A memory is not a thing but an electro-chemical process. This insight--which runs through a lot of contemporary memory theory--is a theme in Daniel Schacter's book Searching for Memory (the one I showed you in class Tuesday night).
Brains don’t store memories, whole and intact. Instead, the nervous system registers experience and is permanently altered by it. An engram, in Shacter's words, is “the enduring change in the nervous system (the ‘memory trace’) that conserves the effects of experience across time.”
According to this idea (which Schacter borrows and adapts from an early 20th-century psychologist, Richard Semon), when something happens to you—a city bus full of people in Halloween costumes whirs by, for example—multiple senses, emotions, and cognitive functions process it, in overlapping "tracks." Neural networks that comprise each track communicate with each other, strengthening their previous connections. The engram is not something you can locate or look at. It’s the strengthened connection at the points of intersection among the networks.
To recall the trace, you’ll require a retrieval cue—say, a bus whirring by, minus the Halloween costumes. When you see someone who looks like an old friend, memories of that old friend are fabricated. When you search for the word on the tip of your tongue, you are firing cues to trigger your brain to respond. When it doesn’t, it’s because it can’t re-connect enough pieces of the engram to spell that particular word. You feel close because the networks are largely there. But a few missing synapses make the word elusive, even if it that word is very familiar, like the name of a neighborhood you lived in just two years ago.
When you perceive an event—say, the sensation of snapping your first lost tooth out of the gum—your synaptic networks respond to nerve stimulus to dose you with pain but also with the curious satisfaction of the snap. In the process, neuromodulators re-shape brain chemistry and structure, subtly but permanently. They chart a relationship between the pain and the satisfaction—and the vast selection of other sensory, emotional, and cognitive experiences the experience engenders. The memory that results is a guide for future action. The members of the snapping tooth network diffuse, wandering off to play their other roles, prepared, though, to call another meeting if the organism loses another tooth, or needs to recall the experience twenty years later to tell a story at a dinner party.
Memories are always approximations, because an engram will never reconvene in its original form, with all members present in their exact configurations. Things change. Cells die, or grow apart, or just get too busy or distracted. This is why a story recounted repeatedly, ritualistically over holiday meals, can become memory. Individual experience can become collective memory because stories resemble experiences enough to trigger identical neural systems. Story and experience can become indistinguishable.