An engram is a unit of cognitive information imprinted in a physical substance, theorized to be the means by which memories are stored[1] as biophysical or biochemical[2] changes in the brain or other biological tissue, in response to external stimuli.
[5] 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, or prove that they exist, using traditional scientific methods.
[9] Such brain parts as the cerebellum, striatum, cerebral cortex, hippocampus, and amygdala are thought to play an important role in memory.
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.
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.