distributed representations

[Ashley Aitken]

G'day,

In the discussion of "Distributed Representations", Max Coltheart writes:
>
> But for nets that are meant to be
> models of cognition, the hidden assumption seems to be that after brain damage
> there is graceful degradation of cognitive processing, so the fact that nets
> show graceful degradation too means they have promise for modelling cognition.
> 
> But where's the evidence that brain damage degrades cognition gracefully? That
> is, the person just gets a little bit worse at a lot of things? Very commonly,
> exactly the opposite happens - the person remains normal at almost all kinds
> of cognitive processing, but some specific cognitive task suffers catastroph-
> ically. No graceful degradation here.

I would suggest that Max is possibly confusing diffuse brain damage with 
catastrophic brain damage. 

Diffuse brain damage is the elimination of a small percentage of neurons 
diffusely from throughout the brain. Examples are the natural death of 
neurons throughout the brain and, perhaps, micro-lesions.

The continual death of an immense number of neurons in the brain, thankfully
only really amounts to the death of a very small percentage of the neurons 
in the brain. In any of the partitioned networks of the brain (say an area 
of the cortex) we would expect only a small number of neurons to die.

If one considers that a neuron may receive in the order of thousands of 
synapses on it's dendritic tree, it can be understood, I believe, how the 
network (thought of as a connectionist network) could continue to function 
if one or two of these were to be eliminated.

I would suggest that this continual death of neurons in the brain with the 
subtle, and often unnoticed, degradation in cognitive performance to be an 
example of (diffuse) brain damage degrading cognition gracefully. Hence, I
believe this type of degradation does show neural networks have promise 
for modelling cogntion.

Of course, this does depend on the the degradation seen in cognition being 
shown to be qualitatively the same as degradation seen in artificial neural 
networks.

Catastrophic brain damage, on the other hand, is the gross elimination of 
neurons (usually relatively localized) from the brain. Examples are lesions
resulting from head injuries or strokes, and ablation.

It would seem that in this case one is most likely seeing the complete (or 
nearly complete) elimination of an entire network (or a critical part of
it) and hence the elimination of it's associated and dependent function(s).
I don't believe anyone would suggest that the brain's function would degrade 
gracefully under such terrorist action.

Max continues:
> 
> I could give very many examples: I'll just give one (Semanza & Zettin, 
> Cognitive Neuropsychology, 1988 5 711). This patient, after his stroke, had
> impaired language, but this impairment was confined to language production
> (comprehension was fine) and to the production of just one type of word: proper
> nouns. He could understand proper nouns normally, but could produce almost none
> whilst his production of other kinds of nouns was normal. What's graceful about
> this degradation of cognition? 

I am definitely no expert neuroscientist but I would suggest that this is an 
example of catastrophic brain damage not diffuse brain damage. Hence, I would 
not expect graceful degradation of cognitive performance. It seems to me that 
this would be too much to ask of all but the most completely holographic-like 
systems.

The interesting point to be made from this example would then be that it 
appears to be evidence for a cortical region involved (directly or in-line) 
with the speech of only nouns. Amazing!

It would also be interesting to test if there is any subtle difference in our
*understanding* of a noun depending upon whether we are receiving (ie hearing
or seeing it) with when we are producing (ie speaking or imagining) it.

If this diagnosis of catastrophic brain damage is correct then I believe this 
example is mute upon whether or not the brain is functionally a Connectionist 
System. Still, the Connectionist System, in my opinion, gets the points for 
the diffuse brain damage. 

Hence Max's concluding suggestion, 

> If cognition does *not* degrade gracefully, and neural nets do, what does this
> say about neural nets as models of cognition?

becomes rather misplaced because cognition does appear to degrade gracefully 
under diffuse brain damage and catastrophically under catastrophic brain
damage. The former providing possible evidence for neural networks as 
models of cognition.


Ashley
ashley at spectrum.cs.unsw.oz.au