Why Behaviour becomes Unreasonable


This article (1) looks at how sufferers of PTSD  have heightened activation of the emotional or fear based part of the brain, the amgydaloid part of the brain.

We have suggested before in various blogs that this hyperactive amgydaloid region takes over from the more reasonable prefrontal cortex in controlling behaviour. So there is an increased tendency to react rather than reflectively act.

In previous blogs we observed how this amgydaloid hyperactivity prompts responding characterized by greater dorsolateral striatal-dependent habit learning, to the detriment of hippocampus-dependent memory.

Moreover, this stress/anxiety-induced habit bias appears to depend on the basolateral amygdala. The idea that a similar mechanism in part underlies the development, persistence and expression of avoidance behaviors in PTSD is supported.

In other words, under certain situations of stress or heightened emotional arousal there is a tendency for the amgydaloid area to prompt the more habit based, automatic and motoric parts of the brain to prepare us to move and to take action – akin to a “fight or flight response” so a PTSD brain is in a sense primed to react.

“Several recent functional neuroimaging studies have provided evidence consistent with amygdala hyperresponsivity during exposure to traumatic reminders in posttraumatic stress disorder (PTSD).1– 6 In addition, PTSD symptom severity is positively correlated with blood flow in the amygdala during such exposure.3,5,7

To characterize the scope of amygdala hyperresponsivity in PTSD, it is important to determine whether amygdala responses to stimuli unrelated to trauma are also exaggerated in this disorder.

Previous research has established that the normal human amygdala is responsive to facial expressions of fear8– 11 and that individuals with PTSD exhibit exaggerated amygdala responses to these facial expressions. Using functional magnetic resonance imaging (fMRI), Rauch and colleagues12 measured blood oxygenation level–dependent (BOLD) signal responses to backwardly masked fearful and happy facial expressions in combat veterans with and without PTSD. In that study, the fearful and happy facial expressions were presented very briefly (33 milliseconds) and were followed immediately by neutral facial expressions.11

Relative to the control group, the PTSD group exhibited greater BOLD signal increases in the right amygdala in response to masked fearful vs happy facial expressions. In addition, PTSD symptom severity was positively correlated with BOLD signal changes in the amygdala.

Several functional neuroimaging studies have demonstrated relatively diminished activation in the subcallosal,13,14 anterior cingulate,15– 17 and medial frontal gyri5,16 in this disorder.”

We have discussed hypoactivation of the anterior cingulate previously as being part of a stress dysregulation mechanism – the hypoactivation is akin to a switch in behavioural control between prefrontal control to dorsal striatal  (via hyperactive amgydaloid activity).

This anterior cingulate hypactivation can also switch memory systems for hippocampal to dorsal, from explicit, “knowledge of things” memory, to habit/procedural/”know how to do things” memory system.

So PTSD can involved implicit (“in the bones”) memory and reaction not just hippocampal “remembering everything that occurred” flashback memory. It can also be a habituated responding to emotionally arousing stimuli without remembering the exact event, more a reliving of the feelings and emotions of the traumatic event instead.

Image not available.

“In addition, PTSD symptom severity is negatively correlated with blood flow in the medial frontal gyrus during traumatic imagery and recollection.5 

In the present study, the PTSD group exhibited exaggerated amygdala responses and diminished medial prefrontal cortex responses to overtly presented fearful vs happy facial expressions. In addition, only in the PTSD group were BOLD signal changes in the amygdala negatively correlated with signal changes in the medial prefrontal cortex. Furthermore, in the PTSD group, symptom severity was negatively related to BOLD signal changes in the medial prefrontal cortex. Finally, relative to the control group, the PTSD group tended to exhibit diminished habituation of fearful vs happy responses in the right amygdala.”

In our next blog we look at how PTSD affects different memory systems and via this control of memory controls behavioural responses and how these may relate to similar mechanisms in addiction…


Shin, L. M., Wright, C. I., Cannistraro, P. A., Wedig, M. M., McMullin, K., Martis, B., … & Rauch, S. L. (2005). A functional magnetic resonance imaging study of amygdala and medial prefrontal cortex responses to overtly presented fearful faces in posttraumatic stress disorder. Archives of general psychiatry,62(3), 273-281.


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Bren Anne

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