Engram (neuropsychology)

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This article is about the concept of the engram in mainstream neuropsychology. For the concept of the engram in Dianetics and Scientology, see Engram (Dianetics).

Engrams are a hypothetical means by which memory traces are stored as physical or biochemical change in the brain (and other neural tissue) in response to external stimuli.

They are also sometimes thought of as a neuronal network or fragment of memory, sometimes using a hologram analogy to describe its action in light of results showing that memory appears to be non-localized in the brain. The existence of engrams is posited by some scientific theories to explain the persistence of memory and how memories are stored in the brain. The existence of neurologically defined engrams is not significantly disputed, though its exact mechanism and location has been a persistent focus of research for many decades.

[edit] Overview

The term engram was coined by Richard Semon and explored by Anton Pavlov. Karl S. Lashley an American behaviorist well-remembered for his influential contributions to the study of learning and memory tried to locate the engram. His failure to find a single biological locus of memory (or "engram", as he called it) suggested to him that memories were not localized to one part of the brain, but were widely distributed throughout the cortex.

Lashley argued for distributed representations as a result of his failure to find anything like a localized engram in years of lesion experiments (src: Early work, Connectionism).

Later researcher, Richard F. Thompson, sought the engram of memory in the cerebellum instead of the cerebral cortex.

Thompson and his colleagues used classical conditioning of the eyelid response in rabbits in their search for an engram. They puffed air upon the cornea of the eye and paired it with a tone. This airpuff normally causes an automatic blinking response. After a number of trials they conditioned the rabbits to blink when they heard the tone even though the airpuff was no longer administered. During the experiment, they monitored several brain cells to try to locate the engram.

One brain region that they monitored that they thought was a possible part of the memory engram was the lateral interpositus nucleus (LIP), when chemically deactivated, it resulted in the rabbits, who were previously conditioned to blink when hearing the tone, to act as if the conditioning never took place; however, when they re-activated the LIP, they responded to the tone again with an eyeblink. This gives evidence that the LIP is a key element of the engram for this behavioral response. (James W. Kalat, Biological Psychology p.391-393)

It is important to stress that this approach targeting the cerebellum, though relatively successful, only examines basic, automatic responses. Almost all animals have these (especially as defense mechanisms) and it is fairly difficult to resist them. Imagine trying to avoid blinking when someone shoots something at your eye. Ideally, research by Thompson and others could eventually lead to isolation of more complicated engrams that control more abstract, declarative memories, like how one remembers one's name or the capital of France. Engrams could also explain why users of psychedelic substances experience "flashbacks", which are probably caused by their endogenous dimethyltryptamine, mostly produced in the pineal gland, because resemblances to previous spiritual experiences in their environment seem to trigger these effects.

The problem here is that considerable studies have shown declarative memories tend to move about the brain between the limbic system (deep within the brain) and the outer cortical areas. This contrasts with the more "primitive" set-up of the cerebellum, which controls the blinking response and receives direct input of auditory information. Thus, it does not need to reach out to other brain structures for assistance in forming simpler memories of association.