Long Delay Lines for Ranging are created by Inhibition in the inferior colliculus of the mustached bat. Saitoh, Isao, and Nobuo Suga. Department of Biology, Washington University , One Brookings Drive, St. Louis, Missouri 63130.
APStracts 2:0036N, 1995.
SUMMARY AND CONCLUSIONS
1. The central auditory system of the mustached bat has arrays of delay-tuned (FM-FM combination-sensitive) neurons in the inferior colliculus, the medial geniculate body, and the auditory cortex. These neurons are tuned to particular echo delays, i.e., target distances. The neural mechanisms for creating the delay-tuned neurons involve delay lines, coincidence detection, and amplification (Olsen, 1986; Suga, 1990). We have hypothesized that delay lines longer than 4 ms are created by inhibition occurring in the anterolateral division (ALD) of the central nucleus of the inferior colliculus. If this hypothesis is correct, suppression of inhibition occurring in the ALD must shorten the best delays of the collicular, thalamic and cortical delay-tuned neurons. The aim of the present study is to test this hypothesis. Responses of single delay-tuned neurons in the FM-FM area of the auditory cortex were recorded with a tungsten-wire microelectrode, and the effects of iontophoretic micro-injections of strychnine (STR) and/or bicuculline methiodide (BMI) into the ALD were examined on the responses of these neurons. 2. STR (glycine receptor antagonist) and/or BMI (GABA-A receptor antagonist) injections into the ALD shortened the best delays of delay-tuned neurons in the FM-FM area with little change in their response patterns. The longer the best delay of a delay-tuned neuron, the larger the amount of shortening. 3. Inhibition mediated by glycine receptors plays a larger role in creating delay lines than that mediated by GABA-A receptors, since STR and BMI respectively shortened the best delay of 91% and 74% of the neurons with best delays longer than 4.5 ms. 4. BMI has no effect on the best delays of delay-tuned neurons which were tuned to echo delays shorter than 4.5 ms. 5. The present data support the hypothesis that long delay lines utilized by delay-tuned neurons are created by inhibition occurring in the ALD of the inferior colliculus. However, the amount of shortening in delay lines by STR and/or BMI was generally smaller than that predicted by a neural network model (Suga, 1990). Therefore, the present study partially answers the questions of where and how long delay lines were created. 

Received 6 December 1993; accepted in final form 1 January 1995.
APS Manuscript Number J618-3.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on  3 April 1995.