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Neural computations of threat affect PTSD symptoms after combat trauma

How the brain and body respond to learning danger and safety depended on the severity of PTSD symptoms in veterans with combat trauma, according to study findings.

During a threat reversal learning experiment, highly affected combat veterans were more sensitive when their predictions about outcomes were wrong, while symptomatic veterans assigned more weight to prediction errors.

Prior research has indicated impaired learning processes in PTSD, such as overgeneralization, heightened contextual anxiety, lower inhibition in response to safety cues and inability to retain extinction learning, according to Philipp Homan, MD, PhD, from the department of psychiatry, Icahn School of Medicine at Mount Sinai, and colleagues.

“These findings link PTSD to basic learning processes, but they do not disambiguate specific aspects of learning that may contribute to the disorder, such as the learning rate or the computation of aversive value,” they wrote in Nature Neurosciences. “It is possible that PTSD-related abnormalities are influenced by learning parameters that we cannot directly observe but are able to infer from observable behavior.”

In their study, researchers used the threat reversal paradigm plus computational modeling to determine latent learning parameters that are relevant for PTSD symptomatology in 55 combat-exposed veterans (24 with PTSD and 30 without) who completed a reversal learning task.

The task presented two visual stimuli (mildly angry faces) paired with a mildly aversive stimulus, according to a press release. Participants learned to associate one face with the mildly aversive stimulus in the first phase of the task then to associate the second face with the aversive stimulus in the second phase.

The investigators found that highly symptomatic veterans showed greater physiological adjustment to cues that did not predict what they had expected (measured by skin conductance responses), which indicated greater sensitivity to prediction errors for negative outcomes. In addition, both smaller amygdala volume and decreased neural tracking of the negative value of the face stimuli in the amygdala independently predicted PTSD symptom severity.

The results also revealed differences in value tracking and associability in other brain regions involved in computation linked to threat learning (ie, the striatum, the hippocampus and the dorsal anterior cingulate cortex), according to the press release.

"What these results tell us is that PTSD symptom severity is reflected in how combat veterans respond to negative surprises in the environment — when predicted outcomes are not as expected — and the way in which the brain is attuned to these stimuli is different," Daniela Schiller, PhD, from the Friedman Brain Institute, Icahn School of Medicine at Mount Sinai, said in the release. "This gives us a more fine-grained understanding of how learning processes may go awry in the aftermath of combat trauma and provides more specific targets for treatment." – by Savannah Demko

Disclosure: The authors report no relevant financial disclosures.