Engram (neuropsychology)

An engram is a unit of cognitive information imprinted in a physical substance, theorized to be the means by which memories are stored as biophysical or biochemical changes in the brain or other biological tissue, in response to external stimuli. Demonstrating the existence of, and the exact mechanism and location of, neurologically defined engrams has been a focus of persistent research for many decades. The term "engram" was coined by memory researcher Richard Semon in reference to the physical substrate of memory in the organism. Semon warned, however: "In animals, during the evolutionary process, one organic system—the nervous system—has become specialised for the reception and transmission of stimuli. No monopoly of this function by the nervous system, however, can be deduced from this specialisation, not even in its highest state of evolution, as in Man." One of the first ventures on identifying the location of a memory in the brain was undertaken by Karl S. Lashley who removed portions of the brain in rodents. In Lashley's experiments, rats were trained to run through a maze and then tissue was removed from their cerebral cortex. Increasing the amount of tissue removed increased the degradation of memory, but more remarkably, where the tissue was removed from made no difference. His search thus proved unsuccessful, and his conclusion – that memory is diffusely distributed in the brain – became widely influential. However, today we appreciate that memory is not completely but only largely distributed in the brain; this, together with its dynamic nature makes engrams challenging to identify using traditional scientific methods. Later, David A. McCormick and Richard F. Thompson sought the engram in the cerebellum, rather than the cerebral cortex. They used classical conditioning of the eyelid response in rabbits in search of the engram. They puffed air upon the cornea of the eye and paired it with a tone. 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.