Disintegration of Conceptual Knowledge In Semantic Dementia

James L. McClelland

Department of Psychology andCenter for Mind, Brain, and Computation

Stanford University

A prize winning scientist

• Graduated college with a 3.99 GPA.• Ph. D. in Mathematical Psychology from one of

the top universities in the US in the 1960’s.• Lead the creation of a research field with the

derivation of a crucial method during the early 80’s.

• Elected to the NAS, won MacArthur ‘Genius’ Award.

A semantic dementia patient

• Left his faculty position in 1999 after an extended period of gradual cognitive decline.

• Diagnosed with fronto-temporal (semantic) dementia, a progressive neurodegenerative disorder affecting semantic knowledge.

• Lives in a facility for persons with ‘memory disorders’.

• Can no longer name or recognize familiar objects, including close relatives and friends.

The General Picture of Semantic Dementia

• Poor performance on tasks requiring use of knowledge of objects.– Naming– Word-picture-matching– ‘Pyramids and Palm Trees’

• With words or pictures

– Delayed but not immediate copying– Object decision

• Progressive deterioration

Delayed Copy

“Camel” “Swan”

Loss of item-specific detailImportation of typical properties of other category members

Object Decision

Gradual deterioration of picture naming in a semantic dementia patient

- Preservation and overgeneralization of highly frequent concepts.- Loss of specific names, preservation of superordinate.

Our Question

• How is conceptual knowledge represented, such that it degrades gracefully in this way?

Our Answer

• Conceptual representations are– learned– distributed

patterns of neural activity

• Such representations– Arise gradually over developmental time– Degrade gracefully with damage or additions of

noise.– Learn and retain typical and shared information

much better than atypical or idiosyncratic information.

• In Rogers and McClelland (2004) we address:

– Emergence of conceptual knowledge in development.

– Many of the phenomena addressed by classic work in psychology on semantic knowledge from the 1970’s:

– Disintegration of conceptual knowledge in semantic dementia

My Talk Today

• Discuss key features of the development of conceptual knowledge, and present a model that addresses these features.

• Address progressive disintegration of conceptual knowledge in semantic dementia, and show how the model addresses these as well.

Two Phenomena in Development

• Progressive differentiation of concepts• Generalization and overgeneralization

Progressive Differentiationin Conceptual Development

Background Principles of Representation, Processing, and Learning of Semantic and

Conceptual Knowledge

• An active semantic representation (e.g., of a yellow Volkswagen) is a pattern of neural activity distributed over brain regions and over neurons within brain regions.

• Neurons in different regions of the neocortex encode different aspects of the object under consideration.

• Activation of a semantic representation occurs through an interactive activation process involving bi-directional propagation of activation among neurons in participating brain regions.

• The knowledge that allows an input to produce the appropriate semantic representations is stored in the strengths of the connections among the participating neurons and acquired through a gradual learning process.

Semantic Learning Model

The Training Data:

All propositions true of items at the bottom levelof the tree, e.g.:

Robin can {grow, move, fly}

Target output for ‘robin can’ input

aj

ai

wij

neti=ajwij

wki

Forward Propagation of Activation

k ~ (tk-ak)

wij

i ~ kwki

wki

aj

Back Propagation of Error ()

Error-correcting learning:

At the output layer: wki = kai

At the prior layer: wij = jaj

…

ai

Very Young

Still Young

Older

What Drives Progressive Differentiation?

Coherent Covariation

Overgeneralization of Frequent Names to Similar Objects

“dog”

“goat”“tree” “bird”

A typical property thata particular object lackse.g., pine has leaves

An infrequent,atypical property

Disintegration of Conceptual Knowledge in Semantic Dementia

• Progressive loss of specific knowledge of concepts, including their names

• Overgeneralization of frequent names• Loss of specific object properties,

overgeneralization of typcial object properties

Picture namingand drawing inSem. Demantia

Grounding the Model in What we Know About The Organization of Semantic

Knowledge in The Brain

• There is now evidence for specialized areas subserving many different kinds of semantic information.

• Semantic dementia results from progressive bilateral disintegration of the anterior temporal cortex.

language

Voxel-BasedMorphometry(Mummery et al, 2000)

Rogers et al (2005) model of semantic dementia

• Simplified neocortical learning system.

• Gradually learns through exposure to information about concepts based on normative data.

• After learning, the network can activate each other type of information from name or visual input.

• Representations undergo progressive differentiation as learning progresses.

• Damage to units within the temporal pole or to connections leads to the pattern of deficits seen in semantic dementia.

name assocfunction

temporal pole

vision

Modeling Features of Semantic Dementia

• Patterns of naming errors as a function of severity of semantic degradation.– Pattern of different error types.– Interaction of error types with category

structure.• ‘Typicalization’ of concepts:

– Omission of distinguishing details.– Intrusion of typical properties.

Severity of Dementia Fraction of Neurons Destroyed

omissions

within categ.

superord.

Patient Data Simulation Results

Errors in Naming for As a Function of Severity

Simulation of Delayed Copying

• Visual input is presented, then removed.

• After several time steps, pattern is compared to the pattern that was presented initially.

• Omissions and intrusions are scored for typicality

name assocfunction

temporal pole

vision

Omissions by feature type Intrusions by feature type

IF’s ‘camel’ DC’s ‘swan’

Simulation results for feature production by lesion severity

Ongoing Investigations

• What is the relationship between semantic and linguistic knowledge?

• How does this relate to the distribution of damage in the brain?– There is some evidence that patients with

more left damage understand and think far better than they can communicate with words.

What does this have to do with Altzheimer’s disease?

Grounding the Model in What we Know About The Organization of Semantic

Knowledge in The Brain

• There is now evidence for specialized areas subserving many different kinds of semantic information.

• Semantic dementia results from progressive bilateral disintegration of the anterior temporal cortex.

• Rapid acquisition of new knowledge (either ‘episodic’ or ‘semantic’) depends on medial temporal lobes, leaving long-term semantic knowledge intact.

language

Proposed Architecture for the Organization of Semantic and Episodic

Memory

colorform

motion

action

valance

Temporal pole

name

Medial Temporal Lobe

Some goals for future research

• Create an integrated model including both the cortical semantic system and the MTL system.

• Include frontal mechanisms necessary for the effective mobilization of mental activity in complex tasks.

• Incorporate crucial neuromodulatory systems.• Model effects of progressive deterioration in

different components to address aspects of AD, SD, and other neurodegenerative conditions.