Download Semantics Without Categorization

Disintegration of Conceptual
Knowledge In Semantic Dementia
James L. McClelland
Department of Psychology and
Center 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 detail
Importation 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 Differentiation
in 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 level
of the tree, e.g.:
Robin can {grow, move, fly}
Target output for ‘robin can’ input
Forward Propagation of Activation
aj
wij
neti=Sajwij
ai
wki
Back Propagation of Error (d)
aj
wij
di ~
Sdkwki
ai
wki
Error-correcting learning:
At the output layer:
At the prior layer:
…
dk ~ (tk-ak)
Dwki = edkai
Dwij = edjaj
Very Young
Still Young
Older
What Drives Progressive
Differentiation?
Coherent Covariation
Overgeneralization of Frequent
Names to Similar Objects
“tree”
“goat”
“bird”
“dog”
A typical property that
a particular object lacks
e.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 naming
and drawing in
Sem. 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-Based
Morphometry
(Mummery et al, 2000)
Rogers et al (2005)
model of semantic
dementia
temporal
pole
name
function
assoc
vision
• 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.
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.
Errors in Naming for As a Function of Severity
Patient Data
Simulation Results
omissions
within categ.
superord.
Severity of Dementia
Fraction of Neurons Destroyed
Simulation of Delayed Copying
temporal
pole
name
function
assoc
vision
• 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
IF’s ‘camel’
DC’s ‘swan’
Simulation results for feature production by lesion severity
Omissions by feature type
Intrusions by feature type
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
name
action
Temporal
pole
motion
color
valance
form
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.