of unconditioned stimulus test intensities (0.1, 0.25, 0.5, 1.0 and 2.0 mA) presented before conditioned stimulus-unconditioned stimulus pairings (Pretest, dotted line), after one, six or ten days of incubation (Post Test1, red line) and again seven days later (Post Test2, blue line).

Figure 3

Mean (± SEM) response frequency (Percent Responding), peak latency in milliseconds, magnitude in millimeters and area in arbitrary units (Magnitude of Area) for paired subjects as a function of the first 20-trial block of unconditioned stimulus test intensities (0.1, 0.25, 0.5, 1.0 and 2.0 mA) presented before conditioned stimulus-unconditioned stimulus pairings (Pretest), after one, six or ten days of incubation (Post Test1) and again seven days later (Post Test2).

Figure 4

The two panels show sample unconditioned response topographies for an individual rabbit from the unpaired (top) group and a rabbit from the paired group (different from the one in Figure 2) as a function of the first 20-trial block of unconditioned stimulus test intensities (0.1, 0.25, 0.5, 1.0 and 2.0 mA) presented before conditioned stimulus-unconditioned stimulus pairings (Pretest, dotted line), after one, six or ten days of incubation (Post Test1, red line) and again seven days later (Post Test2, blue line).