Re: paper on human cochlear tuning (Andrew Bell )

Subject: Re: paper on human cochlear tuning
From:    Andrew Bell  <bellring(at)SMARTCHAT.NET.AU>
Date:    Mon, 25 Mar 2002 20:42:46 +1100

>From the parenthetical part of Fred Wightman's comment, it seems he may now have some doubts about how effective his refutation of Pumphrey and Gold was. Perhaps he could elaborate on those doubts? Notwithstanding Green et al.'s 1975 JASA, I would maintain that Pumphrey and Gold's interpretation is fundamentally correct. No matter whether you invoke pitch or any other effect, and whether one considers the time domain or frequency domain, the stimuli are such that at the end of the day Pumphrey and Gold's statement remains irrefutable: "No frequency analyser [biological or physical, natural or man-made] could distinguish between the two stimuli unless its oscillatory time constants were so large that phase was 'remembered' across the silent interval." The 1975 JASA paper of Green et al. misses the point by looking at the two compound stimuli (A and B) in the frequency domain. Whether one chooses to examine the stimuli in the time or frequency domain is immaterial: there is nothing in one domain that is not implicit in the other. The fact remains that an inverted signal can only be distinguished, after a silent interval, from its antiphase counterpart if the phase is remembered across the interval. Moving the analysis from the time domain to the frequency domain does not change the truth of the statement that _no_ frequency analyser can distinguish the two stimuli unless phase is remembered. Pumphrey and Gold would not dispute that there is a (spectral) difference between the two wavetrains A and B. Indeed, if there were absolutely no difference, then no frequency analyser on earth would be able to tell the difference between them. What Pumphrey and Gold are simply saying is that any difference between A and B can only be perceived if the analyser has a sufficiently high Q. Green et al. attribute that difference to a pitch mechanism; that may be so -- the difference may manifest as pitch or timbre or any other psychophysical percept (clearly, there has to be some psychophysical difference if we can consciously distinguish the stimuli) -- but the pitch differences concerned are only detectable if the detector has a suitably high Q. This is because the magnitude of the spectral components of n wave periods is _precisely_the_same_ as the n periods of its antiphase version (note once again that this is _not_ saying that the spectral components of the compound waveforms A and B are identical). The only exception to the detectability criterion would be if the ear were sensitive to absolute phase, and Pumphrey and Gold exclude this by noting that, if this were true, then a person's ability to distinguish between the two stimuli would be independent of the length of the silent interval -- and this is clearly not so, with the length of the silent interval having a large effect on discriminability. A 10-cycle silent interval gives an obvious difference, whereas with 30 cycles it is hard. Green et al. repeat the Pumphrey and Gold experiments, and it should be noted that their results more or less confirm the earlier ones (confirmed also by Hiesey and Schubert, JASA 51, 1972, 518, who also make the same epistemological error as Green et al. in thinking that because there is a pitch difference this explains away the difference, a position implicit in Fred Wightman's comment to which I am replying). The result remains that since the ear can detect the difference between the A and B waveforms, it must be using a high Q analyser. That it appears as if the pitch of the two waveforms differs is an interesting psychophysical observation, but it does not change the conclusion as to the necessarily high Q of the detector that perceives the pitch difference. To reiterate the statement I made above, whether the difference manifests as pitch or any other psychophysical parameter is beside the point -- the brute fact is that there is a difference, despite identical spectral energy in the n waves and in its delayed antiphasic counterpart. In an interesting modification of the Pumphrey and Gold experiments, Green et al. embed the signals in broad-band noise to produce "[p]robably the most direct test of Gold and Pumphrey's narrow-band estimate of the bandwidth". They looked for a difference in threshold between the A and B waveforms, since the ear should be more able to detect the former because the supposed resonant element could accumulate in-phase energy. Indeed, A was more easily detected, albeit with just a 1 dB advantage. This is said to correspond to a Q value of about 10. The authors note that this is much smaller than the Q values found by Pumphrey and Gold. Although this is true, it is not unexpected, in that the added broad-band noise means the ear is operating at much higher intensities (perhaps 40 or 60 dB SPL? -- the paper does not give us that important information). As we now know, the selectivity of the ear at moderate intensities is much broader than it is at threshold. Now that we have the SFOAE results of Shera et al., does Fred Wightman not believe that the Q of the ear can be as high as 30 (at 10 kHz and 40 dB SPL) or as high as 1000 (based on an SOAE of 1 kHz, 0 dB SPL, with 1-Hz bandwidth)? 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) ________________________________ |>-----Original Message----- |>From: AUDITORY Research in Auditory Perception |>[mailto:AUDITORY(at)LISTS.MCGILL.CA]On Behalf Of Fred Wightman |>Sent: Monday, 25 March 2002 12:02 |>To: AUDITORY(at)LISTS.MCGILL.CA |>Subject: Re: paper on human cochlear tuning |> |> |>What Andrew Bell might have mentioned is that in an |>article that Dave Green, Craig Wier and I published |>in JASA (1975, vol 57, p 935) we argued (convincingly, |>we thought at the time at least) that Pumphrey and |>Gold's result could more parsimoniously be explained |>as a product of simple pitch judgement. In the |>classical tradition of psychoacoustics, we would argue |>that one should look at the stimulus first. |>

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