6. Memory

• information content

• time course

  • short-term (passive sensory trace)

  • long- term (active exploitation of long-term memory)

• temporary activation of updated long-term memory networks for organizing actions in the near term

• associative, provides meaning and purpose,

• has future perspective

• sustained attention focused on an internal representation

• essential for all aspects of executive control & enables cognitive functioning beyond simole input-output mappings

• phonological loop: verbal material

• visuo-spatial sketch pad: visual material

• central executive: control stage

• store information over short time periods

• manipulate information to think and act

• core cognitive function for intelligent behavior

They recorded single-unit activity in the dorsolateral prefrontal cortex of monkeys and found that certain neurons showed persistent firing during delay periods in a delayed-response task. This was one of the first pieces of evidence that the prefrontal cortex maintains information actively over short delays, suggesting a key role in working memory.

They showed that neurons in the dorsolateral prefrontal cortex (dlPFC) of monkeys exhibit spatially selective persistent activity during the delay period of the ODR task.

The neurons fired specifically when the monkey was remembering a particular visual location, linking delay-period activity to spatial working memory and supporting the idea that dlPFC actively maintains goal-relevant spatial information.

-> individual PFC neurons are tuned to locations in space

(Trial: monkey fixate a point, flash is rising, monkey still has to look at fixation, fixation point is out, monkey has to look at the point where the flash was)

Trial Structure:

1. Sample: A visual object is briefly shown to the subject (e.g. a monkey).

2. Delay: The object disappears; the subject must retain it in working memory.

3. Test: A new object is shown. The subject must decide: → Is this the same object as before?

4. Response: Match = reward; Non-match = no reward.

Key Finding:

• Neurons in the inferotemporal cortex (IT) show object-specific activity during the delay period.

• This means the IT cortex can actively maintain object identity in working memory, even in the absence of visual input.

-> individual PFC neurons encode working memory content that is independent of motor commands

Persistent activity refers to the ongoing firing of neurons during the delay period of a task — after a stimulus has disappeared but before a response is required.It reflects the active maintenance of information in working memory.

• intrinsic mechanisms

• network mechanisms

Transient activity model (Lundqvist et al., 2016): Working memory is maintained by brief, rhythmic bursts of neural activity, rather than continuous firing.

Gamma bursts (~30–80 Hz): Encode/decode or reactivate specific items (only found in recording sites with informative spiking)

Beta bursts (~15–30 Hz): steady state, Linked to top-down control; may suppress gamma

Neural firing is intermittent, oscillatory, and sparse

Differs from persistent activity:

– No continuous firing during the delay

– Emphasizes dynamic population coding

– More energy-efficient and biologically plausible

Activity-silent working memory stores information without continuous neural firing. Instead, it relies on temporary changes in synaptic strength or hidden network states.

• No visible delay activity (no spikes or bursts)

• Memory can be reactivated later (e.g., by a cue or TMS pulse)

• More energy-efficient and flexible than persistent firing

They found that working memory is supported by bidirectional, frequency-specific oscillatory synchrony between the prefrontal and parietal cortex:

Beta-band (13–30 Hz): – Parietal → Frontal signaling – Encodes recent inputs (bottom-up flow)

Theta-band (4–8 Hz): – Frontal → Parietal coupling – Differentiates and organizes multiple memory items, controling (top-down control)

Phase coding: – Spikes aligned to different phases of theta oscillations help separate relevant from distracting information

• top-down connectivity in the theta freq band braks dowan after PFC damage

• top-down and bottom-up connectivity established distinct communication channels

Phase-dependent coding means that the timing of neural spikes relative to the phase of brain oscillations (like theta or gamma) determines what content is being represented.

• Different memory items are encoded at different oscillatory phases (e.g., Item A at 0°, Item B at 180°).

• This allows the brain to store multiple items in the same neural population without interference — a form of temporal multiplexing.

• Also helps separate relevant from distracting information.

• Shown in studies like Jacob et al. (2018), where working memory items were separated by theta phase across prefrontal and parietal cortex.

They showed that stimulating the brain at specific frequencies causally affects working memory:

Theta-rTMS (5 Hz) over PFC:

– Enhanced theta synchrony with parietal cortex

– Improved prioritization of relevant memory items

Alpha-rTMS (10 Hz) over parietal cortex:

– Increased alpha power, especially contralateral to distractors

– Boosted suppression of irrelevant information

Conclusion: Working memory control relies on frequency-specific oscillations:

– Theta supports top-down control

– Alpha supports distractor suppression

• working memory content can be decoded from the visual cortex

• responses are similar when stimuli are unattended and task.irrelevant

• working memory tasks engage multiple regions of cortex

• different brain mechansims for storage and selection of working memory information

neuromodulators/chemicals that fine tune neuronal activity

• executive components and percepeptual components

• Preferential innervation

• Topographic organization: Dopaminergic input follows structured pathways related to interoceptive (internal) and exteroceptive (external)information processing.

• Signal modulation: Prefrontal dopamine, via D1 receptors, regulates the strength of working memory signals:

  • – Blocking D1 → weaker signals

  • Stimulating D1 → enhanced signal strength