Memory and trauma

Damage to different areas of the brain can have varied effects on memory. The temporal lobes, on the sides of the brain, contain the hippocampus and amygdala, and therefore have a lot to do with memory transition and formation. Patients who have had injury to this area have experienced problems creating new long-term memories. For example, the most studied individual in the history of brain research, HM, retained his previously stored long-term memory as well as functional short-term memory, but was unable to remember anything after it was out of his short-term memory.[4] A patient whose fornix was damaged bilaterally developed severe anterograde amnesia but no effect on any other forms of memory or cognition.[5]

The Amygdala is known as the "fear center of the brain," and is thought to be activated and regulated in response to stressful situations marked with perceived heightened stimulation. Specifically, the Amygdala is responsible for identifying threats of danger to self and safety.[7] Consistent exposure to trauma and or stress, may lead to over-perception and heighten responsibility and sensitivity to threat.[6] Increased activation in the fear center can impact communication with other circuits in the brain structure including connections between the PFC, amygdala, and hippocampus[6] which can in turn affect how memory are stored by the hippocampus. It is important to note that the amygdala does not fully develop until the late 20's and stress experienced prior to that age may have more extensive impact compared to stress experienced after amygdala is fully developed.[8]

The PFC is a brain structure responsible for executive functioning skills. Included in executive function abilities are emotional regulation, impulse control, mental cognition, and working memory among many other abilities.The PFC is also in charge of modulating response from the Amygdala. However, during high-stress situations, the Amygdala can suppress higher thinking functions of the PFC.[7] Some PFC functions that may be impacted during traumatic stress include; failure of emotion reappraisal, heightened salience of emotional stimuli, failure to inhibit neuro-endocrine response to threat stimuli, inability to maintain or use extinction of conditioned fear.[6]). People who have experienced trauma, especially chronic and ongoing trauma, may be observed to have under-activation of multiple parts of the PFC.[7] Under activation of the PFC can lead to decreased modulation of the amygdala during a stress response.[6] The PFC in most vulnerable to the effects of stress during adolescent and a traumatic event during this time period would have more extensive changes than stress experience when the PFC is fully developed around the mid 20's.[8]

The hippocampus is considered the memory center for the brain and is responsible for storing, encoding, retrieving, and reconsolidating memories.[7] ).Studies indicate that people who have PTSD may have a "shrunken" hippocampus, some estimates indicate that there may be up to a decrease in a range of 5-26%.[9] However, there are alternative explanations to account for the observed decreased hippocampus volume. One study by Gilbertson et al. (2002), suggests that perhaps decrease hippocampal volume may be a pre-existing factor that may predispose people for the development of PTSD.[10] There are conflicting interpretation in understanding if decreased hippocampal volumes are a consequence or a pre-existing vulnerability associated with PTSD.[10][9] While it is unclear if decreased hippocampal volume is evident as a consequence or prior to the traumatic event; there are numerous studies that indicate the hippocampus in under active during traumatic events and potentially also under-active after the event as well.[6][7][9][10] An underactive or dysregulated Hippocampus has many clinical implications including in areas of neurogenesis, disturbances to organization of memory, and ability to impact other endocrine functions such as a stress-response.[7][11][12]

The hippocampus is a major site of neurogenesis, it is where new neurons are born, impact to neurogenesis can have multiple implications. Some studies suggest that blocking of neurogenesis may have the ability to block the efficacy of anti-depressants which are used to treat symptoms of depression.[11] According to the DSM-5 there is comorbidity among depression and PTSD. In addition to comorbidity rates, the symptoms of PTSD and Major Depression Disorder (MDD) also have some overlap. Specifically, both list negative alteration in mood and cognitive disturbances as a symptom, underlying the idea of a "c" factor or a cognitive dysfunction that can be seen as a transdiagnostic dimension of psychopathology.[13] Trauma can impact the hippocampus and may have global implications in mood and symptom progression through the impacts on neurogenesis.[11]

Changes to the hippocampus also may have impact to a person's ability to recall the traumatic experience[7] and produce a trauma narrative .[1] There have been studies that further expand on how trauma can impact victims ability to recall traumatic events.[1][2] These memory difficulties in identifying, labeling, and completely processing the traumatic event can be targets for treatment through psychotherapy.[14] The age of a person when they experience a traumatic event can also modulate the effects of the hippocampus.[8] Particularly, the hippocampus is developing from birth to age 2 and is most vulnerable to the effects of stress during this time period.[8] During adolescence the hippocampus is fully organized and less vulnerable to the effects of stress.[8]

The Hippocampus also has connections with the body's stress response system.[9] The hippocampus is responsible for the negative feedback regulation of the Hypothalamic- pituitary-adrenal axis.[12] Failure to regulate stress response through the HPA axis can have long-term health effects through the experience of chronic stress.[12]

Of the different aspects of memory – working, short-term, long-term, etc. – the one most commonly affected by psychological trauma is long-term memory.[15] Missing memories, changes to memory, intensified memories – all are cases of manipulations of long-term memory.

