|
Oscillation may play a role in time domain central auditory processing Galazyuk AV, Feng AS J Neurosci 2001, 21:RC147 (1-5) Department of Molecular and Integrative Physiology, and the Beckman Institute, University of Illinois, Urbana, Illinois 61801, USA Abstract: To study how sound intensity altered the temporal response pattern of
a unit, we recorded from 92 single neurons in the inferior colliculus
(IC) of the little brown bat and investigated their firing patterns in
response to brief tone pulses (2 msec duration) at the characteristic
frequency of the unit over a wide dynamic range (10-90 dB sound pressure
level). We found two unusual response characteristics at high sound levels
in approximately one-third of the IC neurons investigated. For 16 IC neurons
(17%), an increase in sound level not only elicited a shorter response
latency and an increase in spike count but also transformed the firing
pattern of the unit from phasic to periodic; this pattern was more
pronounced at higher sound levels. The firing periodicity was unit
specific, ranging from 1.3 to 6.7 msec. Twenty-seven IC neurons
(29%) exhibited a longer response latency at higher sound levels compared
with lower sound levels [i.e., paradoxical latency shift (PLS)]. The majority
of this population showed a one or more quantum increase in latency
when sound level was elevated. The quantum shift was also unit specific,
ranging from 1.2 to 8.2 msec. We further investigated the firing
patterns of 14 IC neurons showing PLS before, during, and after iontophoretic
application of bicuculline. For 12 of these neurons, drug application
abolished the PLS and transformed the firing patterns of the unit at high
sound levels from phasic into sustained periodic discharges. Our
results suggest that neural oscillation in combination with ordinary inhibition
may be responsible for the creation of PLSs shown previously to be important
for temporal information processing. Comment: Where and how the pitch of speech and music is extracted by the brain is currently a matter of intensive research. The evidence that has been found so far indicates that it is done in the auditory midbrain by neural frequency filtering of incoming signals that mirror sound frequencies picked up by the inner ear. As to the mechanism of this neural frequency analysis, Galazyuk and Feng have now reached a major breakthrough. In 28 of 92 neurons of the auditory midbrain (inferior colliculus) of a bat species they found unit-specific periodical discharge, with interspike periods ranging from 1.2 to 8.2 msec. As yet, it is unknown if this period tuning is intrinsic, e.g. due to membrane channel constellations, or imported from other neurons. It is clear, however, that period-specific neurons are ideal frequency filters for pitch extraction. These neurons can function as cellular resonators that have their strongest firing probability at signal input of one particular frequency. The periodicity range of the investigated neurons translates into a frequency range of 120 to 830 Hz. In musical terms, this corresponds to the tones from B2 to G#5, that is, three octaves that are well centered in the piano's total tone range. I should be added, though, that period tuning in bats in most cases probably is not used for pitch extraction, but for the identification of sound delays in echolocation. As far as we are concerned, we can safely assume that we have pitch neurons for all tones on the piano. (Comment Martin Braun) |