Why dont we hear/see individual neural firing events??

Michael A. Lebedev, Ph.D. michaellebedev at geocities.com
Tue Sep 24 12:27:27 EST 1996


Lee Kent Hempfling wrote:
> 
> radams2000 at aol.com (RAdams2000) enunciated:
> 
> >Here is a basic question from someone who knows a lot
> >about signal processing but only a little about the human
> >sensory organs. Given that there are about 4000 nerve fibers that
> >connect the basilar membrane in the ear to the barin, and that
> >each fiber only fires at a 500 Hz or so rate, why dont we
> >hear the collection of firing events as noise?? The standard argument
> >seems to be that if you average (filter) some number of these fibers,
> >the "noise" would be eleiminated, but a few basic calculations show
> >that even a few thousand fibers averaged with a time constant on the
> >order of a millisecond would only have a signal-to-noise ratio of
> >40 dB or so; not exactly hi-fi!

The discharges of nerve fibers are entrained to the original signal,
that is, spikes occur at a particular phase of the signal.  During
averaging the neural discharges, in no way you are approximating
a flat level, but rather an oscillatory curve (let's say, a sinusoid).

> >And the signal-to-noise ratio only
> >increases as the square root of the number of fibers, so you dont
> >win very fast by increasing the number of fibers included in the average.

Again, what is noise?  You should distinguish noise during rate coding
(your interpretation) and temporal coding (reality).

Michael



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