Long-term memory is associated with many different areas of the brain including the hippocampus, amygdala, thalamus and hypothalamus, peripheral cortex and temporal cortex. The hippocampus and amygdala have been connected with transference of memory from short-term memory to long-term memory.[16] Thalamus and hypothalamus, located in the forebrain, are part of the limbic system; they are responsible for regulating different hormones and emotional and physical reactions to situations, including emotional stress or trauma. The thalamus is also related to reception of information and transferring the information, in the case of memories, to the cerebral cortex.

Psychological trauma has great effects on physical aspects of patients' brains, to the point that it can have detrimental effects akin to actual physical brain damage. The hippocampus, as mentioned above, is involved in the transference of short-term memories to long-term memories and it is especially sensitive to stress. Stress causes glucocorticoids (GCs), adrenal hormones, to be secreted and sustained exposure to these hormones can cause neural degeneration. The hippocampus is a principal target site for GCs and therefore experiences a severity of neuronal damage that other areas of the brain do not.[17] In severe trauma patients, especially those with post-traumatic stress disorder, the medial prefrontal cortex is volumetrically smaller in size than normal and is hyporesponsive when performing cognitive tasks, which could be a cause of involuntary recollection (intrusive thoughts).[18] The medial prefrontal cortex controls emotional responsiveness and conditioned fear responses to fear-inducing stimuli by interacting with the amygdala. In those cases, the metabolism in some parts of the medial prefrontal cortex didn't activate as they were supposed to when compared to those of a healthy subject.

As with many areas of psychology, most of these effects are under constant review, trial, and dissent within the scientific world regarding the validity of each topic.

Perhaps one of the most controversial and well-known of the psychological effects trauma can have on patients is repressed memory. The theory/reality of repressed memory is the idea that an event is so traumatic, that the memory was not forgotten in the traditional sense, or kept secret in shame or fear, but removed from the conscious mind, still present in the long-term memory but hidden from the patient's knowledge.[19] Sigmund Freud originated the concept of repression and it has developed and changed since his original work.[20] In the eyes of critics of repressed memory, it is synonymous with false memory; however its proponents will argue that these people truly did have traumatic experiences.

Intrusive thoughts are defined as unwelcome, involuntary thoughts, images or unpleasant ideas that may become obsessions, are upsetting or distressing, and can be difficult to be free of and manage. In patients who have suffered from traumatic events, especially those with post-traumatic stress disorder, depression or obsessive-compulsive disorder, the thoughts are not as easy to ignore and can become troubling and severe. These thoughts are not typically acted on; the obsession of the thoughts usually comes from intense guilt, shame or anxiety relating to the fact that the patient is having the thoughts to begin with so they are unlikely to actually act on things they feel so badly about. In trauma patients, the intrusive thoughts are typically memories from traumatic experiences that come at unexpected and unwanted times. The primary difference from other intrusive thoughts for patients is that the memories are real rather than imagined.

Emotion is a large part of trauma, especially near death experiences. The effect emotions have on memory in different instances is an integral part of the effect of trauma on memory. Emotional events tend to be recalled with more frequency and clarity than memories not associated with extreme emotions.[21] Typically traumatic events, such as physical attack or sexual abuse, are interrelated with strong negative emotions, causing these memories to be very strong and more easily recalled than memories not associated with similar emotions, or even those connected to positive emotions. Emotion's strong connection with memory also has to do with how much attention or focus an individual has on the event. If they are heavily emotionally involved in the event, a lot of their attention is directed at what's happening, rendering it a stronger memory. It is also the case with emotionally aroused situations that even if attention is limited, it is more likely that a memory associated with the strong emotion will remain as opposed to some neutral stimulus. Chemically, this is because the emotional and physical stress caused by traumatic events creates an almost identical stimulation in the brain to the physiological condition that heightens memory retention. It excites the neuron-chemical activity centers of the brain that affects memory encoding and recollection.[22] This reaction has been enforced by evolution as learning from high-stress environments is necessary in "fight or flight" decisions that characterize human survival.