Models of Working Memory

Nathan Prosser

Biologically Inspired Intelligent SystemsMay 15, 2007

Requirements For Memory Models

• Short term memory: small-capacity memory which must be actively refreshed to avoid decay

• Long term memory: indefinite capacity, persists for an indefinite period of time

• Procedural memory: the ability to learn coordination & methods of performing relatively complex actions

• Additionally: must be able to store prototypes or representations of objects for categorization

Short Term Memory• Capacity: 5 to 7 “atoms” -atom can be a small image, a number, a word, etc.

• Requires active maintenance -decays after 200 to 500 ms if not actively refreshed

• Memories stored individually

• “Chunking” can increase capacity -it's easier to remember several 3-digit chunks of numbers than it is to remember one 9-digit number

Long Term Memory• Some things are encoded because of significance• Others must be encoded through rehearsal

• Separate semantic and episodic memory -semantic: knowledge of facts and information -episodic: remembrance of events that have happened

• Associative -a current event can trigger a related memory -active retrieval recalls not only the intended memory but also many related memories -episodic memory also associates temporally into episodes

• Procedural memory is often though of as a type of LTM

A Simple Psychological Model• Based on psychological research, does not take into account biological systems

SensoryMemory

Short-termMemory

Long-termMemory

attention

encoding

rehearsal

retrieval

Baddeley's Working Memory Model• In a 1974 paper, Alan Baddeley and Graham Hitch proposed an important componentized model of working memory

• Phonological Loop: Used in processing speech -referred to as an “inner ear” and an “inner speech” -recall things that have been spoken by others or things that are about to be spoken

• Visuospatial scratchpad: Used for visual / spatial processing -remember shapes, colors, & motion -used to formulate complex directions -used for mentally rotating objects

• Central Executive: The processor -processes information stored in memory

Baddeley's Working Memory ModelCentral

Executive

VisuospatialScratchpad

PhonologicalLoop

It's great that we know the limitations and capabilities of

memory, but how does it work?

A Biologically Based Computational Model of Working Memory, a 1999 paper by O'Reilly, Braver, & Cohen attempts to answer this

Structure• Looks at 3 key areas in the brain involved in working memory:

-Posterior Perceptual and Motor Cortex -Hippocampus -Prefrontal Cortex

• These components work together to produce all the characteristics of working memory described in the previous slides

• Types of memory: active neural patterns vs. Hebbian learning

Posterior Perceptual & Motor Cortex

• Learning through axonal weight modification

• Forms generalizations about the world

• Partially responsible for repetitive procedural memory

• Representations are distributed: does not form unique “memories,” learns how to respond to stimulus

Hippocampus

• Performs rapid learning

• Also uses axonal weight modification to learn

• Keeps individual memories more separate than in the PMC -similar information is associated

Prefrontal Cortex• Memory occurs through both neural activity patterns and Hebbian learning

• Activity in the Prefrontal Cortex biases the activity in the rest of the brain

• Representations are completely individual and separate• Representations can be kept active over time even without new supporting stimulus, and even in the presence of novel interfering stimulus• These representations come about through neural activity loops

• Also performs some object categorization functions -through axonal weight modification

Prefrontal Cortex (cont'd)• Active memory: can be modeled as interconnected neurons -activity of a group stimulates another group so that activity is continued along a similar vein -new information can be selectively admitted

•Object prototyping -each prototype is maintained separately -prototypes can be combined to categorize novel ideas -similar to language: a large number of separate words, each with unique, static meaning – can be combined together to form novel concepts

Representations in EachHCMP PFC

PMC

Hierarchy• Arranged from unconscious to conscious, or from automatic to controlled

• PMC: processing happens automatically -highly interconnected both to sensory inputs and to other brain systems -produces impulses based on inputs -most control comes from PFC biasing

• HCMP: higher in the hierarchy -very important to PFC functionality -receives input from PFC and lower brain systems, which derive their input from the senses

Hierarchy (cont'd)

• PFC: highest in this hierarchy -processing & memory is controlled & active -most directly involved in conscious thought

• Hierarchy is not distinct -each system is not at a specific point along the hierarchy -a “continuum” exists, each system contributes along a range of points

Control• The PFC is modeled as a neural network that can excite itself -active memory loop

• There must be a way of allowing or disallowing interference of new stimulus -a gate

• Dopamine control proposed -Midbrain Dopamine Nuclei, under control of the cortex -PFC is primarily self-connected -Dopamine makes afferent connections more significant

So... working memory?• Comes as a result of the interactions between these systems

• We often think of working memory as the same as active memory – which is a function of the PFC

• Controlled processing (and thus meaningful working memory) would not be able to happen without functionality of multiple brain systems

• Active patterns reinforced and made significant by input from the HCMP and PMC