Thursday, October 19, 2006

Significant life events and memories

I ran into this interesting paper (Significant life events and the shape of memories to come: A hypothesis avalaible at from the Department of Psychology at Rutgers written by Tracey Shors that talked about the seemingly strange phenomenon whereby patients who have intense life experiences seem to ‘forget’ large portions of their lives, but still remember in stunning details events that happened to them 20 or 25 years back. She talks about two patients K and F with very large retrograde amnesia after severe neurological insults (one of which was an electrical shock from an oven and the other involved an aneurysm from a temporal lobe hematoma). In the case of Mr. K, he did not remember any part of his life from 1946 to 1980 (he was 53 years old) and Mrs F remembered nothing between 1960 and 1979. Both Mr. K and Mrs. F fairly important life changing events back in 1946 and 1960 (Mr. K’s family house burned down and his family became destitute and Mrs. F was having an illicit baby of a married man).

It was also recorded that they behaved exactly like they were frozen back in time (Mr. K blushed and was skipping and giggling like the teenager he was back in 1946).

Professor Tracy has advanced a theory in her paper that states that hormonal imbalances that inevitably happen when human beings go through intense life altering experiences actually alter neurological substrates like neurons and synapses and thoroughly rewire the brain and result in synaptogenesis (generation of new synapses) and neurogenesis (generation of new nerve cells). In Dr. Tracey’s words,

I began this review by discussing those most unusual cases of K and R, whose life tapes were essentially spliced, leaving them without a decade or so of remembered experience.
You might ask how these examples relate to the theme of this review. Recall that their memory loss went back to times in life associated with stressful and traumatic events.
In the case of K, his memory loss went back to when his family house burned down and his family became destitute. In the case of R, her loss went back to when she become pregnant with a married man’s baby. That these significant life events were associated with the beginning (or end, depending on how you look at it) of the memory deficit suggests that their brains were anatomically altered during those eventful times. Presumably, these two people experienced hormonal changes during these events, which would in turn alter anatomical structures within their brains. Decades later, their brain traumas simply revealed the underlying anatomies into which these memories of life were carved. Under normal circumstances, these anatomies would alter the shape of memories to come.”

This is a good paper and has been accepted by a prestigious journal, but I had a question…

Memory taxonomy consists of two types (1) Declarative (that which stores semantic (factual) and episodic (events)) and (2) Non-declarative (storage of learned behavior like procedural skills that include swimming and cycling and storage of emotional responses regulated by amygloid structures) . If I tend to accept the ‘hormone’ theory hypothesized by Dr. Tracey, then it should have wiped out both declarative and non-declarative memories. How come it wiped out only the declarative memories and hence only the semantic and episodic parts between 1946 to 1980 for Mr. K and between 1960 to 1979 for Mrs. F? Hormone inspired neurogenesis and synaptogenesis will not be able to distinguish between the substrates responsible for the two types of memory described above… This remains unanswered in this otherwise well written review…

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