Speech as raw material

Lonce LaMar Wyse lwyse at copley.bu.edu
Wed Aug 28 18:59:42 EST 1991

RE: Is there a special perceptual mechanism for speech sounds?

The question itself is misleading because it implies that either
a) There is a speech module which takes its input from the
sound-processing front end (which answers the question with "no")
b) There is a two-channel system (one for speech, one for sound), and
a switch that gets toggled according to the listening mode.

Both are extreme simplifications, each has a grain of truth.

In the act of seeing, we control what we see with the eye muscles, but
there are no efferent pathways that terminate on the photoreceptors to
modulate their activity. In contrast, the hearing system does employ
an efferent system that, using the COCB as a final pathway, actually
modulates the neurotransmitter transfer from inner hair cells to
eighth nerve cells.  In addition, the outer hair cells receive
efferent signals which drive them to modulate the dynamics of the
cochlear membrane!

These "active processes" have been cited as being responsible for the
(otherwise mechanically impossible) sharp frequency tuning of the
cochlear membrane. But there is evidence that COCB
(crossed-olivo-cochlear bundle) activity is affected by attention.  It
is even possible that the pattern of vibration of the cochlear
membrane is different giving the same stimulation (say an orchestral
passage) depending upon whether we are attending to the oboe part or
the french horn part!

Anyway, it seems at least plausible to me that listening in a "speech
mode" means doing some reconfiguration of (the one) hearing system via
some centralized process.

						- lonce

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