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Engrams can also be explained as a permanent impression left on protoplasm as the result of a stimulus or a lasting trace left in an organism by psychic experience, or simply the writing left behind in the brain by conscious experience.
They are also sometimes thought of as a neural network or fragment of memory, sometimes using a hologram analogy to describe its action in light of results showing that memory appears not to be 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 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.
Wilder Penfield stumbled upon a two localization hypotheses during his original research of electrical stimulation to discover what was taking place in the brain during a "temporal lobe epileptic episode". In 1939 after much research of no known problems resulting in the brain, he began removing the anterior half of either temporal lobe and then decided to also remove part of the hippocampus as a way to decrease epileptic episodes. No known memory loss was reported until 1951.
Two men, an engineer and a glove cutter, were severely handicapped by memory loss yet had retained the memory of their respective skills. Each of these men suffered an inability to recall the details of recent awareness and a defect of automatic perception. Through extensive research on the men, it was discovered that each had some form of deformity or injury on the other half of the hippocampus resulting in accidental bilateral removal of the hippocampus.
A final conclusion from Wilder Penfield states that "the engram is the permanent impression left behind by psychical experience in the brain's cellular network. This memory trace makes all that came within the focus of a man's attention memorable in one way or another. It may modify or reinforce a skill. It, also, forms a part of the record of the stream of consciousness and may be summoned consciously or automatically for the purpose of recognition, interpretation, perception." 
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.
One region that Thompson's group studied was the lateral interpositus nucleus (LIP). When it was deactivated chemically, the rabbits lost the conditioning; when re-activated, they responded again, demonstrating that the LIP is a key element of the engram for this response.
This approach, targeting the cerebellum, though successful, examines only basic, automatic responses, which almost all animals possess, especially as defense mechanisms.
Studies have shown that declarative memories move between the limbic system, deep within the brain, and the outer, cortical regions. These are distinct from the mechanisms of the more primitive cerebellum, which dominates in the blinking response and receives the input of auditory information directly. It does not need to "reach out" to other brain structures for assistance in forming some memories of simple association.
An MIT study found that behavior based on high-level cognition, such as the expression of a specific memory, can be generated in a mammal by highly specific physical activation of a specific small subpopulation of brain cells. By reactivating these cells by physical means in mice, such as shining light on neurons affected by optogenetics, a long-term fear-related memory appears to be recalled. This suggests that memories may reside in very specific brain cells.