Duration and Capacity of the Memory Stores

A new phone number evaporates the moment someone rattles off a different one because it was only ever sitting in short-term memory, a small holding area that keeps roughly seven items (plus or minus two) active for about 20 to 30 seconds. Before that, the sound of the number briefly existed in sensory memory, an almost photographic or echoic trace that holds a huge amount of raw detail but decays within a second or two. Unless something is done with the information, it simply drops out. That something is rehearsal. Maintenance rehearsal is just repeating the digits silently, enough to dial the phone, but it does nothing to connect the number to anything you already know, so it vanishes once attention shifts. Elaborative rehearsal links the new number to meaning: maybe the last four digits match a friend's birthday. That link gives the brain a retrieval path, which is why elaborative rehearsal moves information into long-term memory, a store with no known capacity limit and a duration that can last decades.

This three-store picture explains why interference tasks are so useful in research. If someone is given three letters and then forced to count backward by threes, that counting occupies the mental workspace short-term memory needs for rehearsal. With rehearsal blocked, the letters fade on their own clock, and recall collapses within seconds rather than minutes. The sharp drop shows that short-term memory's 20-to-30-second window isn't just a rough estimate, it's what happens when nothing refreshes the trace.

This sets up the next question naturally: if rehearsal is the bridge from short-term to long-term storage, what determines whether elaboration actually works, and how is long-term memory organized once information arrives there.