Re: Gold & Pumphrey (Experiment I)

[Andrew Bell <andrew.bell@xxxxxxxxxx>]

Thankyou to Glenis Long for pointing out the importance of threshold
microstructure.

I agree with the implications of microstructure as interpreted by Dan
Ellis. If G&P happened to have used frequencies near microstructural peaks,
or their signal sidebands were more than 15 dB down anyway, then their Q
calculations appear valid. Interestingly, microstructure was only recognised
a decade after G&P's experiments (Elliott, 1958: 'A Ripple Effect in the
Audiogram', Nature 181, 1076), but given G&P's concerns, it is surprising
(with hindsight) that they didn't come across it themselves.

There are some wonderfully detailed examples of microstructural threshold
in Glenis Long's 1984 paper (Hear. Res. 15, 73-87). The figures show that
the peaks and valleys are separated by about 100 Hz in the region near 2
kHz. Note that the 'valleys' (lowest threshold) are in fact the regions of
peak sensitivity, and so the frequencies of the putative resonators
corresponds to the frequencies of the valleys. The -3dB widths of the
valleys are about 20 Hz, so the Q of these peaks is about 2000/20 = 100.
This value is in line with the Q of 80 derived by G&P (although it is
greater than the values of 15-20 derived by Christopher Shera with SFOAEs,
which were discussed earlier on this list).

The simplest interpretation of microstructural thresholds is that the ear
is using a graded bank of resonators, the very notion that G&P were putting
forward and the same perspective implied by Dan Ellis' comments. G&P were
promoting Helmholtz's idea that sound is detected using sympathetic
resonance, and this is a position to which I would also subscribe.

Perhaps the strongest evidence supporting G&P comes from experiments on
microstructural threshold conducted by Thomas in 1975 (JASA 57, S26).
Amazingly, he found that, for stimuli just above threshold, the perceived
_pitch_ of the tone stays _constant_ within the passband of each
microstructural peak. That is, each microstructural passband is associated
with a unique pitch, and as one steadily increases the frequency the pitch
jumps quantally as one moves from one valley to the next. To my knowledge,
this finding, pointing to a fundamental unit of pitch perception and
identifying each passband with a single resonator, has not been followed
up.

Andrew.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Andrew Bell
Research School of Biological Sciences
Institute of Advanced Studies
Australian National University
Canberra, ACT 0200, Australia
andrew.bell@anu.edu.au
phone +61 2 6125 9634
fax +61 2 6125 3808
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


>>> Dan Ellis <dpwe@EE.COLUMBIA.EDU> 05/19/02 12:33am >>>

>>  [Glenis Long said] This proviso rules out most frequencies in a normal
cochlea since
>>  there is almost always some threshold microstructure and often
pronounced
>>  microstructure of up to 15 dB changes within 1/2 critical band.

>This is an interesting twist, new to me.  I'd like to know more details.
>
>Presumably it would be OK if the tests were done with the most
>sensitive frequency in the locality - if that can be easily
>established for a particular listener - since then we can be confident
>that the sidelobes are below threshold.
>
>But, depending on the fineness of the microstructure, doesn't
>microstructure in itself imply high-Q?  If frequencies close to one
>another are being processed differently (i.e. with different sensors
>exhibiting different thresholds), then the individual sensor
>structures (mechanical and neural) must be finely enough tuned to
>separate these close frequencies, implying high-Q.  Half a critical
>band wouldn't give you a Q of 300, but the details might be telling.
>