[Comp-neuro] "realistic models"

Harry Erwin harry.erwin at sunderland.ac.uk
Thu Aug 21 17:19:59 CEST 2008

On 20 Aug 2008, at 17:09, james bower wrote:

> The issue here is simple - it isn't whether the brain is for sure  
> KCC, it is a question of what assumptions one makes to begin with.   
> In the Marrian case, where one regards the brain as not necessarily  
> the optimal, but one of a number of implementations of a set of  
> algorithms, then one would clearly want to work on the set of  
> algorithms first - and ignore the brain -- the more one suspects  
> that the brain is not just one implementation - but perhaps a KCC  
> implementation -- then one pays more attention to the structure of  
> the brain -- In the end do these two approaches converge -- we can  
> hope -- but I would say they will only converge if the abstract  
> modeler accepts as an assumption (as I know, for example, that Bard,  
> Larry Abbot, and others do), that the brain, in the limit, is the  
> common reference point.

The reason physical science works--Paul Davies suggests--is that the  
aspects of the universe addressed by physical science are not KCC. A  
scientific field that deals with a KCC phenomenon is basically stamp- 
collecting. On the other hand, I believe there are areas of science  
that are materialist but not reducible--what intervenes is Stewart and  
Cohen's 'ant country'. There's reducibility--lawfulness--but it's an  
incomplete theory, and the phenomenon has to be understood at its own  
level, too.

> I also completely agree with you on the need to remember that we  
> classify things (neurons, motor-systems, etc), and the brain does  
> not care in the least about our classifications.  Again, in  
> principle, a structurally realistic model of the nervous system also  
> inherently does not have such classifications.  However the more  
> removed your modeling efforts are from that physical structure, the  
> more functionally modular models become.  Something I consider to be  
> a big problem.
> I actually think that it is critical to develop a better  
> understanding of what the brain does -- but, my assertion (or going  
> in assumption) that the brain is KCC means that what the brain does  
> is actually reflected fundamentally in its real structure -- another  
> argument for realistic modeling.
> I can give you a particular for instance - returning to the  
> cerebellum, looking at its circuitry, Marr / Albus proposed that,  
> basically, the climbing fiber 'taught' the Purkinje cell what  
> pattern of active parallel fibers to recognize.  Thus, the Purkinje  
> cell is seen as a parallel fiber pattern recognizer.  Based on our  
> realistic models and related experimental work, we have provided  
> evidence that the Purkinje cell doesn't even respond to parallel  
> fiber input, and instead, the the parallel fibers, by modulating  
> local regions of the Purkinje cell dendrite, provide contextual  
> information for the Purkinje cells 'evaluation' of inputs it  
> receives from excitatory inputs from synapses associated with the  
> axon of the granule cell as it courses vertically into the molecular  
> layer -- this is a COMPLETELY DIFFERENT type of computation --  
> Although first proposed almost 30 years ago, no current abstract  
> model of cerebellar function even includes this ascending axon  
> input.  When they do, you can bet that it will be included in such a  
> way as to maintain the core notion of Purkinje cell instructive  
> learning.
> Jim

Let me get this straight--granule cell axons first synapse strongly on  
a single Purkinje cell and then split into parallel fibres, synapsing  
weakly (both directly and indirectly via interneurons) on a large  
number of other Purkinje cells. So the output of a Purkinje cell is  
dominated by a few granule cell axons and is tuned by a large  
population of parallel fibres. How do the climbing fibres fit in?

I understand that a major part of the cerebellum is used to propagate  
motor plans forward (and backwards). How does the cerebellar  
architecture support that function?

"If academic research is not devoted to finding the truth, it is a  
form of propaganda, and not necessarily to be preferred to other  
forms, much cheaper and perhaps more persuasive." (Russell 1993)

Harry Erwin

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