"pure place code" and "pure phase-locking code"

[Martin Braun <nombraun@xxxxxxxxxxxxxxx>]

Annemarie wrote:

"I had in mind the study by Steinschneider et al., JASA, 104 (5), 1998,
2935 ff. who found that at the level of the primary auditory cortex
phase locked responses occurred only at sites with high best frequencies
up to about 200 Hz (stimuli: alternating polarity click trains),
............

Does that mean that the temporal code might not play a role at all in
the low frequency channels or is it more likely that phase locking had
been transformed into a rate-place code before the A1 (perhaps in the
midbrain)?..........."

Answers:

1) As soon as a harmonic is resolved in the cochlea, spectral coding takes
place and then runs along the complete auditory pathway.

2) If the spectral information is poor in the cochlea, as with click
stimuli, it is also poor anywhere else in the auditory system.

3) Current evidence indicates that f0 in the main speech and music range is
transcoded from a temporal to a place code in the central nucleus of the
inferior colliculus (ICC). In other words, time-locking in this f0 range
disappears above the ICC, and the extracted f0-pitch is coded at its
frequency place by discharge rate, as most other information that is
transported into and around the cortex. (See references below)

4) Phase-locking to acoustic frequencies recorded in the cortex possibly is
not related to pitch extraction at all. It may be a by-product of other
functions of the auditory system, e.g. orientation in space.

In conclusion:

A) In the cortex, f0-pitch in the main speech and music range is coded
purely spectrally. (No phase-locking in pitch coding)

B) Up to the ICC, f0-pitch in the main speech and music range can be coded
purely temporally, but for all natural, i.e. non-laboratory, complex tones
it is coded spectrally and temporally. (Phase-locking necessary for pitch
coding)


Langner, G., 1992. Periodicity coding in the auditory system. Hear. Res. 60,
115-142.

Schreiner, C.E., Langner, G., 1997. Laminar fine structure of frequency
organization in auditory midbrain. Nature 388, 383-386.

Langner, G., Schreiner, C.E., Biebel, U.W., 1998. Functional implications of
frequency and periodicity coding in auditory midbrain. In: Palmer, A.R.,
Rees, A., Summerfield, A.Q., Meddis, R. (Eds.), Psychophysical and
Physiological Advances in Hearing. Whurr, London, pp. 277-285.

Braun, M., 1999. Auditory midbrain laminar structure appears adapted to f0
extraction: further evidence and implications of the double critical
bandwidth. Hear. Res. 129, 71-82.

Braun, M., 2000. Inferior colliculus as candidate for pitch extraction:
multiple support from statistics of bilateral spontaneous otoacoustic
emissions. Hear. Res. 145, 130-140.


Martin


Martin Braun
Neuroscience of Music
Gansbyn 14
S-671 95 Klässbol
Sweden
nombraun@post.netlink.se