Connectionism and models of memory and amnesia
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Connectionism and models of memory and amnesia. Jaap Murre University of Amsterdam [email protected] http://www.memory.uva.nl.

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Connectionism and models of memory and amnesia

Connectionism and models of memory and amnesia

Jaap Murre

University of Amsterdam

[email protected]

http://www.memory.uva.nl


Connectionism and models of memory and amnesia

The French neurologist Ribot discovered more than 100 years ago that in retrograde amnesia one tends to loose recent memoriesMemory loss gradients in RA are called Ribot gradients


Overview

Overview

  • Catastrophic interference and hypertransfer

  • Brief review of neuroanatomy

  • Outline of the TraceLink model

  • Some simulation results of neural network model, focussing on retrograde amnesia

  • Recent work:

    • Mathematical point-process model

  • Concluding remarks


Catastrophic interference

Catastrophic interference

  • Learning new patterns in backpropation will overwrite all existing patterns

  • Rehearsal is necessary

  • McCloskey and Cohen (1989), Ratcliff (1990)

  • This is not psychologically plausible


Osgood surface 1949

Osgood surface (1949)

  • Paired-associates in lists A and B will interfere strongly if the stimuli are similar but the responses vary

  • If stimuli are different, little interference (i.e., forgetting) occurs

  • Backpropagation also shows odd behavior if stimuli vary but responses are similar in lists A and B (hypertransfer)


Hypertransfer

Hypertransfer

Learned responses

StimuliTarget responses (after three learning trials)

Phase 1: Learning list A

rist munk twup

gork gomp toup

wemp twub twup

Phase 2: Learning interfering list B

(after five learning trials)

yupe munk muup

maws gomp twup

drin twub twub

Phase 3: Retesting on list A

rist munk goub

gork gomp tomp

wemp twub twub


Problems with sequential learning in backpropagation

Problems with sequential learning in backpropagation

  • Reason 1: Strongly overlapping hidden-layer representations

  • Remedy 1: reduce the hidden-layer representations

    • French, Murre (semi-distributed representations)


Problems with sequential learning in backpropagation1

Problems with sequential learning in backpropagation

  • Reason 2: Satisfying only immediate learning constraints

  • Remedy 2: Rehearse some old patterns, when learning new ones

    • Murre (1992): random rehearsal

    • McClelland, McNaughton and O’Reilly (1995): interleaved learning


Final remarks on sequential learning

Final remarks on sequential learning

  • Two-layer ‘backpropagation’ networks do show plausible forgetting

  • Other learning networks do not exhibit catastrophic interference: ART, CALM, Kohonen Maps, etc.

  • It is not a necessary condition of learning neural networks; it mainly affects backpropagation

  • The brain does not do backpropagation and therefore does not suffer from this problem


Models of amnesia and memory in the brain

Models of amnesia and memory in the brain

  • TraceLink

  • Point-process model

  • Chain-development model


Neuroanatomy of amnesia

Neuroanatomy of amnesia

  • Hippocampus

  • Adjacent areas such as entorhinal cortex and parahippocampal cortex

  • Basal forebrain nuclei

  • Diencephalon


The position of the hippocampus in the brain

The position of the hippocampus in the brain


Hippocampal connections

Hippocampal connections


Connectionism and models of memory and amnesia

Hippocampus has an

excellent overview

of the entire cortex


Trace link model structure

Trace-Link model: structure


System 1 trace system

System 1: Trace system

  • Function: Substrate for bulk storage of memories, ‘association machine’

  • Corresponds roughly to neocortex


System 2 link system

System 2: Link system

  • Function: Initial ‘scaffold’ for episodes

  • Corresponds roughly to hippocampus and certain temporal and perhaps frontal areas


System 3 modulatory system

System 3: Modulatory system

  • Function: Control of plasticity

  • Involves at least parts of the hippocampus, amygdala, fornix, and certain nuclei in the basal forebrain and in the brain stem


Stages in episodic learning

Stages in episodic learning


Dreaming and consolidation of memory

Dreaming and consolidation of memory

“We dream in order to forget”

  • Theory by Francis Crick and Graeme Mitchison (1983)

  • Main problem: Overloading of memory

  • Solution: Reverse learning leads to removal of ‘obsessions’


Dreaming and memory consolidation

Dreaming and memory consolidation

  • When should this reverse learning take place?

