The Journal of Neurophysiology Vol. 84 No. 3 September 2000, pp. 1475-1487
Copyright ©2000 by the American Physiological Society
1Department of Neurobiology, Duke University Medical Center, Durham, North Carolina 27710; and 2Department of Psychology, University of Washington, Seattle, Washington 98195
Casseday, John H.,
Daphna Ehrlich, and
Ellen Covey.
Neural Measurement of Sound Duration: Control by
Excitatory-Inhibitory Interactions in the Inferior Colliculus. J. Neurophysiol. 84: 1475-1487, 2000. In the inferior
colliculus (IC) of the big brown bat, a subpopulation of cells
(~35%) are tuned to a narrow range of sound durations. Band-pass
tuning for sound duration has not been seen at lower levels of the
auditory pathway. Previous work suggests that it arises at the IC
through the interaction of sound-evoked, temporally offset, excitatory
and inhibitory inputs. To test this hypothesis, we recorded from
duration-tuned neurons in the IC and examined duration tuning before
and after iontophoretic infusion of antagonists to gamma-aminobutyric
acid-A (GABAA) (bicuculline) or glycine (strychnine). The
criterion for duration tuning was that the neuron's spike count as a
function of duration had a peak value at one duration or a range of
durations that was >= 2 times the lowest nonzero value at longer
durations. Out of 21 units tested with bicuculline, duration tuning was
eliminated in 15, broadened in two, and unaltered in four. Out of 10 units tested with strychnine, duration tuning was eliminated in four, broadened in one, and unaltered in five. For units tested with both
bicuculline and strychnine, bicuculline had a greater effect on
reducing or abolishing duration tuning than did strychnine. Bicuculline
and strychnine both produced changes in discharge pattern. There was
nearly always a shift from an offset response to an onset response,
indicating that in the predrug condition, inhibition arrived
simultaneously with excitation or preceded it. There was often an
increase in the length of the spike train, indicating that in the
predrug condition, inhibition also coincided with later parts of
excitation. These findings support two hypotheses. First, duration
tuning is created in the IC. Second, although the construction of
duration tuning varies in some details among IC neurons, it follows
three rules: 1) an excitatory and an inhibitory event
are temporally linked to the onset of sound but temporally offset from
one another; 2) the duration of some inhibitory event must be linked to the duration of the sound; 3) an
excitatory event must be linked to the offset of sound.