Re: missing fundamental references

[Peter Cariani <peter@xxxxxxxxxxxxxxxxxxxx>]

It is an interesting question.

That they see periodicities in the FFR's that are consistent with the
pitches of
inharmonic complex tones (first effect of pitch shift, de Boer's rule) I
think is
strong evidence for a population-based temporal code for pitch
at the level of the midbrain.

One of the main doubts that I have about time-to place conversions in the
midbrain (via units with bandpass MTF's is that the behavior of
MTF-tuned responses
would not be expected to retain the autocorrelation-like character of
these pitch shifts.
The other is that the MTF tunings appear to broaden at higher SPL's,
which is
not at all consistent with how the pitch percept behaves. A priori I
think it is highly
unlikely that the auditory system "pays attention to" one subpopulation
of neurons
at low levels and another at high levels, but it is difficult to rule
out such hypotheses.

So yes, I think Greenberg's FFR observations are entirely consistent
with
a temporal mechanism for pitch that operates at least as far up as the
IC.

Why these periodicities are not seen with latencies for other brainstem structures,
I don't know (certainly one would see them in averaged cochlear
potentials and
certainly all the major cell populations in the cochlear nucleus are
driven, some
better than others, by the fine time structure of the stimulus. I have
seen the interval
correlates of the pitch shifts of inharmonic AM tones in primarylike, chopper,
and pauser responders, although these patterns are weaker for pausers.
Maybe it
has to do with their electrode placement and/or cancellation technique
-- I would
have to go back and look more closely at the papers.

As for temporal representations for pitch above the IC, we should
remember that
the FFR is giving us information about the synchronized component of the
population response and that timing information can be present in single units
with a range of latencies such that it would not be seen in a scalp
averaged gross
potential (for example, my guess is that the upper limit of scalp
cortical AEP's that is
seen is probably between 100-200 Hz, but Steinschneider's group has seen
periodicities up to 300 Hz in current source density analysis of local field
potentials, and there are occasional reports of phase locking in
cortical neurons
to still higher periodicities (Semple's group at Neurosciences reported
in their
poster a cortical unit that phase-locked to 1 kHz). So, I believe that
this question is
still unresolved (will let you know if we find anything).

It could be the case that (not in any order of plausibility)
1) the temporal information is converted into a form that we do not yet understand
        (e.g. something like Hopfield's relative latency code)
2) the temporal information is in fact transformed into MTF-tuned
activation profiles
        in a manner we don't fully understand (such that the representation
takes care
        of de Boer's rule and doesn't degrade at high levels)
3) the timing information is put onto more and more neurons in a sparser
and sparser
        way and multiplexed with other information (e.g. re: location) as one proceeds
        up the pathway (a million neurons each with one interval at the fundamental)
4) that the computations that make use of timing information  involve
top-down/bottom up
        interactions at the level of IC and/or thalamus that we don't
understand (some sort
        of time-time comparison that requires reverberating circuits)
5) it's all a covert rate-place code, and we have been looking at the
wrong neurons

Take your pick.

Peter


Martin Braun wrote:
>
> Dear Peter,
>
> encouraging to see that we both (warmly) recommended Greenberg, S., et al.
> (1987) to Ani.
>
> I also added three papers by Galbraith & Co (see below), who replicated and
> extended Greenberg's results. I didn't send them to the list but to him
> personally (11-29 that was), because Ani had announced he would send his
> harvest to the list anyway.
>
> But the fact that you mailed yours via the list gives me the chance to a
> possibly interesting question.
>
> I am a bit surprised that you put a special emphasis on these findings. The
> FFR to f0 is only seen with a latency corresponding to wave V of the ABR,
> never with a latency corresponding to the earlier waves. Wave V has its
> origin in the upper brainstem, most likely the IC.
>
> Question:
> Are these relations compatible with your hypotheses on extraction of
> f0-pitch ?
>
> Martin
>
> Galbraith, G.C., 1994. Two-channel brain-stem frequency-following responses
> to pure tone and missing fundamental stimuli. Electroenceph. clin.
> Neurophysiol. 92, 321-330.
>
> Galbraith, G.C., Doan, B.Q., 1995. Brainstem frequency-following and
> behavioral responses during selective attention to pure tone and missing
> fundamental stimuli. Int. J. Psychophysiol. 19, 203-214.
>
> Galbraith, G.C., Bhuta, S.M., Choate, A.K., Kitahara, J.M., Mullen, T.A.,
> 1998. Brain stem frequency-following responses to dichotic vowels during
> attention. Neuroreport 9, 1889-1893.
>
> Martin Braun
> Neuroscience of Music
> Gansbyn 14
> S-671 95 Klässbol
> Sweden
> nombraun@post.netlink.se