Questions: Working Memory Neural Circuits

Question 1

During a delayed-response task, a monkey sees a stimulus location and then waits several seconds before responding. What do neurons in the dlPFC do during the delay, and what mechanism sustains this activity?

• A. They go silent and reactivate when the response cue appears, driven by hippocampal replay
• B. They fire continuously throughout the delay via recurrent synaptic connections that keep the population active after input disappears
• C. They receive sustained input from sensory cortex, which keeps them firing at a low rate
• D. They gradually reduce their firing rate in proportion to the length of the delay

Answer: (B)

Delay-period activity — continuous firing with no ongoing stimulus and no motor response — is the defining neural signature of working memory maintenance. The mechanism is recurrent connectivity: dlPFC neurons have dense reciprocal synapses that form reverberating circuits. Once activated by the stimulus, the population sustains itself like a tuning fork that keeps vibrating after being struck. Sensory cortex, lacking this dense recurrent architecture, goes quiet when the stimulus disappears — which is why dlPFC, not sensory cortex, maintains the representation.

Question 2

Why does overloading working memory degrade ALL currently held items rather than simply causing the most recently added item to be dropped?

• A. The phonological loop runs out of rehearsal time and begins dropping the oldest items first
• B. Each item requires a dedicated reverberating population; too many populations compete for recurrent bandwidth, degrading all representations simultaneously
• C. The hippocampus cannot consolidate more than 3-4 items and discards extras randomly
• D. Attention gates only one item at a time, so additional items never enter working memory at all

Answer: (B)

Working memory capacity is limited by competition between dlPFC neural populations, each sustaining a separate item through recurrent firing. When too many populations are active simultaneously, they interfere with each other's reverberation — the recurrent excitation that should sustain each population is degraded by mutual inhibition or resource competition. The result is parallel degradation of all representations, not sequential dropping. This is distinct from models that treat capacity as a serial bottleneck.

Question 3

Damage to the hippocampus produces working memory deficits while leaving long-term memory intact.

• T. True
• F. False

Answer: False

The dissociation is the opposite. Hippocampal damage (as in H.M.'s famous case) impairs the formation of new long-term memories (anterograde amnesia) while leaving working memory largely intact — H.M. could hold a conversation normally despite being unable to form new lasting memories. PFC damage produces the reverse: working memory deficits with relatively preserved long-term memory. This double dissociation is strong evidence that working memory and long-term memory rely on distinct neural substrates.

Question 4

Sustained delay-period firing in dlPFC neurons represents a direct neural correlate of 'holding something in mind' — it is active in the absence of a stimulus and before any motor response.

• T. True
• F. False

Answer: True

This is what makes delay-period activity so significant. It cannot be explained as sensory processing (stimulus is absent) or motor preparation (no response is occurring yet). It is maintenance-specific — the neuron is actively representing information that exists only in the animal's working memory. This was a landmark empirical finding because it provided a concrete, measurable mechanism for the cognitive concept of 'keeping information online' in the brain.

Question 5

How does the recurrent connectivity architecture of the dlPFC enable it to maintain information even after the original stimulus has disappeared?

Model answer: The dlPFC contains dense reciprocal synaptic connections among its neurons. When a stimulus activates a population of dlPFC neurons, those neurons excite each other through their recurrent synapses, creating a self-sustaining reverberating circuit — each neuron's firing drives its neighbors to fire, which in turn drive it to keep firing. This positive feedback loop maintains the firing pattern even after the external input is removed, like a loop of dominoes that keeps cycling. The stability of this reverberating state encodes the absent stimulus as a sustained pattern of activity. Sensory cortex lacks this architecture; it is feedforward-dominant and responds to current inputs rather than maintaining past ones.

The distinction between recurrent (dlPFC) and feedforward (sensory cortex) architectures is the mechanistic key. It explains why working memory is localized to prefrontal regions, why it is disrupted by stress or competing inputs (which destabilize reverberation), and why pharmacological agents targeting recurrent circuit dynamics (e.g., D1 dopamine receptor modulators) can improve or impair working memory performance.