Jacek Debiec, ‘Memories of trauma are unique because of how brains and bodies respond to threat’, https://theconversation.com/memories-of-trauma-are-unique-because-of-how-brains-and-bodies-respond-to-threat-103725, The Conversation (September 24, 2018)

Two-roads model amygdala

Biological research over the past few decades has made significant progress in understanding how the brain responds to threat. Defense responses are controlled by neural systems that human beings have inherited from our distant evolutionary ancestors. (…) One of the key players is the amygdala, a structure located deep in the medial temporal lobe, one on each side of the brain. It processes sensory threat information and sends outputs to other brain sites, such as the hypothalamus, which is responsible for the release of stress hormones, or brain stem areas, which control levels of alertness and automatic behaviors, including immobility or freezing. (…) Research in animals and more recently in people suggests the existence of two possible routes by which the amygdala receives sensory information. The first route, called the low road, provides the amygdala with a rapid, but imprecise, signal from the sensory thalamus. This circuit is believed to be responsible for the immediate, unconscious responses to threat. (…) The high road is routed through the cortical sensory areas and delivers more complex and detailed representations of threat to the amygdala. Researchers believe the high road is involved in processing the aspects of threats of which a person is consciously aware. (…) The two-roads model explains how responses to a threat can be initiated even before you become consciously aware of it. (…) Studies show that the amygdala is critical for encoding and storing associations between a harmful and neutral stimuli, and that stress hormones and mediators – such as cortisol and norepinephrine – play an important role in the formation of threat associations. (…) Researchers believe traumatic memories are a kind of conditioned threat response. (…) Memories are biological phenomena and as such are dynamic. Exposure to cues that trigger the recall or retrieval of traumatic memories activates the neural systems that are storing the memories. This includes electrical activation of the neural circuits, as well as underlying intracellular processes. (…) Reactivated memories are susceptible to modification. (…) It’s possible to link the trauma to other experiences and diminish its destructive impact. Psychologists call this post-traumatic growth.