  • During REM sleep

    • Normal input is deactivated

    • Semi-random activations from the brain stem

    • REM sleep may have lively hallucinations


Consolidation may also strengthen memory

Consolidation may also strengthen memory

  • This may occur during deep sleep (as opposed to REM sleep)

  • Both hypothetical processes may work together to achieve an increase in the definition of representations in the cortex


Recent data by matt wilson and bruce mcnaughton 1994

Recent data by Matt Wilson and Bruce McNaughton (1994)

  • 120 neurons in rat hippocampus

  • PRE: Slow-wave sleep before being in the experimental environment (cage)

  • RUN: During experimental environment

  • POST: Slow-wave sleep after having been in the experimental environment


Wilson en mcnaughton data

Wilson en McNaughton Data

  • PRE: Slow-wave sleep before being in the experimental environment (cage)

  • RUN: During experimental environment

  • POST: Slow-wave sleep after having been in the experimental environment


Some important characteristics of amnesia

Some important characteristics of amnesia

  • Anterograde amnesia (AA)

    • Implicit memory preserved

  • Retrograde amnesia (RA)

    • Ribot gradients

  • Pattern of correlations between AA and RA

    • No perfect correlation between AA and RA


Connectionism and models of memory and amnesia

Normal forgetting

anterograde

amnesia

retrograde

amnesia

x

past

lesion

present


An example of retrograde amnesia patient data

An example of retrograde amnesia patient data

Kopelman (1989)News events test


Retrograde amnesia

Retrograde amnesia

  • Primary cause: loss of links

  • Ribot gradients

  • Shrinkage


Anterograde amnesia

Anterograde amnesia

  • Primary cause: loss of modulatory system

  • Secondary cause: loss of links

  • Preserved implicit memory


Semantic dementia

Semantic dementia

  • The term was adopted recently to describe a new form of dementia, notably by Julie Snowden et al. (1989, 1994) and by John Hodges et al. (1992, 1994)

  • Semantic dementia is almost a mirror-image of amnesia


Neuropsychology of semantic dementia

Neuropsychology of semantic dementia

  • Progressive loss of semantic knowledge

  • Word-finding problems

  • Comprehension difficulties

  • No problems with new learning

  • Lesions mainly located in the infero-lateral temporal cortex but (early in the disease) with sparing of the hippocampus


No consolidation in semantic dementia

No consolidation in semantic dementia

Severe loss of trace

connections

Stage-2 learning proceeds

as normal

Stage 3 learning strongly

impaired

Non-rehearsed memories

will be lost


Semantic dementia in tracelink

Semantic dementia in TraceLink

  • Primary cause: loss of trace-trace connections

  • Stage-3 (and 4) memories cannot be formed: no consolidation

  • The preservation of new memories will be dependent on constant rehearsal


Connectionist implementation of the tracelink model

Connectionist implementationof the TraceLink model

With Martijn Meeter from the University of Amsterdam


Some details of the model

Some details of the model

  • 42 link nodes, 200 trace nodes

  • for each pattern

    • 7 nodes are active in the link system

    • 10 nodes in the trace system

  • Trace system has lower learning rate that the link system


How the simulations work one simulated day

How the simulations work: One simulated ‘day’

  • A new pattern is activated

  • The pattern is learned

  • Because of low learning rate, the pattern is not well encoded at first in the trace system

  • A period of ‘simulated dreaming’ follows

    • Nodes are activated randomly by the model

    • This random activity causes recall of a pattern

    • A recalled pattern is than learned extra


Patient data

(Patient data)

