How Working Memory Works

Have you ever put your keys down and then quickly forgotten where to find them? When you try to recall where you might have left them, you are drawing on working memory, which is the ability to maintain and manipulate small amounts of information in your mind over a short period of time. Only recently has research begun revealing how the brain pulls off this feat.

A study recently reported in Nature pinpoints interactions between the frontal part of the brain that controls cognition—the mental actions involved in thinking, planning, and sustained focus—and the hippocampus, a structure nestled deep inside the brain and critical for memory formation and storage. Brain waves passing between the two regions trigger a group of brain cells called phase-amplitude coupling (PAC) neurons that help coordinate working memory. The findings provide fresh insight into not only how the brain manages memory but also how we might improve attention, decision-making, and memory retrieval.

What Is Working Memory?
Think of working memory as a filing cabinet that holds a few documents (about seven items) for a brief period ranging from a few seconds to several minutes, depending on the task and the individual’s cognitive abilities. You hold a phone number in your mind, for example, just long enough to dial it. Unless it’s a number you will call often, you do not want it to take up valuable storage space, so it slips out of memory soon afterward. If any documents are deemed essential and frequently revisited, they are stored for long-term use.

But working memory is fragile; the brain is constantly multitasking, holding and manipulating many snippets of information at once, all competing with working memory and distracting us from the task at hand. That number you were trying to remember? Poof, gone. Mental focus —technically termed “cognitive control”—is what allows us to intentionally foreground specific information relevant to a task or a particular social context while suppressing possible distractions.

A breakdown in either the temporary storage of information as memory or the cognitive control to retrieve and foreground the information long enough to complete a set task risks interrupting working memory. If a filing cabinet does not contain the desired document, there is no way of proceeding. Similarly, if the cabinet is in a state of complete clutter, with many other documents floating around, it will be hard to find the one that’s needed.

How the Brain Makes Memories
Memories are initially encoded in the hippocampus. The process hinges on neural plasticity—the ability to strengthen the connections, or synapses, between neurons through repeated stimulation. (That is why flashcards and other rote learning techniques work; continual exposure helps solidify new information at the neuronal level.)

Throughout the process, the hippocampus and prefrontal cortex communicate via brain waves of different frequencies. Faster gamma waves reinforce correct associations, while slower theta waves weaken incorrect ones, guiding what the brain learns.

The two types of brain waves work together—theta-gamma phase-amplitude coupling—creating a certain rhythm. They become synched so that when one gets stronger, so does the other. Such coordination is thought to facilitate working memory by enabling us, on the one hand, to maintain temporary storage of information and, on the other, to keep this information at the front of our minds until we have used it for its intended purpose.

The idea was put to the test in 36 patients undergoing surgery to treat their drug-resistant epilepsy. As their brains were monitored, they observed 140 rounds of one or three differing pictures they had to hold in memory in order to later identify whether a new picture matched any previously seen.

The study revealed a direct link between theta-gamma phase-amplitude coupling and the firing patterns of single neurons. When patients responded quickly and accurately, researchers saw two types of neurons activate: One set, category neurons, latched on to the category of the image —say, a human face; the other, phase-amplitude coupling neurons, helped direct focus.

Neurons that show phase-amplitude coupling synchronize with frontal theta waves, particularly when demands on working memory are high, improving cognitive control and facilitating recognition of a previously seen image. The phase-amplitude coupling neurons also fire in time with gamma waves, which are closely linked to information processing.

Categories and Control
The phase-amplitude neurons firing in tandem with the category neurons help home in on the correct information in the brain. The category neurons keep an eye on the content of the memory (like someone yelling, “Human face”), while the phase-amplitude neurons make sure that the information is kept in focus (like someone yelling, “Remember!”). Lead researcher Ueli Rutishauser compares the effect to an orchestral performance: “The two groups of neurons create a harmony superimposing their messages,” leaving us with the final signal: Remember/human face.

Like the label on a file, the category neurons provide a quick outline of the information to be remembered. The phase-amplitude coupling neurons, like a clear set of instructions, detail where and how to locate the file.

Theta and gamma brain waves help stimulate category neurons and phase-amplitude coupling neurons, which together scaffold working memory. Category neurons hold onto the information to be kept in mind, while phase-amplitude coupling neurons make sure this information is foregrounded and prioritized over possible distractions. Combined, the neurons coordinate the frontal and hippocampal regions of the brain so that you can effortlessly find your car keys.

Hippocampus Helpers

Nutrition is one known way to influence neuronal plasticity in the hippocampus of adults. Animal studies show that what you eat can help or hamper memory.

Positive effects:

• Curcumin supplements
• Polyunsaturated fatty acids, including omega-3 fatty acids
• Polyphenols
• Intermittent fasting
• Caloric restriction

Negative effects:

• High-fat diet
• High-sugar diet

William A. Haseltine, Ph.D., is chair and president of the nonprofit Access Health International.