Short-Term & Working Memory

Short-Term & Working Memory

• Short-term memory is for information currently held "in mind" and has limited capacity.
• Long-term memory stores information that doesn't need to be readily accessed, has virtually unlimited capacity.
• Information in LTM can enter STM if it's brought to mind.
• Working memory is better term because it captures the idea that the information currently in mind is being manipulated, unlike the static/passive store impression given by STM.
• Working memory denotes the active manipulation of information within a STM store to serve high cognitive functions like comprehension and reading.
• Working memory is critical for performing complex tasks or behaviours as well as daily life activities. If it fails, so too does the ability to carry out many activities of daily living

Baddeley's (2000) Model of Working Memory

• Baddeley's model proposes is the idea that STM stores have an interacting relationship with LTM
• The Baddeley model includes a central executive, visuospatial sketchpad, episodic buffer, and phonological loop
• Separate STM stores and an executive system manipulate and control information within the stores.
• Other approaches argue for temporary activity within LTM, rather than separate STM stores for working memory.

Neural Correlates of Phonological Loop

• The phonological loop contains a phonological store and a rehearsal mechanism.
• Paulesu et al. (1993) used a PET study to study the phonological loop, participants performing tasks engaging short-term memory for letters activating the store and rehearsal components. Alternatively, participants were performing rhyming judgments of letters activated the rehearsal system only.
• The phonological store is associated with the left supramarginal gyrus, and that the rehearsal system is associated with Brodmann's area 44 (Broca's area).
• According to the Baddeley model, the phonological loop contains

Neural Correlates of Visuospatial STM

• Ranganath et al. (2004) explored visual working memory maintenance and long-term associative retrieval.
• In Ranganath's experiment participants used STM to determine if a probe was a match.
• Participants also retrieved faces previously seen with a building from LTM to determine if the probe face was a match.

Ranganath et al., (2004) Visual WM Results

• Activity within category-selective regions of inferior temporal cortex was associated with actively maintained information in both associative and working memory tasks.
• Maintaining a single object in STM involves activating ventral stream representations.
• Regions are functionally connected to frontal and parietal regions during the delay period.

Working Memory and Prefrontal Cortex (PFC)

• The experiment involved a delayed-response task, measuring working memory in monkeys.
• The animal had to retain the location of unseen food during a delay period to test working memory.
• Prefrontal lesions impair performing this task.
• Animals are able to perform the task is this relies on forming associations, but not working memory alone. Associative memory is also tested by this paradigm.

Working memory vs associative memory

• A similar paradigm can be used to test associative memory.
• The food is paired with a visual cue (plus sign)
• The task measures the animal's is ability to retain long-term rules.
• No need for the animal to retain visuospatial information durng the delay period (as in the working memory task)
• During the delay period when testing asscoaitive memory, there is no need to store anyhting becaisde they have already learnt the association between th eplus and food
• PFC damage disrupts a but not b
• Damage in prefronal cortex can perform associative memory task but not wm task

PFC Neurons and Task Stages

• Prefrontal neurons respond differentially to different stages of an experiment (cue - delay - response).
• Neurons active during the delay period provide a neural correlate for keeping a representation active after a triggering stimulus is no longer active.
• Neurons are active during the delay period and provide neural correlates for keeping a representation active.
• Neurons remain active only if the animal needs the information for an imminent action.
• Neurons become responsive to a new set of stimuli if task conditions are changed.

What is the Role of the PFC?

• The PFC activity may reflect the representation of a task goal.
• Most theorists and empirical evidence agree that the PFC activity reflects representation of the task goal

PFC activity and memory

• Druzgal & D'Esposito (2003) looked at the function of the Pre Frontal Cortex in working memory activity

PFC in Working Memory for Faces

• Participants needed to memorise a set of faces, after a delay period a probe was shown
• Participants were asked whether it matches any of the faces in the trial.
• The study showed there is some sustained activation during delayed period.
• Activity was greater during the delay period the more faces had to be kept in mind
• The prefrontal cortex represents information we hold in our mind at the service of working memory and PFC responds to increases in the demands of this task

Working memory and PFC

• Prefrontal cortex activation reflects a representation of a task goal.
• Working memory relies on the interaction between the PFC and other parts of the brain with perceptual and long-term knowledge relevant to achieve a goal.
• PFC acts as a guide, selecting relevant information for a specific response or decision.

Petrides' Theory of Working Memory

• Petrides' model assumes division of PFC into at least two separate processes: maintenance and manipulation.
• DLPFC is active during manipulation and monitoring of information.
• VLPFC is active during activity maintenance and information retrieval.
• Short-term retention of spatial information has been shown correlated with the VentroLateral PFC, while Retention + update new location had Dorsolateral PFC activated.

Summary: Working/Short-term Memory

• Working memory involves manipulating information held within STM and is linked to the prefrontal cortex functions.
• STM might be considered distinct from LTM, though and alternative perspective suggest STM is the temporary activation of long term memory.
• Working memory relies on the PFC and other brain parts that contain perceptual and long-term knowledge important a goal.
• Petrides discriminated between DLPFC and VLPFC, with DLPFC controlling manipulation and monitoring, and VLPFC maintaining activated representations.
• PFC helps select info relevant to the current goal in posterior cortex that needs activation.

PFC and LTM

• The PFC is diverse, with functions associated with higher cognitive functions, such as executive abilities.
• The PFC is identified as the central executive site.
• The PFC is responsible for information maintenance and active control in LTM systems.
• Functions of the PFC are prevalent in purely LTM tasks like encoding and retrieval

PFC and Memory Encoding

• Lateralised responses in PFC at encoding depend on materials.
• Encoding verbal or semantic material recruits the left PFC.
• Encoding spatial information or faces is connected with the right PFC.

Subsequent Memory Effect: Clustered Recall

• Dorsolateral prefrontal cortex (DLPFC) activation predicts subsequent semantic clustering.
• Ventrolateral prefrontal cortex (VLPFC) activation predicted subsequent recall, whether clustered or unclustered.

PFC and Retrieval

• PFC regions aid in organizing, selecting, monitoring, and evaluating processing durnf retrieval.
• PFC regions do this when retrieving memories from memory
• Higher activity in this region is associated with the need for increased monitoring durng retrieval
• E.g., free recall, recall (vs recognition), low confidence judgements will have increased activity from the DLPFC
• Damage to the PFC results in greater impairment when doing free recall as compared to recognition

PFC and Free Recall

• In free recall, minimal cues are available to help memory performance.
• Greater strategic search, organization, selection, and retrieved information evaluation occur in free recall as compared to cued recall.
• Because of this, the conribution of PFC is mnimal

Source Monitoring & PFC

• Source monitoring relates to recall and recollection that stresses the ability to attribute retrieved memories to their original context.
• The PFC is involved as placing an event in context requires active evaluation process before we are able to access the origin of the memory.

Source Monitoring and PFC Damage

• Damage to the PFC causes issues in placing memories in their spatiotemporal contexts
• It subjetively causes the experience of remembrance (e.g. they can even perform a recgniton task), but they fail to retrieve the correct source
• In cases of PFC damaged the subjects often fabricate or confabule events not actually present

PFC and Memory

• The PFC contributes to maintaining information in working memory, selecting environmental information to focus on, providing retrieval cues, and evaluating memory content.