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Auditory Impulse Travel and Distance

Tristan Davies tbd at neuro
Fri Jun 21 08:54:13 EST 1991

>>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.
>(brp at 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 at 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

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