Kopelman (1989)News events test


A simulation with tracelink

A simulation with TraceLink


Frequency of consolidation of patterns over time

Frequency of consolidation of patterns over time


Strongly and weakly encoded patterns

Strongly and weakly encoded patterns

  • Mixture of weak, middle and strong patterns

  • Strong patterns had a higher learning parameter (cf. longer learning time)


Transient global amnesia tga

Transient Global Amnesia (TGA)

  • (Witnessed onset) of severe anterograde and retrograde amnesia

  • Resolves within 24 hours

  • Retrograde amnesia may have Ribot gradients

  • Hippocampal area is most probably implicated


Connectionism and models of memory and amnesia

Transient Global Amnesia (TGA)


Other simulations

Other simulations

  • Focal retrograde amnesia

  • Levels of processing

  • Semantic dementia

  • Implicit memory

  • More subtle lesions (e.g., only within-link connections, cf. CA1 lesions)


The memory chain model a very abstract neural network

The Memory Chain Model: a very abstract neural network

With Antonio Chessa from the University of Amsterdam


Abstracting tracelink level 1

Abstracting TraceLink (level 1)

  • Model formulated within the mathematical framework of point processes

  • Generalizes TraceLink’s two-store approach to multiple neural ‘stores’

    • trace system

    • link system

    • working memory, short-term memory, etc.

  • A store corresponds to a neural process or structure


Learning and forgetting as a stochastic process 1 store example

Learning and forgetting as a stochastic process: 1-store example

  • A recall cue (e.g., a face) may access different aspects of a stored memory

  • If a point is found in the neural cue area, the correct response (e.g., the name) can be given

Forgetting

Successful

Recall

Unsuccessful

Recall

Learning


Neural network interpretation

Jo Brand

Neural network interpretation


Single store point process

m

a

Link system

Retrieval

Survival probability

Single-store point process

  • The expected number of points in the cue area after learning is called 

  • This  is directly increased by learning and also by more effective cueing

  • At each time step, points die

  • The probability of survival of a point is denoted by a


Some aspects of the point process model

Some aspects of the point process model

  • Model of simultaneous learning and forgetting

  • Clear relationship between signal detection theory (d'), recall (p), savings (Ebbinghaus’ Q), and Crovitz-type distribution functions

  • Multi-trial learning and multi-trial savings

  • Currently applied to over 250 experiments in learning and forgetting, since 1885


Forgetting curve

Forgetting curve

If we need to find at least one point we obtain the following curve (one-store case):

m is the intensity of the process (expected number

of points) and a is the decay parameter

We predict a flex point when the initial recall is

at least


Connectionism and models of memory and amnesia

Example: Single-store model fitted to short-term forgetting data

R2 = 0,985


Multi store generalization

Multi-store generalization

  • Information about the current event passes through many neural ‘stores’

  • The retina, for example, holds a lot of information very briefly

  • The cerebral cortex holds very little information (of the current event) for a very long time


General principles of the ppm multi store model

General principles of the PPM multi-store model

  • A small part of the information is passed to the next store before it decays completely

  • Subsequent stores hold information for longer time periods: slower decay rates in ‘higher’ stores


Two store model

Two-store model

  • While neural store 1 is decaying (with rate a1) it induces new points (representations) in store 2

  • Induction rate is linear with the intensity in store 1 and has induction rate m2

  • The points in store immediately start to decay as well (at a lower rate a2)


Example of two neural stores

Example of two neural stores

  • Store 1: firing neural groups

  • Store 2: synaptic connections between the neural groups

  • Other interpretation are possible as well, e.g.:

    • Store 1: hippocampus

    • Store 2: cerebral cortex

Skip


Example of two neural stores encoding phase

Example of two neural stores: encoding phase

Additional

cue area

Store 1

Stimulus

H


Storage phase decay of neural groups and hebbian learning

Storage phase: decay of neural groups and Hebbian learning

Store 2


Connectionism and models of memory and amnesia

Q

R

?

