By Richard Restak, M.D., The George Washington University School of Medicine and Health Sciences

Edited by Kate Findley, The Great Courses Daily

Have you ever wondered why phone numbers are structured the way they are? Dr. Restak describes our brain’s capacity for memorizing numbers and how we can give our brains—and memory—a boost.

Short-Term Memory

Memory falls into two categories: short-term and long-term. Long-term memories are distant memories such as meeting your spouse or ingrained skills such as riding a bike. Short-term memory, by contrast, encompasses events in the near past and is maintained through active rehearsal—for example, you might keep going over science facts immediately before a test.

Some of these short-term memories eventually get converted into long-term memories, and you then have that information stored in your brain and no longer have to make an effort to retrieve it. Every day of our lives, we can strengthen our memory through active efforts to learn new information by reading or taking courses.

Digit Span Exercise

Dr. Restak recommends the digit span exercise to strengthen auditory and visual short-term memory. Start with auditory recall. Read a series of five, six, seven, and eight-digit strings into a recorder. 

You’ll probably want to group them so that all the five-digit strings are together, the six-digit strings are together, and so on. Next, put them aside for a while and clear your mind. 

Several hours later, listen to the lists, pause, and then write down as many of those strings that you can recall. Then play the recording to check for accuracy.

Next, you can do this exercise visually by writing your lists on a piece of paper. Write the lists, turn them face down, look at them quickly one at a time, and then try to remember the sequence as you saw it. You can check at the end by turning them face up again.

There are benefits of doing both visual and auditory spans. First of all, it’s a quick test of whether you are a visual or auditory learner. It focuses attention, and it focuses concentration. 

How Many Digits?

A digit span can be increased by practice—you start at five and work your way up. A normal adult digit span is between five and seven. For children, the ability to memorize one digit per year of age is normal, so a two-year-old can remember two digits, a three-year-old can remember three, and so on.

You may be wondering, then, why the average digit span of adults is that of a seven-year-old child. As it turns out, there is a limit to how many digits your brain can naturally hold.

In his 1955 paper “The Magical Number Seven, Plus or Minus Two,” George Miller, a Harvard psychology professor, discusses a principle called chunking. He found that numbers are easier to memorize if you chunk the numbers by taking long strings and breaking them up into parts. 

That’s what we do with telephone numbers. We remember the first three numbers, then we have a space, and then we remember the last four numbers. 

The brain doesn’t spontaneously encode more than seven items into short-term memory. If you want to improve your digit span, you have to practice it.

Boost Your Memory

On the surface, the digit span might seem like a trivial exercise. However, the digit span is a predictor of math and reading proficiency and enhanced performance in attention, concentration, sequencing, and auditory and visual short-term memory. 

“Don’t be fooled by the apparent simplicity of this exercise,” Dr. Restak said. “This is a powerful way of improving brain function in multiple areas.”

Short-term memory needs to be in active rehearsal in order to be moved to long-term memory. Your attention must be focused on material. If your attention is disrupted, like when somebody asks you a question while studying, then you lose track of the information.

However, rehearsal alone isn’t sufficient. Depth of processing is required. Rote repetition is not as effective as working with the information. 

Memorization of dates, for example, is not as effective as writing an essay about what happened on that particular date. Depth of processing increases the web of connections among stored memories, increasing neuronal connections so that you can more easily recall this information in the future.

This article was edited by Kate Findley, Writer for The Great Courses Daily.

Dr. Richard Restak is Clinical Professor of Neurology at The George Washington University School of Medicine and Health Sciences. He earned his M.D. from Georgetown University School of Medicine. Professor Restak also maintains an active private practice in neurology and neuropsychiatry in Washington, D.C.