In 1956, the renowned cognitive psychologist George Miller published one of the field’s most widely cited papers, “ The Magical Number Seven, Plus or Minus Two.” In it, he argued that although the brain can store a whole lifetime of knowledge in its trillions of connections, the number of items that humans can actively hold in their conscious awareness at once is limited, on average, to seven.

Those items might be a series of digits, a handful of objects scattered around a room, words in a list, or overlapping sounds. Whatever they are, Miller wrote, only seven of them can fit in what’s called working memory, where they are available for our focused attention and other cognitive processes. Their retention in working memory is short-lived and bounded: When they’re no longer actively being thought about, they’re stored elsewhere or forgotten.

Since Miller’s time, neuroscientists and psychologists have continued to study working memory and its surprisingly strict limitations. They have found that the limit may really be closer to four or five items than seven. And they have studied the ways in which people work around this constraint: We can remember all the digits of a phone number by “chunking” digits (remembering 1, then 4, as the single item 14, for instance), or develop mnemonic devices for shuffling random digits of pi out of longer-term storage.

But the explanation for why working memory starts to falter at such a seemingly low threshold has been elusive. Scientists can see that any attempt to exceed that limit causes the information to degrade: Neuronal representations get “thinner,” brain rhythms change and memories break down. This seems to occur with an even smaller number of items in patients who have been diagnosed with neurological disorders, such as schizophrenia.

The mechanism causing these failures, however, has remained unknown until recently.

In a paper published in Cerebral Cortex in March, three scientists found that a significant weakening in “feedback” signals between different parts of the brain is responsible for the breakdown. The work not only provides insights into memory function and dysfunction, but also offers further evidence for a burgeoning theory of how the brain processes information.

Synchronized Humming in the Brain

Earl Miller, a neuroscientist at the Picower Institute for Learning and Memory at the Massachusetts Institute of Technology; Dimitris Pinotsis, a research affiliate in his lab; and Timothy Buschman, an assistant professor at Princeton University, wanted to know what sets the capacity limit of working memory so low.

They already knew that a network involving three brain regions—the prefrontal cortex, the frontal eye fields and the lateral intraparietal area—is active in working memory. But they had yet to observe a change in neural activity that corresponded to the steep transition between remembering and not remembering that comes with exceeding the working memory limit.

Earl Miller, a neuroscientist at the Picower Institute for Learning and Memory at the Massachusetts Institute of Technology, studies the interplay of brain waves and their role in working memory. Courtesy of Earl Miller

So they returned to a working memory test that Miller’s lab had performed a few years earlier, in which the researchers showed monkeys a series of screens: first, a set of colored squares, followed briefly by a blank screen, and then the initial screen once more, this time with the color of one square changed. The animals had to detect the difference between the screens. Sometimes the number of squares fell below their working memory capacity, sometimes above. Electrodes placed deep in the monkeys’ brains recorded the timing and frequency of brain waves produced by various populations of neurons as they completed each task.

These waves are essentially the coordinated rhythms of millions of neurons that become active and go quiet simultaneously. When brain areas exhibit matching oscillations, both in time and in frequency, they’re said to be synchronized. “It’s like they’re humming together,” Miller said. “And the neurons that hum together are talking.” He likens it to a traffic system: The brain’s physical connections act like roads and highways, while the patterns of resonance created by these oscillating brain waves “humming” together are the traffic lights that actually direct the flow of traffic. This setup, researchers hypothesize, somehow seems to help “bind” active networks into a firmer representation of an experience.