Prefrontal cortex (PFC) maintains task-relevant information across delays through sustained firing of pyramidal cells organized into functional subgroups. Different PFC regions encode task rules, stimulus-response mappings, and expected outcomes. Working memory capacity (~7 items) is limited by the balance between signal strength and noise.
Use delayed-response tasks; record PFC during memory delay. Map population codes for maintained information.
Working memory is stored in PFC—PFC maintains activity patterns. Attention makes memories permanent—sustained attention is required.
You already know that the cerebral cortex is organized into functionally distinct regions, and that the primary motor cortex generates the commands that drive voluntary movement. The prefrontal cortex (PFC) sits anterior to motor areas and serves a fundamentally different purpose: rather than executing actions, it holds information "online" so you can use it to guide those actions. Think of it as a mental workspace — a whiteboard where you temporarily pin the facts, rules, and goals needed for whatever you are doing right now. This capacity is called working memory, and it is what allows you to remember a phone number long enough to dial it, follow the thread of a conversation, or keep track of which step you are on in a multi-step procedure.
The neural basis of working memory is sustained firing. When a piece of information enters working memory, a population of pyramidal neurons in the PFC continues to fire throughout the delay period — even after the original stimulus is gone. Imagine a delayed-response task: a monkey sees food hidden under one of two cups, then waits through a delay before being allowed to reach. During that delay, specific PFC neurons fire persistently, encoding "left cup" or "right cup." If those neurons stop firing — because of distraction, interference, or experimental disruption — the animal reaches for the wrong cup. The information literally exists as ongoing neural activity, not as a stored trace the way long-term memories are consolidated in hippocampal and cortical circuits.
Different subregions of the PFC maintain different kinds of information. The dorsolateral prefrontal cortex is particularly involved in maintaining spatial locations and task rules — the "what am I supposed to do" aspect of a task. The ventrolateral PFC contributes more to maintaining object identity and feature information. And the orbitofrontal and medial prefrontal regions encode expected outcomes and reward values, helping the system decide which information is worth holding onto. This division of labor means working memory is not a single box but a distributed workspace with specialized compartments.
A critical feature of working memory is its strict capacity limit — famously around seven items (plus or minus two), though more recent estimates suggest the true limit is closer to four independent chunks. This limit arises from the biophysics of sustained firing: each maintained item requires a group of neurons to keep firing against a background of noise and competing signals. As more items are loaded, the signals interfere with each other, degrading the fidelity of each representation. This is why you can hold a seven-digit phone number in mind but struggle with a ten-digit one, and why any distraction during the maintenance period can cause the information to collapse. The PFC does not passively store information — it actively fights to maintain it, and that active maintenance is the bottleneck that makes working memory both powerful and limited.
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