Biology 202
1998 Third Web Reports
On Serendip

Determining the Intensity of a Memory

Karen Taverna

Two people can have the same exact experience but remember the event differently. This has always amazed me, especially when I am reminiscing with a friend and our memories of our relationship are slightly different. For example, we went on a road trip to the shore and when I recall the vacation, my first thought is of the car ride and how much fun it was. My friend thinks of our night at Seaside Heights. It isn't that I have repressed the memory of the rickety old rides and carnival games, I just don't think of that part of the vacation first. For some reason, the experience of singing along with our "road trip" cassette tape with our hair blowing in the wind made more of an impression on me. There are obviously different kinds of memories. There are memories concerning facts and figures, learned skills, emotions, faces, survival skills, long term, short term, vacations, etc. All of these types of memories are different for all people, and are different from experience to experience. This is due to the fact that the brain is always changing, and no two situations are identical. This is further clarified by the cliche; "You only get one chance to make a first impression." Once a person has an experience, her brain is permanently altered.

Memory is a vast topic and only the very surface will be scratched here. Several areas are mentioned, repressed memories and False Memory Syndrome, and Post Traumatic Stress Disorder. These topics enabled me to learn how some memories are purposely "forgotten", how memories effect behavior in the aftermath, and what dictates memory storage. Two structures of the brain linked to memory are the hippocampus and amygdala. Both are discussed with respect to their role in memory storage and what trends are noticed in subjects who are damaged in some manner either by experience or for the purposes of research. All this information is used to address the following question. Why are some experiences remembered so clearly and vividly while others are completely forgotten?

As I mentioned above in my personal anecdote, some memories seem to be "at the back of the mind" and are not recalled as strongly as others. Repressed memory is an extreme example of this. The person basically forgets a traumatic experience and this is associated with victims of physical and sexual assault, rape, and other devastatingly traumatic experiences. This repression may be the only way the victim knows how to deal with the circumstances. Perhaps the only way the victim can emotionally survive in their present environment especially if the abuser is a member of the family. There are advantages and disadvantages to memory repression. The advantage is that the victim can forget the painful experience but the disadvantage is that behavior can change. The experience did happen, and therefore, the chemistry of the brain did change even if the person doesn't "remember" the cause.

There is a large controversy over whether repressed memories are accurate or a symptom of False Memory Syndrome. This debate has a feminist spin on it because obviously women are victims of sexual and physical violence but their claims in most cases contested by the accused. The male example of traumatic experience is war and although the justness of a war can be argued, no one says to a veteran that the war didn't happen. (1) The accuracy of repressed memory is questioned. J Freyd addresses this question and asserts that recovered memories are just as accurate as other memories. (1) The corollary to this statement is that continuous memory is not "perfectly accurate" and is often distorted. With this in mind, repressed memories are just like continuous memories except for the fact that the experience has been forgotten by the I-function.

It is clear that an experience alters behavior and therefore the brain. The alteration may not be drastic or even noticeable but it exists. Post Traumatic Stress Disorder, (PTSD) is an example of memory so intense that the person has flashbacks and behavior is noticeably different. Typical symptoms are flashbacks or dreams where the experience is relived where the victim has a sense of numbness, detachment from others, unresponsiveness to surroundings, and avoidance of situations similar to that of the trauma. Also, rarely there are instances of "shell shock" where a stimuli triggers a re-enactment of the trauma. (2) This is clearly a different extreme of memory. The problem is that the memory keeps pushing its way to the forefront so that the experience is repeated. This disorder is a clear example of how experiences and the memory of them result in behavioral alterations.

Memory repression, and PTSD show two ways a memory can be integrated into a person's life either fully and vividly or seemingly not at all. The implications of this are that our experiences control us, we don't control them. So how does the brain change? What changes occur within the brain? New neurons are being synthesized and new synapses are forming continuously. The connections are influenced by past, present and suspected future experiences. Various neurochemicals have different effects on neurons. It was found that the storage of information in the brain is modulated by endogenous hormone systems or that the surrounding chemicals influence the new connections being made. (3) Traumatic experiences are high stress situations and particular chemicals are released to cause the fear and the anxiety that the victim experiences. In fact, people with PTSD tend to have abnormal levels of key hormones involved in stress response, in particular, cortisol present at lower levels and both norepinephrine and epinephrine are present at higher levels. (4) So what part of the brain signals for the release of these stress hormones? Evidence suggests that the control point for this is the amygdala, more specifically the region of the basolateral nucleus. (4)

The magnitude of how an experience is remembered is effected by the emotional state of the individual. From my anecdote, I was extremely happy in the car singing my favorite oldies and laughing with my friend. This is obviously why that part of the trip was so memorable for me. Apparently, my friend just wanted to get to the beach. Another example of how emotion effects memory deals with fear conditioning of rats. A study trained rats to fear an electric shock when a particular light appeared. When the amygdala of the conditioned rats was removed, the rats did not exhibit fear when the light went on. (5) The light stimulated the memory of fear until the amygdala was removed. Apparently, this made the rat forget that he was supposed to be afraid.

