Neural tuning to sound duration in the inferior colliculus of the big brown
bat, Eptesicus fuscus.
DAPHNA EHRLICH, JOHN H. CASSEDAY, AND ELLEN COVEY.
Department of Neurobiology, Duke University Medical Center, Durham, NC,
27710.
APStracts 4:0040N, 1997.
ABSTRACT
Neural tuning to different sound durations may be a useful filter for
identification of certain sounds, especially those that are biologically
important. The auditory midbrains of mammals and amphibians contain neurons
that appear to be tuned to sound duration. In amphibians, neurons are tuned to
durations of sound that are biologically important. The purpose of this study
was to characterize responses of neurons in the inferior colliculus (IC) of
the big brown bat, Eptesicus fuscus, to sounds of different durations. Our
aims were to determine what percent of neurons are duration tuned, how best
durations are correlated to durations of echolocation calls and to examine
response properties that may be relevant to the mechanism for duration tuning,
such as latency and temporal firing pattern; we also examined frequency tuning
and rate-level functions. We recorded from 136 single units in the central
nucleus of the IC of unanesthetized bats. The stimuli were pure tones,
frequency modulated (FM) sweeps and broad-band noise. The criterion for
duration tuning was an increase in spike count of 50% or more at some
durations compared to others. Out of the total units sampled, 36% were tuned
to stimulus duration. All of these units were located in the caudal half of
the IC. Best duration for most units ranged from <1 ms to 10 ms, but a few had
best durations up to 20+ ms. This range is similar to the range of durations
of echolocation calls used by Eptesicus. All duration tuned neurons responded
transiently. The minimum latency was always longer than the best duration.
Duration tuned units have best durations and best frequencies that match the
temporal structure and frequency range of the echolocation calls. Thus the
results raise the hypothesis that neurons in the IC of Eptesicus, and probably
the auditory midbrain of other vertebrates, are tuned to biologically
important sound durations. We suggest a model for duration tuning consisting
of 3 components: 1) inhibitory input that is correlated with the onset of the
stimulus and is sustained for the stimulus duration; 2) transient excitation
that is correlated with the offset of the stimulus; 3) transient excitation
that is correlated with the onset of the stimulus but is delayed in time
relative to the onset of inhibition. For the neuron to fire, the two
excitatory events must coincide in time; non-coincident excitatory events are
not sufficient.
Received 19 March 1996; accepted in final form 8 January 1997.
APS Manuscript Number J229-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 5 February 1997