Memory trace

Engrams are theorized to be means by which memories are stored as biophysical or biochemical changes in the brain (and other neural tissue) in response to external stimuli.

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 their exact mechanism and location has been a focus of persistent research for many decades.

The term engram was coined by the little-known but influential memory researcher Richard Semon.

Karl S. Lashley's search for the engram found that it could not exist in any specific part of the rat's brain, but that memory was widely distributed throughout the cortex. One possible explanation for Lashley's failure to locate the engram is that many types of memory (e.g. visual-spatial, smell, etc.) are used in the processing of complex tasks, such as rats running mazes. The consensus view in neuroscience is that the sorts of memory involved in complex tasks are likely to be distributed among a variety of neural systems, yet certain types of knowledge may be processed and contained in specific regions of the brain. Overall, the mechanisms of memory are poorly understood. Such brain parts as the cerebellum, striatum, cerebral cortex, hippocampus, and amygdala are thought to play an important role in memory. For example, the hippocampus is believed to be involved in spatial and declarative learning, as well as consolidating short-term into long-term memory.

In Lashley's experiments (1929, 1950), rats were trained to run a maze. Tissue was removed from their cerebral cortices before re-introducing them to the maze, to see how their memory was affected. Increasingly, the amount of tissue removed degraded memory, but more remarkably, where the tissue was removed from made no difference.

Later, Richard F. Thompson sought the engram in the cerebellum, rather than the cerebral cortex. He used classical conditioning of the eyelid response in rabbits in search of the engram. He puffed air upon the cornea of the eye and paired it with a tone. (This puff normally causes an automatic blinking response. After a number of experiences associating it with a tone, the rabbits became conditioned to blink when they heard the tone even without a puff.) The experiment monitored several brain regions, trying to locate the engram.

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