Traumatic events are encoded (converted) differently than more routine, everyday experiences in life. It is well known within the scientific and psychological communities that human memory and recall do not function like a tape recorder, faithfully recording events later to be recalled on command. Our memories are fallible and have gaps and inconsistencies. As a result, we recall and narrate traumatic events differently than routine events.
Memory and Recall: Some General Points
Memory is essentially the capacity for storing and retrieving information. Three processes are involved in memory: encoding, storage, and recall.
First we receive the information (e.g., from what we see, hear, and understand). Then we convert the information so it can be stored in various parts of the brain. There are three main ways in which information can be encoded: visual, acoustic and semantic. When encoding an event, we focus more attention on aspects that our brain appraises as important and less on those deemed insignificant.
This differential focus is what memory scientists refer to as central versus peripheral details. Memory retrieval refers to the access, selection, reactivation, or reconstruction of stored internal representations. Additionally, over time memory works to edit information, and we lose memories, forget some details of memories we do retain, and modify aspects of other memories as the result of repeated retrievals.
Our brain-based memory systems have been sculpted to function adaptively. Memories of trauma are like normal memories in these respects, but they have important characteristics that make them much different from normal, everyday memories.
However, after being traumatized certain central events may be remembered forever and this is an adaptive outcome. The brain has learned “this is important, remember this because it could later save your life.” To understand this more fully it is necessary to look at two key brain structures, the hippocampus and the amygdala.
The Hippocampus and the Amygdala: Encoding and Consolidating Memory
The hippocampus and the amygdala are two brain structures that encode memory. The hippocampus is responsible for putting experience into chronological order and into perspective; it is necessary for forming new explicit memories. Explicit memory is the conscious, intentional recollection of factual information, previous experiences, and concepts.
For explicit memory, we need the hippocampus. This part of the brain is responsible for integrating the raw sensory data into a coherent picture, putting a time tag on it, and transferring it into long-term episodic memory, where it can be retrieved later. Over time, when memory is consolidated, its long-term storage is distributed in different parts of the neocortex.
The amygdala, part of the limbic system, catalogues past sensory experiences (threats, anger) as implicit memories, memories that are unconscious but can affect thoughts and behaviors.
The amygdala, part of the limbic system, catalogues past sensory experiences (threats, anger) as implicit memories, memories that are unconscious but can affect thoughts and behaviours. These memories are associated with intense arousal making them readily primed in order to quickly associate them with future situations that are stressful or threatening. This is a critical survival feature of implicit memory, enabling an instant response to danger.
How Threat and Highly Stressful Events Affect Memory
Normally, the amygdala neurons encode fear memory traces (or fragments) while the hippocampus learns about the context of the fear. But when faced with threatening experiences, this emotionally arousing information increases amygdala activity. That activity correlates with more deeply remembered memory traces in the amygdala.
Stress and fear heighten activation of the amygdala. This reinforces and intensifies traumatic memories while at the same time impairing hippocampal function, which is involved in episodic or explicit memory. Victims whose memories are not integrated into their hippocampus and cortical circuitry have implicit or limbic memory traces (or fragments). This happens because the amygdala activates the HPA axis, resulting in a flood of neurohormones that interfere with hippocampal learning. This is why, after a stressful situation, people have trouble remembering some specific details, and say things like, “It was all a blur.”
Implicit memory, also called procedural or sensorimotor memory, refers to behavioral knowledge of an experience without conscious recall. It is not a memory we can reflect on or think about. These memories are impossible to verbalize. They are often fragmented in time, and for the most part consist of primary sensory information (images, smells, sounds) that are linked to physiological fear symptoms.
Intensified Traumatic Memories: Flashbulb Memories and the Hippocampus in Overdrive
The effect of fear, threat or states of intense stress on memory can result in intensified memory recollection, or it can result in fragmented or impaired memories. Both are the result of the stress hormones released (the HPA axis) when the defense circuitry is activated.
Some elements of traumatic memories are more acutely remembered. The adrenal glands release adrenaline, which has been shown to help encode memories to the hippocampus more intensely.
A burst of adrenaline is thought to enhance memory storage of the events closer to the onset of a traumatic or highly stressful event. This strengthens memory pathways and creates what are referred to as “flashbulb memories.”
Experiences with emotional significance are more likely to be consolidated into episodic memory and made available for intentional, conscious recollection than those with little or no emotional significance. The brain encodes what it pays attention to. During a threatening event, the brain focuses on what is central to survival so it does not focus on insignificant and peripheral details, so it does not encode them.
During a threatening event, the brain focuses on what is central to survival so it does not focus on insignificant and peripheral details, so it does not encode them.
From Intensified to Fragmented Memories
When high levels of the stress hormone cortisol are secreted, along with adrenaline, the hippocampus super-encodes these intense early moments of the event. Following this, if the threat or fear continue, the hippocampus continues to be flooded with stress hormones and it is temporarily impaired and there may be minimal encoding. That is how the hippocampus goes from flashbulb mode to fragmentary mode.
Not only can the hippocampus not integrate various systems of attention and memory, it also disrupts the storage of information. The hippocampus can disrupt memory encoding for conscious explicit memory when it is blocked or damaged by stress hormones or inhibited by intense amygdala activation.
The hippocampus can disrupt memory encoding for conscious explicit memory when it is blocked or damaged by stress hormones or inhibited by intense amygdala activation.
The amygdala is critically involved in calculating the emotional significance of events. When it perceives a threat, it creates emotional arousal. This is intended to alert us to pay attention and be ready to respond. The amygdala also has a selective effect on the particular stimuli we notice and encode. Fear focuses one’s attention on a few details at the expense of a lot of others.
Fear focuses one’s attention on a few details at the expense of a lot of others.
During states of fear (high arousal), the hippocampal and amygdala networks can become dissociated, resulting in a disconnection between the emotional memories of the amygdala and explicit hippocampal processing.
Sensation, emotion, behaviour, and conscious awareness, which are usually integrated with one another, can be disconnected from their context in time and space. As a result, few peripheral details, little or no context or time‐sequence information, and no words or narrative surrounding the memory may be recalled.
How Attention and Memory Affect Recall of Traumatic Events
Much of what is remembered of a traumatic or threatening event functions as if existing in separate islands of memory.
Information encoding and storage are impaired for aspects of the experiences that are not considered essential for survival or are of little emotional importance. This includes the sequence of events as well as peripheral details. This often results in a disorganized and incomplete narrative memory.
Enhanced Traumatic Memory Coexists with Incomplete Memory
Some elements of traumatic memories are actually more acutely remembered than others. These are called enhanced memories. They are etched more deeply in our memories precisely because they are traumatic and overwhelming to us.
It is neither realistic, nor rational, to expect victims of sexual assault to recall all aspects of their traumatic experiences with detailed accuracy from start to finish. That is not how the brain works when the defence circuitry has kicked in. Understanding this is part of what a trauma-informed criminal justice system requires if justice is to be done and fair trials are to be conducted for the accused and for victims.