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From: tbd@neuro (Tristan Davies)
Newsgroups: bionet.neuroscience
Subject: Re: Auditory Impulse Travel and Distance
Summary: what a marvelous thing is the brain!...
Message-ID: <22420@duke.cs.duke.edu>
Date: 21 Jun 91 13:54:13 GMT
References: <9106171949.AA20716@genbank.bio.net> <1991Jun18.033141.11511@agate.berkeley.edu>
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>>More simply put, do louder sounds travel further along
>>auditory pathways than sounds which are more quiet?
>
>Any sound which we perceive, whether loud or soft, must travel into the 
>auditory cortex, hence all the way along the pathway.  The intensity of the
>sound doesn't affect whether the signal is propagated.  If it is transduced
>at the cochlea, it will be transmitted.  Some neurons are level dependent,

Absolutely correct!!

>i.e. their discharge probability is a function of input intensity, but many
>are not.  These will convey the signal (almost) independent of its intensity.
>
>
>bruce
>(brp@bandit.berkeley.edu)

Thank you for the simplest, most elegant answer.  I have an additional fact
which y'all might find interesting.  The range of sensitivity of an
auditory neuron is measured by a *tuning curve*, which is a graph of
sound frequency (x axis) vs. intensity to cause firing (i.e., threshold)
(y-axis).  When recording the activity of single auditory neurons, 
physiologists find that most neurons have a tuning curve which is roughly
V-shaped, indicating that the neuron has the lowest threshold at a single
frequency and its ability to respond to a pure tone decreases as the
frequency of that tone is farther from the preferred frequency of the
neuron.  Get it?

Here's the neat thing: some neurons have **circular tuning curves**!
That is, they respond only to a narrow range of both frequency and
intensity.  While some neurons prefer loder noises, there are also neurons
that prefer soft sounds, and will not fire in response to a loud sound,
even if that sound is at the preferred frequency.  Thus the loudness
of a sound is probably encoded in *which* neurons fire more than the rate 
at which they fire.

BTW, I have encountered a couple of these neurons during a lab rotation
where I recorded from the inferior colliculus in bats, so I'm fairly
certain they exist...

Hope this helps!

Tristan Davies
Dept. of Neurobiology, Duke Univ.
Go Blue Devils!!!

e-mail: tbd@neuro.duke.edu

"The brain is truly an impressive organ.  It starts working the instant
we get up in the morning and doesn't stop working until we get to 
the office." --paraphrased from an unknown source