ENVELOPE CODING IN THE LATERAL SUPERIOR OLIVE II. CHARACTERISTIC DELAYS AND
COMPARISON WITH RESPONSES IN THE MEDIAL SUPERIOR OLIVE.
Joris, Philip X.
Department of Neurophysiology, University of Wisconsin Medical School,
Madison, Wisconsin 53706.
APStracts 3:0133N, 1996.
SUMMARY AND CONCLUSIONS
1. Spike rates of cells in the cat's lateral superior olive (LSO) depend on
interaural level differences (ILDs) and envelope interaural time differences
(ITDs) of amplitude-modulated tones presented to both ears. We previously
proposed (Joris and Yin 1995) that these sensitivities arise from a common
mechanism, which is the IE binaural interaction ( I nhibited by the
contralateral and E xcited by the ipsilateral ear). As a further test of that
proposal and to gain a better understanding of the importance of this ITD-
sensitivity, responses to monaural and binaural modulation are here compared
over a range of modulation frequencies. 2. At low modulation frequencies, LSO-
IE cells respond maximally when the envelopes of the amplitude-modulated
stimuli at the two ears are out-of-phase by a half-cycle. This phase
difference changes in a systematic way, which varies from cell to cell, when
modulation frequency is increased. Mean interaural phase, measured over a
range of modulation frequencies, was subjected to a characteristic delay
analysis (Yin and Kuwada 1983). Two measures were extracted: characteristic
delay, which reflects differences in conduction delay between ipsi- and
contralateral pathways, and characteristic phase, which reflects their sign of
interaction. Most characteristic delays were within the physiological range of
ITDs. There was a small bias towards positive delays, indicating a longer
conduction time for the contralateral pathway. Characteristic phases were
tightly distributed around 0.5 cycles, consistent with the proposed IE
mechanism for ITD-sensitivity. 3. Increases in the modulation frequency of
binaural stimuli beyond 300 Hz consistently caused a profound decrease in
average spike rate, as well as a decrease in the modulation of spike rate by
ITD. The upper limit of ITD-sensitivity was 800 Hz. Sensitivity to envelope-
ITDs is therefore limited to a much lower range of frequencies than
sensitivity to ITDs in fine-structure, e.g. as found in the medial superior
olive (MSO), which operates up to several kHz. 4. A small sample of high-
frequency EE cells (excited by both ears) in MSO was also tested with binaural
amplitude-modulated stimuli. MSO-EE cells showed weak envelope ITD-sensivity
over a limited range of modulation frequencies. Consistent with the EE
interaction, characteristic phases clustered around 0 cycles. 5. Mean
interaural phase was compared with the phase of responses to monaural
modulation. The difference between the ipsilateral and contralateral phase
correlated well with the phase measured binaurally, both for LSO and MSO
cells. 6. Many features of LSO-IE responses were mimicked by the simplest
possible computer model, consisting of subtraction and rectification of low-
pass filtered envelope waveforms. Differences between model and physiological
results are suggestive of a temporal limitation in the binaural interaction
that creates the ITD-sensitivity. 7. These results provide additional evidence
for LSO ITD-sensitivity paralleling human psychophysical results. The stimulus
boundaries within which ITD-sensitivity occurs suggest that it has a limited
role in free-field conditions. It is traditionally thought that, in order to
contribute to the perceived change in spatial location of a sound source, the
LSO needs to show a change in overall firing rate summed across cells. This is
achieved with small ILDs, but requires large ITDs, because the latter cue is
less potent in single cells and has varied effects across cells by virtue of
differences in characteristic delay.
Received 18 May 1995; accepted in final form 3 June 1996.
APS Manuscript Number J333-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 28 June 96