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Connectionism and models of memory and amnesia. Jaap Murre University of Amsterdam [email protected]

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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]

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
  • 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)

Learned responses

Stimuli Target 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
Hippocampus has an

excellent overview

of the entire cortex

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
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
Normal forgetting









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


Stage-2 learning proceeds

as normal

Stage 3 learning strongly


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

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
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







single store point process


Link system


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

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


example of two neural stores encoding phase
Example of two neural stores: encoding phase


cue area

Store 1






Recall phase: retrieval through cue Q


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

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


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

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
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
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