[Comp-neuro] From Socrates to Ptolemy

james bower bower at uthscsa.edu
Thu Aug 7 17:56:22 CEST 2008


Bard, smirky  (hard to believe).  ;-)   (is it time for a limerick  
Bard??).  I am sure that 'noise' presents many opportunities,  
especially with respect to my postings  - :-)

Yes, with respect to thermal channel noise -- the amount of energy  
invested by neurons in precisely controlling spike timing both  
indicates that spike timing is important, and likely is intended to  
specifically reduce the influence of the inevitable thermal noise in  
physical systems.

By analogy, Brownian noise is a physical reality one cannot avoid --  
so the cochlea operates just above this physical limit, but then the  
brain has internal mechanisms of hyper acuity to, in effect, increase  
its resolution.  Also applies to vision, electro-reception, etc.

But the original point I was trying to make about noise, is related to  
the fact that accumulated spike train intervals produce what looks  
like random distributions of intervals -- it is a mistake to conclude,  
however, that neuronal spike trains are "noisy", especially given  
Shannon's noisy-channel coding theorem.

While "noise" is a convenient parameter to play with in abstract  
modeling for a whole bunch of reasons --  (see simulated annealing for  
example), its use as a fundamental mechanism in real biological system  
runs against what seems to be the predisposition of biology to  
implement very specific (and sophisticated) control processes.

That was all I was trying to say.

Jim

BTW, I was aware I was being a bit unfair to Bard. He is agnostic  
about whether there is "real" noise or not.




On Aug 7, 2008, at 7:19 AM, G. Bard Ermentrout wrote:

> John:
>
> This was a smirky comment I made after 2 long weeks of back and  
> forth on noise. Basically, there was a paper in Nature Reviews  
> Neuroscience, some months ago in which the authors said noise was  
> very bad. Nathan Urban, Roberto Galan, and I wrote a TINS paper (Aug  
> 2008) on a possible functional role for noise in synchrony and Jim  
> sort of tweaked the community (as he is wont to do) which started  
> this rather long discussion. We meant by noise - any broadband  
> signal which has short correlation times and did not mean noise in  
> the sense of something to get rid of. For example, set up a simple  
> recurrent sparsely connected network (ala Brunel, Sompolinsky, ...)  
> and the synaptic inputs will have exactly this type of character. In  
> any case, as you point out, there is also real thremal noise as in  
> th opening of channels, but (forgive me if I say this incorrectly),  
> I think that Jim was pointing out that channel noise cannot play  
> that much of a role since the AP of a mammalian (and other neurons)  
> neuron is extremely well-regulated.
>
> Bard
>
> On Thu, 7 Aug 2008, John Hallam wrote:
>
>>
>> On Fri, 1 Aug 2008, G. Bard Ermentrout wrote:
>>
>>> We've established that there is no "noise" in the nervous system.
>>
>> 	I am intrigued by this.  Engineers in my experience mean one of  
>> two things by "noise":  either the effects of stochastic processes  
>> in the physical world which affect the system they are working  
>> with, or phenomena in the physical world unaccounted for in the  
>> model they are using. The first kind of noise clearly exists in the  
>> nervous system, since there are processes (e.g. channel gates)  
>> operating at the thermal noise level...
>>
>> 	So evidently you mean something different by "noise" -- could you  
>> elucidate, or send me a pointer to the paper where this is  
>> established, please, so that I can see how the term is used in  
>> practice by comp-neuro folk?
>>
>> Thanks in advance,
>>
>> 	John Hallam
>>




==================================

Dr. James M. Bower Ph.D.

Professor of Computational Neuroscience

Research Imaging Center
University of Texas Health Science Center -
-  San Antonio
8403 Floyd Curl Drive
San Antonio Texas  78284-6240

Main Number:  210- 567-8100
Fax: 210 567-8152
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