This shows how the amygdala influences memory recollection. The light would trigger the amygdala to release fear-causing chemicals so that the rat would remember the fear associated with the appearance of the light and prepares himself for a possible electric shock. The connection from the amygdala to the cortex is much stronger than the connection from the cortex to the amygdala. This indicates the importance of the "emotion" response to stimuli.(6) This implies that once the amygdala sends out its fear signal, the cortex can't shut if off right away because the connection is substantially weaker. The signal neurochemicals are released until the cortex can send a sufficient termination signal.

The amygdala influences memory storage and recollection by releasing hormones which effect other parts of the brain involved in memory functions. The second structure of the brain examined in this essay is the hippocampus which is clearly influenced by the amygdala. Sustained stress can damage the hippocampus. (7) The stress glucocortoids over time accelerate the death of hippocampus neurons. A study was done using magnetic resonance imaging to measure hippocampal volumes in patients who were victims of abuse. This study showed that the left side of the hippocampus was markedly reduced by an average of 5% its normal volume and that those subjects with smaller hippocampus, were more likely to have PTSD. (8) A study of PTSD patients indicated a hippocampal reduction of 13%. (8) This is clear evidence that experience changes the brain. The hippocampus is particularly sensitive to cortisol which after a stressful experience, can continue to circulate for extended periods of time. The experience caused a chemical change within the brain which altered the structure of the hippocampus.

This system seems counterproductive. An individual is in a stressful life -threatening situation. The brain has to initiate survival tactics. Apparently part of survival, is killing off a portion of the brain. When an army goes to war they don't say, "our army is just too large, Unit 345 line up before the firing squad!" So why does the brain do this? Is it a way of housekeeping? While in the stressful situation, the individual can either run or fight. Isn't that how survival works? Perhaps memories of past situations get in the way. When there is an instant to react, should the individual remember every experience of her life, analyze them, and then decided on a proper course of action? Of course not, what woman would ask her attacker, "Excuse me, can you wait a minute, I have only recalled the first half of my life. Could you give me some more time to decide what to do?" The amygdala jumps in signaling to the rest of the brain that there is danger and survival tactics are initiated.

Perhaps the implications of this "cleaning house" only effect humans because we have a sense of self that is so dependent on remembering our experiences and the part that is controlling those responses, the amygdala, is part of the old brain. He doesn't care about the new kid on the block, the I-function. The amygdala reacts with evolution-tested mechanisms, even if portions of the hippocampus are destroyed in the process.

The database of experiences is clearly altered by the chemical changes that are caused by emotions like anxiety and fear. Because portions of the hippocampus are under the influence of the stress chemicals, the storage of the experience happening at the time is effected. For example, the memory could be lost as in repression, or amplified as in PTSD. These are extreme examples but I refer again to my anecdote. The car ride was most memorable to me because I had a feeling of pleasure at the time. I don't remember what kind of cars were around me on the highway, or what clothing I was wearing, or even the date. I know that the ride was fun and the weather was beautiful. These are clearly emotionally influenced memories. Did I learn anything during the trip? Perhaps I learned the words to a song on the radio, or that to maintain 65mph I have to hold my foot at a particular angle on the accelerator, but these aren't what I remember about the ride. The non-declarative section of my brain surely remembers these things but I don't. My amygdala decided what information was appropriate for the use of my I-function and influenced the storage of my memories accordingly.

Over time memories fade. Is this due to neuron death in the hippocampus? Have those memories served their purpose and the neuron connections are degraded to make way for new memories? This examination of memory has created more questions but it is clear that the emotion felt at the time of the experience effects the intensity of the memory.


(1) Women's Review Letter by J Freyd.

(2) Post Traumatic Stress Disorder: European Description.

(3) Research Interests: Neurobiology of Learning and Memory. James L. McGaugh.

(4) Research Brings Hope for Veterans....who Suffer from Post Traumatic Stress Disorder. Press Release National Institute of Mental Health.

(5) While you were sleeping: Recent Discoveries About Amnesia. Chang, Tawen.

(6) Emotion, Memory, and the Brain: What the Lab does and why we do it Ledoux Lab. New York University.

(7) Neuroendocrinology. Robert M. Sapolsky.

(8) Hidden Scars: Sexual and other abuse may alter a brain region. Mukerjee, Madhusree. Scientific American. Reprinted from Oct1995.

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