Recall phase: retrieval through cue Q

Skip


The contributions of s individual neural stores can simply be added

The contributions of S individual neural stores can simply be added


Two store model retention function r 12 t r 1 t r 2 t

Two-store model retention function: r12(t)= r1(t)+ r2(t)


The retention function for the third store of a three store model

The retention function for the third-store of a three-store model


Recall probability p t as a function of different learning times l

Recall probability p(t) as a function of different learning times l

n is the learning rate

l is the learning time

r(t) is the decline function

t time since learning


Saturation assumption

Saturation assumption


Hellyer 1962 recall as a function of 1 2 4 and 8 presentations

Hellyer (1962). Recall as a function of 1, 2, 4 and 8 presentations

Skip

Two-store model with saturation. Parameters are

m1= 7.4, a1= 0.53, m2= 0.26, a2= 0.31, rmax= 85; R2=.986


Amnesia animal data

Amnesia: animal data

Retrograde amnesia


Cho kesner 1996 mice r 2 0 96

Cho & Kesner (1996). (mice) R2=0.96


Summary of animal data

Summary of animal data


Frankland et al 2001 study

Frankland et al. (2001) study

  • a-CaMKB-dependent plasticity (in neocortex) switched off in knock-out mice

  • No LTP measurable in neocortex but LTP in hippocampus was largely normal

  • Forgetting curves with different levels of initial learning were measured

  • A learning curve was measured

  • Assumption: use r1[2](t) for knock-out mice


Forgetting after 3 shocks using three parameters

Forgetting after 3 shocks, using three parameters


Using the same three parameters and a massed learning correction

Using the same three parameters and a massed-learning correction.


Controls receive 1 shock experimental animals 3 shocks no new free parameters

Controls receive 1 shock, experimental animals 3 shocks (no new free parameters).


Repeated learning for experimental animals no new free parameters

Repeated learning for experimental animals (no new free parameters)


Summary of cortical amnesia using only 4 parameters for all curves r 2 0 976

Summary of ‘cortical amnesia’. Using only 4 parameters for all curves (R2 = 0.976).


Amnesia human data

Amnesia: human data


Application to retrograde amnesia

Application to retrograde amnesia

  • Data on clinical tests cannot be used for direct modeling

  • The reason is that remote time periods in these tests are typically made easier

  • Data for the different time periods are therefore not equivalent

  • Our model may offer a solution here: the relative retrograde gradient or rr-gradient


Sometimes this problems occurs with animal data as well

Sometimes this problems occurs with animal data as well

  • Wiig, Cooper, and Bear (1996)

  • Used non-counterbalanced stimuli


Wiig cooper bear 1996 rats r 2 0 28

Wiig, Cooper & Bear (1996). (rats) R2=0.28


Wiig cooper bear 1996 rats with rr gradient r 2 0 84

Wiig, Cooper & Bear (1996). (rats) with rr-gradient: R2=0.84


Define the relative retrograde gradient or rr gradient

Define the relative retrograde gradient or rr-gradient


Rr gradient continued

rr-gradient (continued)


The rr gradient does not have parameters for learning strength m 1 or cue strength q

The rr-gradient does not have parameters for learning strength m1 or cue strength q


Recall probability p t must transformed to retention r t

Recall probability p(t) must transformed to retention r(t)


Albert et al 1979 naming of famous faces

Albert et al. (1979), naming of famous faces


Squire haist and shimamura 1989 recall of public events

Squire, Haist, and Shimamura (1989), recall of public events


Concluding remarks

Concluding remarks

  • In this presentation, we have shown models at two levels of abstraction:

    • Mathematical, based on point processes

    • Computational, based on simplified neural networks


Concluding remarks1

Concluding remarks

  • These models incorporate data from:

    • Neuroanatomy and neurophysiology

    • Neurology and neuropsychology

    • Experimental psychology

  • The aim is to integrate these various sources of data into a single theory that is implemented in a series of coordinated models


Concluding remarks2

Concluding remarks

  • Given that the brain is exceedingly complex, we need models at various levels of abstraction to aid our understanding

  • This is especially true when trying to unravel the link between the brain and human behavior, which is extremely complex itself

  • Hence, models are of particular use in the new, interdisciplinary field of cognitive neuroscience


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