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).


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