Wednesday, December 19, 2012

Brain Rule #6: Long Term Memory

My summaries of the book Brain Rules continue...
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Chapter 6: Long Term Memory
In summarizing this chapter, it is perhaps easiest to begin where it ends.  It ends by returning to H.M., who was mentioned in chapter 5.  In the early 1950s, H.M. had his hippocampus removed in the center of his brain.  As a result, he cannot convert short term memory into long term memory.  He has no memories since the 1940s and doesn't recognize his own face in the mirror.

What is interesting is that he does not remember anything since about eleven years before his surgery.  The implication is that it can take over a decade for short term memories to solidify fully.  Even though long term memory is apparently stored in various parts of the cerebral cortex, the outer part of our brain, the hippocampus continues to be involved in its solidification for as much as a decade after an event occurs.  The process seems to work something like the following.

First, long term memory comes as a result of ongoing interactions between the hippocampus and the cerebral cortex, "multiple reinstatements" of the same memory (141).  It starts with "working memory," which has auditory, visual, and executive components.  The executive component keeps track of how the various components of working memory fit together.

Secondly, "These reinstatements are directed by the hippocampus perhaps for years" (141). Like the components of working memory, there appear to be different systems of long term memory as well, although the categories are less agreed on.  However, there may be "semantic" memory systems for details (126).  There may be "episodic" memory that have to do with past events.  An important subset of episodic memory may be "autobiographical" memory about your own past.

The key to conversion from short term to long term memory seems to be repetition.  "Repetition, doled out in specifically timed intervals, is fixative" (130).  When it comes to experiences, talking about an event immediately after it occurs significantly enhances the long term memory of the event (131). Otherwise, a great deal of memory loss occurs in the first hour or two after initial exposure.

After the initial learning, you should deliberately re-expose yourself to information in fixed, spaced intervals (133).  Do so "more elaborately" as you re-expose yourself.  "Increasingly limited exposures can result in increasingly stronger responses" (134), like a whiff of someone's perfume or music you associate with someone.

The first spark of memory will fade in about 90 minutes if not re-fired.  In the formation of long term memory, its "consolidation," there will have to be continued communication over years between the hippocampus and the cortex.  This can happen even while you sleep.  Neurologists aren't quite sure where exactly the memories are as the long term consolidation takes place.  One person called it "nomadic memory" as it may wander around the brain's "neural wilderness" (140). A memory may actually come to final rest in the same region as it started.

So, thirdly, a memory eventually becomes permanently stored in the cortex (141-42).  H.M. could remember things from 11 years before his surgery, memories that did not depend on the hippocampus at all.  But, fourthly, retrieving long term memories for the rest of us immediately makes them changeable again (142).  "When previously consolidated memories are recalled from long-term storage into consciousness, they revert to their previously labile, unstable natures" (127).  Unfortunately, "permanent storage exists in our brains only for those memories we choose not to recall."

For short term memory retrieval, we recall like someone going through the stacks in a library to find a book (reproductive retrieval).  But as time goes by, retrieving a memory is more like a Sherlock Holmes investigation (128). We have fragments of memory that we try to piece together and re-enact (fragmentary memory).  "Over time, the brain's many retrieval systems seem to undergo a gradual switch from specific and detailed reproductions to this more general and abstracted recall" (129).

Meanwhile, "forgetting allows us to prioritize events" (143).  Without the ability to forget, we would not be able to see the big picture.  Forgetting allows us to find the common points between things and discover larger, repeating patterns.

What impact does an understanding of long term memory have?  Medina suggests an experimental learning regiment that is quite intriguing.  He would divide teaching moments into 25 minute modules which would be repeated in 90 minute intervals throughout the day. Then every third or fourth day they would review material from the previous 72-96 hours. Then critical information would be repeated on a yearly or semi-yearly basis.  He even thinks it would be helpful to bring practitioners in the field into dialog with the university, creating life long memory refreshment.

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