How memory makes waves

Reports on a study which explored the reasons for the limited capacity of human short-term memory. Brain waves as determinants of the memory's capacity; Mental time-sharing operation of short-term memory.

By PT Staff, published on September 01, 1995

Look up a phone number and most of us can remember it long enough to dial.But tack on an unfamiliar area code and we're in trouble. Seven chunks of info is about all our memory can handle at one time.

Why is our short-term memory so feeble? The answer lies in our brain waves, researchers at Brandeis University believe. They've devised a new theory that may transform how we think of short-term memory. Forget those computer metaphors--short-term memory may operate more like a mental time-share system.

While long-term memories are based on the patterns of connections between neurons, the cells that keep track of short-term memories do so by electrical signals. As long as they're retaining a memory, the cells fire in cycles, or oscillations, that last between 20 and 40 milliseconds, says biologist John Lisman, Ph.D.

This is where the time-sharing comes in. Our brains also produce longer electrical signals--alpha and theta waves--that last about 200 milliseconds. Divide those long waves into slots, and you can fit about seven of the 20-to-40-millisecond cycles.

If Lisman and physicist Marco Idiart, Ph.D., are right, each memory takes its place in a designated slot of the longer brain waves. Computer simulations confirm that a neural network operating under these conditions can, with appropriate input and feedback, store about seven memories.

This new view, reported in Science, "addresses for the first time the function of brain oscillations in short-term memory," Lisman says. "We're pulling together biophysical ideas, neural network ideas, and traditional psychology."

Indeed, it was a 30-year-old psych experiment that convinced Lisman and Idiart they were on the right track. In a famous study, Saul Sternberg presented subjects with a list of one to six digits (thereby introducing the numbers into short-term memory) and then asked whether a particular digit was on the list. Subjects took about four-tenths of a second to answer--plus an extra 38 milliseconds for each item on the list.

That added delay, Lisman says, tells us how long it takes to scan through each short-term memory--and it corresponds to the 20 to 40 millisecond storage cycles.

PHOTO (COLOR): Ocean wave