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Re: Control of jitter
Charles S. Watson wrote:
> I certainly do not disagree in principle, nor did Lloyd who was well
> aware of the potential power of the law of large numbers, averaging
> across lots of neural elements, etc. But still, 10 microsec thresholds
> (for auditory localization/lateralization) are pretty darned small, and
> the combined jitter of, say, 4 to 8 synapse crossings (not to mention the
> temporal dispersion created by a wide variety of diameters).... but most
> of all, there DOES seem to be a delay-to-place mapping at the level of
> the olivary nucleus, as LAJ predicted.
I wasn't taking issue with the notion that, if possible, one wants
to do fine timing analysis as early in the pathway as possible --
that, all other things being equal, fewer steps in the
transmission chain are better.
I was taking issue with the common (entropic) assertion that
timing must <inevitably> deteriorate as one ascends the
auditory pathway. The counterexamples that I mentioned
suggest that in at least some systems, timing can be
preserved at higher stations -- but note this is not to imply
that timing must be preserved in all pathways......
(By the way, there could also be other additional reasons why binaural
cross-correlators are situated low in the auditory pathway
that have to do with getting rapid motor response times for orienting
one's ears and one's body to moving sounds. One may not want
to wait 100 or 200 msec for these decisions to be made at
The rest are more general ruminations................
Of course, we all think that the auditory system is special and
is exquisitely adapted for the processing of temporal information.
However, when I look into the literature on other sensory systems
there are phenomena that look very similar to what we see in
audition. My understanding is that in somatoception, one can
distinguish by apparent location on the body time differences
between two electrical impulses at different loci
down to < 1 msec apart (Bekesy's experiments).
I do think we need to look more seriously at exactly how much
timing information might be available in higher auditory centers.
There is clearly a huge overabundance of timing information related
to frequency in the auditory nerve, so that even if a small
fraction of the information were preserved (maybe 5-10%) in the
primary auditory cortex, this could well be enough to account for
frequency discrimination. (It continually astonishes me that
the argument is made that timing information is "too good" to
account for the frequency resolution of the human auditory system,
since it's extremely easy to throw away one kind of information
in the pathway in order to include other kinds.) If there is
enough timing information available for frequency/periodicity
discrimination, then some of the terrible complexities of
saturating and nonmonotonic rate-level functions and
the multiplicity of factors that can drive cortical discharge rates
can be avoided by using timing instead.
The dominant impression one gets from the literature is that there
is barely any timing information above 200 Hz in the IC and barely
anything above 25 Hz in the auditory cortex. Most of the time
these assertions are about the lack of
obvious, synchronized responses that hit you
over the head (like they do in the auditory nerve). On the
other hand, if you look more carefully, you see more. At ARO,
Shamma's group presented pooled population interval
data from their ferret ICs that showed robust encoding of
1200 Hz periodicities (they thought it might go up to
about 2 kHz), and this cutoff is consistent with the conclusions
that were reached for IC by Steven Greenberg in his analysis of
his FFR data. So it's entirely possible that the periodicity
information doesn't need to be stuffed into modulation-tuned
rate-based periodicity detectors as early as possible. The
temporal representations may still be better, even at the IC.
This remains to be seen experimentally.
Maybe timing information is distributed onto more and more
neurons -- sparse temporal coding -- as one goes up the
pathway.......and it becomes less and less obvious. We need
to look with this in mind.
At primary auditory cortex averaged potentials can
show periodicities up to 200 Hz or more,
which is only the synchronized component
of the response, and there are accounts of single unit activity
showing locking from several hundred Hz to 1 kHz (de Ribaupierre).
Whatever the upper limit, this is a far cry from 25 Hz. What it
means is that the modulations that we care about in speech,
from voice pitch to VOT down to rhythm and still larger patterns
could all be directly temporally coded at the level of the cortex
in time patterns of onset and offset responses.
Maybe, as in the IC with periodicity pitch,
rate profiles of modulation-tuned units aren't
the (only) way that these modulations can be represented......
Here are Joris and Phillips references that were requested:
Joris, P.X., Carney, L.H., Smith, P.H., and Yin, T.C.T.
Enhancement of neural synchronization in the
anteroventral cochlear nucleus. I. Responses to tones at
the characteristic frequency. J. Neurophysiol. 71: 1022-1036, 1994.
Phillips, D.P., and Hall, S.E. Response timing constraints on the
cortical representation of sound tIme structure. J. Acoust. Soc. Am.
88: 1403-1411, 1990. (Variability of cortical first-spike latencies.)