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Re: pitch neurons (2)



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