There is a century old debate in psychology regarding the method the brain employs to store memories. A recent article in the Scientific American (February, 2013) contained an article that spoke to this very debate. It attempts to revive the theory that the human brain stores memories in specific neurons. Put another way, any memory encoded is programmed into a discrete cluster of neurons within the brain. For example, the authors reported they discovered the exact brain cells that encoded the image of Jennifer Aniston (actress) in someone’s brain. Quite impressive. The competing theory is that the features of any memory are distributed across the brain. Rather than being stored with Ms. Aniston in a particular area of the temporal lobe, this same area is conjectured to be the site where the location of each sense memory is stored. For example, the name of Jennifer Aniston would be stored in the area of the brain that retains words, her face would be stored in a different area that stores faces, and her body type would be stored in an area that retains spatial details. This retrieval strategy has similarities to the library card catalogue system. The actual books (content) are not stored in the library cards, but rather the cards store the location where the content may be found.
This article is perhaps most notable for its contradictions than it is for presenting new ideas. It is very seductive to believe there is an exact area of the brain that serves as a storage sight of each memory. It corresponds to the way we store objects in our everyday life. Nothing is easier or more attractive than to relate complicated processes to our everyday experience. Contrary to this end, the authors (to their credit) admit that brain cells that fired to a picture of Jennifer Aniston also fired to a picture of another blond actress. This tends to disprove their contention of discrete cellular representations of each memory. Another objection advanced by the authors is that there is not enough brain cells to encode every new experience in the human brain. Essentially, our brain would fill with memories until the bucket could hold no more. The authors believe the solution to this contradiction is that a “typical person remembers no more than 10,000 concepts.” Their notion of “concept” is problematic, even beyond their vague definition as to what defines a “concept.” The notion that all information regarding objects and their relations should hover around 10,000 bits is difficult to accept.
The authors advance another contradiction as proof of their theory. They cite the famous case of a man whose hippocampus was surgically destroyed. The hippocampus is a medial temporal lobe structure where the authors believe all our memories are stored. This unfortunate man could not encode new memories at all, but he enjoyed complete retention of all his old memories-until the time of the surgery. This strongly contradicts the authors contention of exact cellular storage of each memory. Given that the hippocampus was destroyed by a surgeon, this man should lose all his old memories as well. He would literally become a tabula rasa; incapable of even drawing anything new upon the slate.
Even without a detailed knowledge of neuroscience, a critical reading of this article would alert the reader to fundamental problems. In psychology it is called “internal consistency.” This refers to a story having a logical progression that avoids contradictions. The authors brought up the contradictions in order to dispel these arguments; long advanced by psychologists. They tended to reframe the contradictions in an effort to prove their theory; rather breath new life into an old theory. This is a perfectly normal error that bedevils all human thought. Once humans formulate a goal, we will tend to gloss over the contradictions to obtain that goal. It may be termed “messaging the data” to ensure that data proves the desired outcome. Science is the bulwark protecting us from ourselves. Always maintain a critical eye. These authors have a bit of wool over theirs!
Memories are typically useful and pleasant. It is fortunate that relatively few are unpleasant and aversive. Both pleasant and unpleasant memories are sustained by the same brain system, though the strength of the memories may be unequal. It is advantageous for a person to encode emotionally charged information over neutral memories. Events that provoke an emotional reaction tend to leave a stronger trace than those that do not leave an emotional impression. Remembering where food poisoning was contracted is likely more important than the lyrics to a song. The first memory avoids illness or death while the second avoids boredom. Obviously, survival of the organism is more dependent on the first memory than the second. The trouble begins when the emotionally charged memory trace is over-activated. It transforms the memory from helpful and adaptive to unpleasant and aversive. Moreover, the overactive memory may decrease adaption and jeopardize survival. The memory of contracting food poisoning may make the person afraid of all food sources. The once useful memory may become a liability.
Posttraumatic Stress Disorder evolves by this mechanism. Being harmed or confronted with death is highly emotionally charged, and will certainly make a lasting impression. This is a necessary adaption in order to avoid harm in the future. Avoidance of specific situations that risk harm to the organism is necessary for survival. Avoidance of all situations that are even remotely related to the original is called over-generalization. The more the fear is generalized to related stimuli, the greater the overall impairment in the person’s functioning. A man holding an umbrella may generate as much fear as that of a man holding a gun. The over-generalization becomes increasingly entrenched. Repeated alarming experiences that are close to the initial trauma are reinforced. It is not only the gun that that generates anxiety and fear, but any object that remotely resembles a gun. Each time the person has frightful reactions to related stimuli, the maladaptive memories become stronger and stronger. Like any road, furrows widen and deepen with use. The longer the maladaptive memory persists, the wider and deeper it becomes.
The act of filling in the maladaptive furrows with neutral recollection is termed extinction. It is a very difficult process for the patient, in that it entails approaching the feared stimuli. Moreover, it is very common for the extinguished traumatic memory to reappear months later. This can be very disconcerting, as the patient and doctor believe the trauma is extinguished, only to reappear at a later date. It is only natural for the patient to become increasingly hopeless that their suffering will be alleviated. They feel powerless in the face of their traumatic memory. Recently, there has been progress towards increasing the effectiveness of extinction training; both behavioral and medicinal.
It appears that the extinction of traumatic memories may be accelerated by drugs that block the physical aspects of an emotional response. The beta blocker propranolol blocks a neurotransmitter that induces excitation in the brain. It may help the brain to reconsolidate traumatic memories in a less fearful way. Unfortunately, the research has been inconclusive as to its efficacy during extinction training. Some studies indicate that it is useful in this regard, others do not. A new behavioral approach has received greater experimental support, and does not have the typical disadvantages of medication. Researchers (principally Elizabeth Phelps at NYU) have included an extra reminder trail ten minutes prior to the usual extinction trial. For example, an extinction trial for the person with food poisoning may consist of pairing pictures of the food with its odor. As the trials proceed, the odor is gradually removed until the patient no longer displays a reaction to the feared object-say a hot dog. It appears that merely showing the picture without the odor, ten minutes prior to the extinction trial, significantly increases the effectiveness of the procedure. It is believed that the reminder trial opens the brain to change during the extinction trial. The effectiveness of the extinction is enhanced if the brain is already considering the feared stimuli. There may be a window during recollection where memories are vulnerable to alteration.
It is hoped that this new technique will significantly enhance behavioral treatment of Posttraumatic Stress Disorder. The reminder session during extinction may shed light on how humans consolidate their memories. This process has been studied intensively for decades, and yet remains murky. If this new technique receives additional experimental support, it may not only help those suffering with traumatic memories, but reveal a fundamental property of how we form memories. It’s two revelations for the price of one. A bargain at any price.