Re: pitch neurons (2) (Eckard Blumschein )

Subject: Re: pitch neurons (2)
From:    Eckard Blumschein  <Eckard.Blumschein(at)E-TECHNIK.UNI-MAGDEBURG.DE>
Date:    Fri, 11 Oct 2002 10:22:42 +0200

Dear all, I will try and agree with each other most of the messages I got concerning the pitch issue by brushing them sometimes a little bit against your intention. Lets's trace the auditory pathway in alphabetic order beginning at base of cochlea with Dr, then Eli, Flatmax, Jean-Sylvain Lienard, Jont, Martin, Nicolas Grimault, and Peter as to better unite physiology and psychology. 'Dr J Stevenson's research assistant' reminded me of aliasing. Well, hearing is fundamentally different from technical signal processing in that it is not based on Fourier analysis and not on regular sampling. Eli, please don't take it amiss if I am permanently objecting against misuse of such common terms like acoustic energy. I accept your caveat that periphery is not predominantly important. Nonetheless, I believe many paradoxes already reside in OHCs, IHCs, and auditory nerve. Perhaps, elucidation of periphery will succeed first. Many experts like you are convinced that cochlear frequency resolution sets a limit to acuity of any further neural analysis. I don't think so. Matt, I would like to humbly join the queue of those who share your opinion that (as expressed by Steinschneider et al. 1998) there is evidence for (at least) two mechanisms subserving pitch. However, I don't support your idea concerning the efferents. Jean-Sylvain Lienard, you are quite right in that pitch is a conscious perception, not a physical quantity. Please reconsider whether periphery actually extracts fo. Jont, I am not sure if it makes sense to ascribe a frequency limit to a single auditory nerve fiber, while I am rather sure that refractory time does not differ much among species. The very small bat Myotis lucifugus (just 8 g body weight) has been described to have a ratio of afferent fibers to IHCs of 70:1, a very high value as compared to about 4:1 to 6:1 in the basal cochlea region of Rhinolophus, Pteronotus, and cat. Why? Martin, you are definitely close to the heart of discussion. I am still fascinated by Langner who demonstrated that sounds of quite different temporal as well as spectral shape evoke the same pitch. Patterson et al. found indications for much neural activity at the level of IC or below. Here you added the argument: 'The advantage of detecting a pitch is that it helps the animal to identify and follow a sound source. In order to do this, the animal needs ONE pitch detector, not several. More than one would only lead to a more diffuse pitch.' Why not seeing it the other way round? Once the animal is consciously aware of features matching to a source, they focus at these. Having two eyes we are unable to split our conscious focus. Sometimes I can hear different pitches out of a steady complex signal one by one. If Nicolas Grimault already could offer a simple and convincing explanation how the two mechanisms act together, perhaps we all would very appreciate it. Peter, you are offering two alternative final conclusions: 1) The cortex plays the role of organizing processing in the lower centers rather than a locus for the representations themselves. 2) A central representation is "covert" and sparsely distributed across many neurons (either in a spatial or temporal code) Do they actually contradict each other? I tend to favor the temporal code for the central representation rather than a 'mechanistic nature' of it. Peter, I don't share the 'common misconception that single ANFs must fire every cycle in order to encode a tone's frequency'. What about the volley principle, I think it points into the right direction but we have to learn much more about stochastics. I also strongly support your attitude concerning the role of intervals except you seem still to deny the role of first order intervals which was not just shown by Kaernbach & Demany but also more recently confirmed by Carlyon. Peter, you wrote: 'We all agree that there are neurons in the IC that have bandpass MTF functions.' I don't question the behavior, but I hesitate interpreting it as a function. I share your doubts: 1) IC MTF functions generally have broad tunings to begin with when compared with pitch discriminations, 2) they flatten out at higher SPLs when pitch percepts remain highly salient, 3) Bandpass MTFs cannot explain pitch equivalence between pure and complex tones 4) they cannot explain the pitch shifts of inharmonic AM tones I could add octave enlargement. . . You are certainly correct: 'We need to expand the range of possible kinds of mechanisms that are considered. We implicitly first want to see nice, spatially ordered maps of periodicity-tuned elements. Failing that, we would like to see mosaics of units with the right kinds of response properties that would account for pitch (comb filter tunings, temporal autocorrelators). Failing that, we might think that there is some kind of covert, distributed processing going on in which combinations of units are analyzed (how this leads to the low-dimensional structure of pitch space and pitch classes is quite unclear). Failing that, maybe we should consider that the representation lies in relations between spikes rather than in patterns of units being activated --i.e. the whole idea of feature detectors is suspect.' Peter, just your last comment sounds not sound to me. I recommend taking to heed all your well founded criticism, and I suggest to distinguish step by step between pitch and periodicity, then between periodicity in general and periodicity, already detected at the level of IC, etc. Kind regards, Eckard

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