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Basic Neuron Questions

Didier A. Depireux didier at tango.isr.umd.edu
Sat Apr 12 12:50:51 EST 2003

r norman <rsnorman_ at _comcast.net> wrote:
> On Thu, 10 Apr 2003 15:44:25 +0000 (UTC), "Didier A. Depireux"
> <didier at rai.isr.umd.edu> wrote:

> Didier's comments are quite sensible, but I disasgree that 'it is
> really quite straightforward."  

That was a bit tongue-in-cheek. The original poster said he would like
someone to tell him it _is_ straightforward... I wanted to oblige. 

> Even putting aside the conceptual
> difficulties of how to do it, the experimental difficulties of finding
> a preparation on which to test any hypothesis and then actually carry
> out the work is currently out of the question.

I wouldn't quite take it that far. When I characterize a cell in auditory
cortex, and I hypothesize that it encodes spectral envelope linearly, then
0) I characterize the cell's properties with a set of sounds. 
1) I verify that I characterized the cell properly by presenting new sounds
and predicting fairly accurately what the response will be to these
non-trivial, speech-like sounds. 
So in a sense I verified a hypothesis with a live, awake animal. In
another, I have not shown that the cell's job _is_ to extract or encode
spectral envelope, I have only shown that I can correlate very well the
firing of the cell with some properties of the sounds I am interested in. 

> Certainly there are specific cases where timing of nerve activity,
> even to the sub-millisecond level, is critical.  

That is true, and typically it's periferal. For instance, fibers of the
auditory nerve can code pure tones' phase up to about 2 kHz (more or less
depending on who you read). This implies an accuracy in firing time in the
100 microsec range. By the time you get to cortex, though (which I thought
was the original question), things are a lot more sluggish and noisy.
Present the same stimulus 20 times and you will never get 20 times the same
response, in most parts of cortex. 


Didier A Depireux         ddepi001 at umaryland.edu  didier at isr.umd.edu
685 W.Baltimore Str      http://neurobiology.umaryland.edu/depireux.htm
Anatomy and Neurobiology                      Phone: 410-706-1272 (off)
University of Maryland                                      -1273 (lab)
Baltimore MD 21201 USA                             Fax: 1-410-706-2512

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