Subject: Re: paper on human cochlear tuning From: Andrew Bell <bellring(at)SMARTCHAT.NET.AU> Date: Sun, 24 Mar 2002 18:41:12 +1100Christopher: Thankyou for bringing this paper to our attention. Using SFOAEs at 40 dB SPL, you and your colleagues show that the human ear can display Qs of 15-30, increasing with frequency from 1 to 10 kHz -- sharper than the standard experiments indicate. It well demonstrates the simplicity and power of SFOAEs. One thing your paper omits to do is acknowledge psychophysical studies which, more than 50 years ago, came to about the same conclusion as you do. In 1948, Gold and Pumphrey concluded that "the selectivity of the resonant elements... of the human ear is in fact roughly proportional to frequency and is very much higher than has generally been supposed" [Proc. Roy. Soc. B, 135, 462]. This sounds similar to your statement on p.3322 that "human filter bandwidths are considerably narrower than previously believed throughout the cochlea." The only difference is that Pumphrey and Gold's figures, derived from working at lower intensities than you did, show even higher Qs. In fact, your paper can be seen as really underlining how far ahead of their time Pumphrey and Gold were in the late 1940s. I think it would have been appropriate to make reference to their pioneering and revolutionary work. In Nature 160 (1947), 124 [Transient Reception and the Degree of Resonance of the Human Ear], Pumphrey and Gold give a description of a revealing experiment in which they measured the Q of the human ear psychophysically and found that, at SPLs near threshold, the Q is in the range 200-350 from 2.5-10 kHz, falling to 50 at 1 kHz. This fundamental experiment involved an observer comparing the detectability of a continuous tone (at threshold) with a similar wave-form that comprised only a finite number of cycles. Clearly, the latter will need to have a higher intensity to be audible, and this difference will depend on the Q of the system. The higher the Q, the more cycles will be needed to get the resonant elements ringing at audible amplitude; at the same time, for a fixed number of cycles, a higher intensity will be needed. From the trade-off between number of cycles and intensity increase, Pumphrey and Gold calculate the equivalent Q. A very nice experiment. An even more elegant piece of work is reported soon after in Nature 161 (1948), 640 [Phase Memory of the Ear: a Proof of the Resonance Hypothesis], in which Pumphrey and Gold use a different paradigm based on a series of wavetrains separated by an integer number of cycles of silence. If an observer can distinguish that stimulus from a similar one in which every second wavetrain is of inverted polarity, then the ear must be able to 'remember' the inversion across the silent gaps. Again, the experiments were done at low intensity -- about 20 dB above threshold -- and the resulting Q values were similar to before: the ear could remember up to 30 cycles, translating to a Q of at least 100 and as high as 250 at 5 kHz. At this point it is worth emphasising your statement that human auditory frequency selectivity is determined at the level of the periphery (that is, frequency discrimination is all there physically in the cochlea, and neural sharpening is a concept to be discarded). It would be interesting if you were to repeat your experiments at 20 dB SPL, and see whether you again need to raise the Q values attributed to the human ear. This is particularly so in the light of your comment [p.3321] that "previous behavioural measures of frequency selectivity [meaning Fletcher's paradigm, actually, not Pumphrey and Gold's] greatly overestimate the bandwidths of peripheral cochlear filters..." due to suppression by the masker or self-suppression. One can only expect Qs to increase as SPLs decrease. For example, it is not unusual for low-level SOAEs between 1-2 kHz to have bandwidths of less than 1 Hz, implying an equivalent Q of more than 1000. This example is relevant to your comment on p.3322: "The development of an objective, noninvasive measure of cochlear tuning solves a long-standing problem in the hearing sciences." Granted, your work has given us another tool on the objective front, but haven't simple measurements of SOAE bandwidths already given us the same sort of information? And given that SFOAEs are entraining the same oscillators as give rise to SOAEs, I would venture to say that SFOAE-derived Q values will undoubtedly rise as SPLs diminish. The Pumphrey and Gold experiments are of course measuring the very same resonators. I am surprised that Pumphrey and Gold's findings were not taken up at the time, and their work still seems to be overlooked. The title of their second paper remains a challenge to today's auditory theorists. It seems clear that SOAEs represent the continuous ringing of the ear's resonant elements for which Pumphrey and Gold were searching. They have been proved right about their seminal idea of a "regenerative receiver" mechanism in the cochlea; why then, with detailed knowledge of SOAEs, does the resonance theory, for which they offered solid evidence, continue to be ignored? Can I suggest that list members again take time to read Pumphrey and Gold, particularly the more detailed companion papers in Proc. Roy. Soc. B, 135 (1948), 462-498. The quote on the first page throws down the gauntlet: "Previous theories of hearing are considered, and it is shown that only the resonance hypothesis of Helmholtz... is consistent with observation." In this context, your own recent JASA paper adds an important perspective to the issue. You say that "These results [the phase-constancy of BM responses to clicks -- the intensity of which range over almost the entire dynamic range of hearing] contradict many, if not most, cochlear models in which OHC forces produce significant changes in the reactance and resonant frequencies of the partition" [JASA 110 (2001), 332]. Perhaps you could offer us a list of which models remain in the running? You point out that this result implies, physically, that the local resonant frequencies of the cochlear partition are virtually independent of intensity. I take this to mean that the resonance theory would be on your list? Thankyou again for raising this important issue. Andrew. ________________________________ Andrew Bell PO Box A348 Australian National University Canberra, ACT 2601 Australia Phone {61 2} 6258 7276 Fax {61 2} 6258 0014 Email bellring(at)smartchat.net.au ________________________________ Endnote: For the record, here are other specific places in your paper where due recognition of Pumphrey and Gold calls for changes. p.3318. "Filter bandwidths in humans... have relied on psychophysical (i.e., behavioral) measures of filter bandwidth based on the phenomenon of masking". You have overlooked the Pumphrey and Gold threshold-detection approaches. p.3321. "To obtain the most accurate behavioral measures of cochlear tuning that psychophysics can currently provide..."; again Pumphrey and Gold measured these parameters at lower intensities and arrived at superior Q values. They commented that the scatter of their results was small and that agreement with the theoretical slope was good. |>-----Original Message----- |>From: cochlea-admin(at)auditorymodels.org |>[mailto:cochlea-admin(at)auditorymodels.org]On Behalf Of |>Christopher Shera |>Sent: Thursday, 21 March 2002 1:04 |>To: auditory(at)lists.mcgill.ca; cochlea(at)auditorymodels.org |>Subject: [Cochlea] paper on human cochlear tuning |> |> |>Dear list members, |> |>If you can forgive the shameless advertisement,* |>some of you may be interested in the following paper |>recently buried in an out-of-the-way location: |> |>"Revised estimates of human cochlear tuning |>from otoacoustic and behavioral measurements" |>C.A. Shera, J.J. Guinan, Jr, and A.J. Oxenham |>Proc. Natl. Acad. Sci. USA 99:3318-3323. |> |>http://www.pnas.org/content/vol99/issue5/#PHYSIOLOGY |>