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Nope, this time it is not mine. This is the regularly scheduled June 6th blog, and the topic is how memory fails.
There are several means by which memory fails -
(A) retrograde amnesia - the inability to recall a stored memory
(B) anterograde amnesia - the inability to store memory
- either of which can be caused by
(C) disease states that cause A or B
(D) head/brain injury
(E) abnormal memory (drug abuse and PTSD)
Retrograde (backward) amnesia is the inability to recall information that was previously remembered. This is the classic "hit on the head and forgets the last 20 years" type of soap-opera amnesia discussed in this blog: http://teddysratlab.blogspot.com/2011/03/amnesia-who.html. Anterograde (forward) amnesia is the failure of the association and storage network for making new memory. Both types of memory are affected by the hippocampus, but in particular, hippocampal damage results in (B) with very little effect on (A).
Many disease states cause amnesia - Alzheimer's disease, of course, results from gradual damage to neurons first to those neurons in hippocampus and Temporal Lobe, causing anterograde amnesia, then more widespread involvement causes retrograde amnesia. Senility, alcoholism and Huntington's Disease cause retrograde amnesia, by damaging the retrieval circuits in diencephalon and Frontal Lobe. (Compare with amyotrophic lateral sclerosis (ALS) which affects the nerves into and out of the spinal cord, leaving all of the higher brain functions intact, but cutting that brain off from a functioning body) Pituitary or optic nerve tumors can press on the hippocampus and cause anterograde amnesia. Other brain tumors, stroke or aneurysm can rob brain areas of blood and oxygen (see: http://teddysratlab.blogspot.com/2011/03/give-me-air.html) thus causing amnesia depending on the precise area affected.
Head injury, traumatic injury, surgery, all cause compression, physical damage and alteration of blood flow to the brain. Contrary to the TV and movie cliche, a simple blow to the back or top of the head won't do it. The brain is suspended in a fluid, between "dura mater" a thick membrane lining the skull, and "pia mater" a light thin (permeable!) membrane lining the surface of the brain. Cerebrospinal fluid is made at specific sites where blood vessels cluster at the origin of several fluid-filled channels called "ventricles" in the brain. This combination of membranes and fluid cushions the brain. To damage it, you need to penetrate the membranes (trauma or surgery), infect them (encephalitis or meningitis), or make the brain "slosh" and strike the inside of the skull. Very strong blunt force blows to the very front, side or lower back part of the skull will cause the brain to move and strike the inside of the skull opposite the impact. If the Frontal or Temporal Lobes hit the inside of the skull, they get bruised, like any other body part. Small blood vessels break, tissue swells, and the neurons don't function, resulting in amnesia (usually retrograde).
In addition, any time the brain is physically injured, it swells. Swelling, tumors, infections, all compress blood vessels and injure neurons. If temporary, the amnesia is only temporary, but if the injury is permanent, or goes on for too long, teh amnesia could be permanent. It's become a very common problem in troops returning from war zones - too many time of "having their bell rung" (concussive head injury due to roadside bombs) leads to permanent memory loss. This is also becoming better understood as a long-term problem following surgery, radiation or chemotherapies for brain tumor or cancer.
All of the above oare problems with memory not working, but in some cases it works too well, and the result is a memory that cannot be forgotten. I covered this at length in an earlier blog (http://teddysratlab.blogspot.com/2011/03/it-came-to-me-in-flash.html) and is the root of PTSD and some aspects of drug abuse. In both cases, the associated information stored with the memory triggers a strong recall - in the PTSD-sufferer, the traumatic stress incident - in the drug user, the desire to seek out drug stimulation. Every time the memory is recalled, it is rewritten/restored and can be hard to break - however, like the "flashbulb memory mentioned in that earlier blog - the problem is also (possibly) part of the solution - change enough of the association cues, and it may be possible to dilute the memory.
The final entry is interference, and this can take many forms. When trying to remember a phone number "three-six-oh-four. Three. Six. Oh. Four." repetition and no distraction is the key. If a friend asks you to meet them at Four O'Clock, it can disrupt the remembered chain "Three-six-four-oh." This is called "proactive interference, and is a failure of the brain to keep information separated. Another example is the famous parking lot problem: Every day you park in the same place in the parking lot at school/work. Today your spot was taken, and you parked somewhere else far away. You have trouble finding your car because the previous habit interferes with the current (different) memory.
There are other types of interference. To put a memory into long term storage requires time, and the create the proteins and molecules necessary to alter connections between neurons. A lot of this is done by the brain passively replaying information during sleep (a partial explanation for dreams). Interrupt the sleep, and you interrupt the storage process. Likewise disease, illness, nutritional deficits and head injury will also interrupt the process - one reason why a head injury patient may *never* recover memory of the accident - it just never got stored!
The final type of interference, is if the brain damage occurs to the associational areas - a lack of sense of smell, hearing or vision will interfere with both storage and recall of memories with those components. Injuries to Wernicke's area (language) can cause memory failures for specific words (aphasia). Visual or sensory damage can cause the patient to ignore parts of the body or environment (agnosias, or "neglect"). Motor cortical damage can cause "apraxia" where the knowledge of a skill, movement or speech is present, but the patient cannot perform the necessary movements.
From this wonderful complexity of memory, comes the knowledge that there is much to be lost. Our memories are a large part of who we are, as individuals. As any person who has lost a loved one to Alzheimer's can attest, the greatest loss is of the self.
So tonight, I close - as I do so often - with the *serious* appeal: Take care of your brain - it's the only brain you've got.
It's the only memory you've got.
It's the only *self* you've got!