MEMORY [ back to Glossary Index ]
Memory is a critical faculty for survival, because it records lessons from experience to guide future behavior in an adaptive manner. Persons with severe memory dysfunction do not learn from experience, no matter how negative the consequences of their behavior. They cannot integrate their past and future, and live in an eternal present. TBI tends to disrupt short term memory and leaves long term memories of events before the trauma intact, including personal experiences and acquired general knowledge such as vocabulary. By virtue of laboratory studies, memory has been classified into various types. Working Memory is processed in the frontal lobes, which "register" a bit of information, like a 7 digit phone number, and enable us to hold it in mind for a brief moment of say 5-10 seconds. Short Term Memory is held deep inside the brain in the hippocampus of the medial temporal lobe. It "encodes" information and enables us to remember something, like a person's name, for a short time, say 30-60 minutes.

The hippocampus can process information held in short term memory for long term storage elsewhere in the brain. This conversion of working to long term memory requires among other things a minimum supply of acetylcholine. People with Alzheimer's Disease, stop making sufficient acetylcholine to have a functioning short term memory, but can be helped by drugs which block the breakdown of this neurotransmitter and leave extra quantities at the synapses. The conversion also requires release of norepinephrine, which enables us to selectively attend with great concentration to a single fact while blocking out unimportant stimuli. If we rehearse the answer to an exam question which we know will appear on an important test tomorrow you will pay more focused attention and will more likely remember it, than what I was wearing or what we ate while studying for the test. People with a TBI may have depleted supplies of norepinephrine which renders them unable to block out unimportant stimuli., and making them highly distractible. Another aspect to this conversion process is the strengthening of synaptic connections by thickened growth of dendritic-axonal contacts between nerve cells. Repeated exposure to the same material, like reading a French-English dictionary over and over, spurs this process. It is believed that the REM phase of sleep involves replaying of material registered during the day so that it becomes "learned." Long Term Memory is not located in any one place in the brain. It is a "spatio-temporal firing pattern" of neurons in different parts of the brain. Since it is "stored" in pieces throughout the brain, long term memory is very durable and resistant to the effects of aging and traumatic injury. Thus,  elderly persons or much younger persons with a TBI who cannot recall what they ate for breakfast, can remember events long past like their 5th birthday party. Memory is also classified without regard to the time element, in terms of what is remembered. Declarative or Episodic Memory is the conscious memory of a specific event bound to a certain time and place, like taking a tennis lesson on a June day at summer camp with a teacher named Bob. Procedural Memory is the subconscious knowledge behind an acquired skill like riding a bicycle or playing tennis. A person who knows how to play a good game of tennis, may not recall that he learned it from Bob. A famous patient of the Canadian neurosurgeon Wilder Penfield named H.M. had most of his hippocampus removed on both sides of his brain to stop his severe epileptic seizures. Afterwards he had immediate memory but no short term declarative memory. For example, he could say "You're Bob" 5 seconds after meeting a man named Bob, but would have no recollection of meeting this person 30 minutes later. On the other hand, he could be taught skills which objectively improved his performance at various activities, without his being able to recall receiving any lessons. Destruction of his hippocampi did not impair his procedural memory. Hippocampal nerve cells are extremely sensitive to oxygen deprivation, and are the first cells to die when oxygen is cut off to the brain from a near drowning, heart attack or inhalation of carbon monoxide.

Memory is also differentiated in the following ways. Recognition vs. Recall. Recognition, which is much easier than recall, involves the sense of having seen someone or something before. Recall requires a firmer grasp on the material, such that you can place the person and name him. Lab tests have shown that the average person can recognize 10,000 or more faces shown rapidly to him in photographs weeks earlier. Recognition can grow into recall with repeated exposure to the same place, event or person. Retrospective vs. Prospective. Memory of a past experience, like having gone to the store to buy a mango, is retrospective. Remembering one's past intention to go to the store to buy a mango, is called prospective memory. Persons with a TBI tend to have trouble with both, and this is a big part of what is called executive function disorder. It is hard to run a household efficiently when you are standing in the produce section of the grocery store and forget what you wanted to buy or when you buy mangos twice in the same day, because you forgot your already bought them. Visual vs. Verbal. Visual memory of faces and scenes is located more on the right side of the brain, while verbal memory of poems, speeches and sayings is located more on the left side of the brain. When damage from a TBI is located more on one side than the other side of the brain, we may well see a discrepancy in these types of memory capacities. There are many other distinctions which neuroscientists makes in types of memory, but these are some of the most central ones.