Working Memory

Question 1

1. WM storage controversy

Answer 1

Early Views based on lesion studies: WM is primarily PFC

Problem of studies: activity during memory delay or memory impairment after lesions to a region do not necessarily mean that regions STORES the WM content  could just hold processes important to that storage

Both low-level and high-level cortices involved  distributed representation – but why?
• PFC could contain a duplication of sensory information
o Redundant, but might help in robustness against distraction
• However: differences in functional roles of regions
o Differing levels of abstraction in sensory regions (simple features) and PFC (abstract or verbal format)
o Differing functional roles of representation, from representation of sensory input to planning behavioural response
•  there seems to be a division of labour between low and high level regions
o Advantage: no necessity to duplicate low-level feature information in PFC
o PFC can then build flexible and task-dependent representations

Question 2

2. WM capacity controversy, slots vs resources

Answer 2

WM capacity is very limited
 There must be a restriction on the number of memories per node or the number of active nodes
 Is this restriction due to properties of individual storage regions or of the distributed network?

Competition for representation
• Multiple items compete within the same representational map, interfering with each other during encoding or delay period
• Evidence: visual WM capacity correlates with individual V1 volume, persistent stimulus selective activity per item decreases when number of items increases
• However: differing sensory modalities can impair each other, e.g. memorization of visual features reducing ability to retain auditory features

Distributed storage
• Items that are currently in the focus of attention are stored in a detailed way in the sensory cortex, secondary items for later use in a more abstract way in anterior regions
• Fits findings on neural activity

Task-relevant detail
• Level of detail required has implications for capacity
• Posterior to frontal gradient: complexity reduction, chunking
•  higher areas can retain larger chunks of low-level features

Question 3

3. Working memory active representation controversy (activity-silent WM)

Answer 3

What is the neurocomputational mechanism of retention? Two main theories:
• Activity-based retention
• Activity-silent retention
o = Stimulus-selective representations are maintained as a pattern of synaptic weights
• Might be a combination of both

Question 4

4. Persistent stimulus-selective activity

Answer 4

Necessary properties of regions encoding WM content:
• Stimulus selectivity: different memory contents should lead to different patterns of activity
• Persistent or delay-period activity: stimulus selective activity should persist over delay

Electrophysiological recordings on non-human primates
• Some findings on content specific delay period activity in the PFC
• But also in modality-specific sensory cortices
• Furthermore some other frontal, parietal and temporal areas

MVPA in humans
• Found delay-period stimulus specific activity in sensory cortical regions, frontal areas (e.g. FEF and PFC)
• Also EEG studies found activity in the PFC

 WM content is stored in a distributed way across sensory, parietal, temporal and frontal cortices

Persistent activity in the PFC
• reflects both retrospective memory and a prospective action plan
• Increases in strength until right before the response
• Should be highly task-specific

Question 5

5. Definitions of distributed representations

Answer 5

1. Local pattern information: referring to decoding stimuli from patterns of brain activity – distributed representation means information is distributed across a local population of neurons within one area
2. Separable information in multiple brain regions: multiple local response pattern exist in parallel and can be independently used to decode stimulus information
3. Inseparable information across multiple brain regions: information is encoded in global patterns of brain activity, and single areas alone would not allow decoding

How to disentangle this?
• Difference between 2 and 3 is the question whether information in multiple areas is redundant – does one region contain enough information already to decode a stimulus feature? Probably gradual rather than categorical
• Lesion studies
• Looking at choice probabilities: which signal is maximally predictive of a behavioural choice

Question 6

6. WM gradient

Answer 6

Posterior Frontal
Sensory cortices Increasing levels of processing Frontal cortex
sensory information Abstracted information,
(pure and detailed) transformed to support behaviour

Areas in between might maintain intermediate transformations

Which regions exactly are involved depends on the task – which format is the most suitable?
- e.g. is detailed sensory information required, or more abstract?