Working memory · Grey Matter

Working memory is the brain's small mental workspace that holds a few items for seconds while they are used: a phone number between reading and dialling it, the intermediate result of a sum done in the head, the rule of the task at hand.

Working memory. Working memory is the brain's small mental workspace that holds a few items for seconds while they are used: a phone number between reading and dialling it, the intermediate result of a sum done in the head, the rule of the task at hand.

Its capacity is small, around four meaningful chunks by most modern estimates, and its contents vanish when attention moves elsewhere. It depends on a network more than a store. The dorsolateral prefrontal cortex maintains the goal and protects the contents from distraction, and the posterior parietal lobe holds spatial and numerical content; sensory areas keep the details of what is held (a tone in auditory cortex, a shape in visual cortex). The prefrontal cells keep firing across the delay, and that persistent activity, among other mechanisms, is what maintains the item.

It is a bottleneck for thinking. Reasoning, mental arithmetic and understanding long sentences all depend on how much can be held at once, which is why its capacity correlates with fluid intelligence.

It works without the hippocampus for short spans. The patient H.M. could repeat a number for as long as he kept rehearsing it, and lost it once distracted.

It is sensitive to the body's state. Stress, fatigue and sleep loss reduce it through their effects on the prefrontal cortex, one route by which bodily state colours thought.

Questions: Which areas hold a phone number in mind for the seconds it takes to dial it? Holding the number is working memory, run by the dorsolateral prefrontal cortex together with the posterior parietal cortex, with the speech and auditory areas keeping the sound of the digits rehearsed. Prefrontal neurons keep firing during the delay, which protects the digits from interference, and silent repetition refreshes them. Seven to ten digits are close to the limit, and grouping them into chunks (three, three and four) is what makes a longer number fit.