The word engram refers to “a presumed encoding in neural tissue that provides a physical basis for the persistence of memory.”
Neuroscientists presume that engrams exist in the same way that biologists once presumed that a feature of living cells contained the mechanism of inheritance. Since we know that memories are persistent, there must be a molecule or substance that records and stores them. That would be the engram, but we haven’t found it yet.
Current theories about the engram assume that all the information the brain stores is input by the senses. Thus, the memory of an apple is a collection of sight, smell, taste, touch and sound data that becomes recorded in the tissue of the brain somehow. Some experiments, however, have shown that the mysterious engram stores simple data that the senses do not transmit — specifically, time intervals (such as the interval between a stimulus and a response). This implies that the engram has a numerical functionality. It can store and process numbers in some raw form we don’t understand and cannot explain.
This simple observation, however, may be useful in our effort to discover the engram itself. Given the biological material that exists in the cell, how could a numerical storage device be constructed? If we can imagine the design, we can begin to look for the thing.