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July 2003 Vol 4 No 7 REVIEW
Nature Reviews Neuroscience 4, 540 -550 (2003); doi:10.1038/nrn1136
NEW ROLES FOR SYNAPTIC INHIBITION IN
SOUND LOCALIZATION
Benedikt Grothe about the author
Preface
The arrival times of a sound at the two ears are only microseconds
apart, but both birds and mammals can use these interaural time
differences to localize low-frequency sounds. Traditionally, it was
thought that the underlying mechanism involved only coincidence
detection of excitatory inputs from the two ears. However, recent
findings have uncovered profound roles for synaptic inhibition in the
processing of interaural time differences. In mammals, exquisitely
timed hyperpolarizing inhibition adjusts the temporal sensitivity of
coincidence detector neurons to the physiologically relevant range of
interaural time differences. Inhibition onto bird coincidence
detectors, by contrast, is depolarizing and devoid of temporal
information, providing a mechanism for gain control.
Summary
 Interaural time differences (ITDs) are the main cue for localizing low-
frequency sounds. As they are only in the range of microseconds, ITD
detection requires the most elaborate mechanism for temporal
processing in the mammalian or avian brain.
 The textbook view of how ITD processing is achieved has been
dominated by the seminal model put forward by Jeffress in 1948. This
model incorporates excitatory projections from both ears that faithfully
time-lock to the temporal structure of sounds and converge onto
binaural coincidence detector neurons. These fire maximally when the
two inputs arrive simultaneously. The model also assumes that a
systematic arrangement of the length of the input fibres (delay lines),
can produce different conductance delays that tune different
coincidence detector neurons to different favoured ITDs. Such a
system could then create a map of best ITDs, and hence of azimuthal
space.
 ITD-sensitive coincidence detector neurons have been found in the
mammalian medial superior olive (MSO) and in its avian analogue, the
nucleus laminaris. In the bird ITD-processing circuits, delay lines have
been described structurally and functionally, and there is good
evidence for a topographic representation of ITDs (and, hence, of
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azimuthal space) at least in the barn owl auditory system. By
contrast, the existence of delay lines in mammals has been
controversial, and convincing evidence for topographic maps of ITDs is
lacking. Recent evidence indicates that, in mammals, the
representation of azimuthal space calculated from ITDs might be
organized in a fundamentally different way than proposed by Jeffress.
 Our view of the representation of ITDs in mammals, and also of the
mechanism of ITD processing itself, is undergoing marked changes. In
birds and mammals, profound inhibitory inputs have to be added to
our picture of ITD processing. These inhibitory inputs have entirely
different functions in birds and mammals. Whereas in birds tonic
depolarizing GABA ( -aminobutyric acid)-mediated inhibition accounts
for differential gain control and a general improvement of the
coincidence detection mechanism, temporally precise inhibition onto
ITD-sensitive neurons in the mammalian MSO actively contributes to
the ITD tuning itself.
 The ITD-processing circuits in birds and mammals are an excellent
example of how evolution of the vertebrate nervous system can
produce very different functional solutions for the same computational
problem.
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