LONG DELAY-LINES FOR RANGING ARE CREATED BY INHIBITION IN THE INFERIOR COLLICULUS OF THE MOUSTACHED BAT

1. The central auditory system of the mustached bat has arrays of dela y-tuned (FM-FM combination-sensitive) neurons in the inferior collicul us, the medial geniculate body, and the auditory cortex. These neurons are tuned to particular echo delays, i.e., target distances. The neur al mechanisms for creating the delay-tuned neurons involve delay lines , coincidence detection, and amplification. We have hypothesized that delay lines longer than 4 ms are created by inhibition occurring in th e anterolateral division (ALD) of the central nucleus of the inferior colliculus. If this hypothesis is correct, suppression of inhibition o ccurring in the ALD must shorten the best delays of the collicular, th alamic, and cortical delay-tuned neurons. The aim of the present study is to test this hypothesis. Responses of single delay-tuned neurons i n the FM-FM area of the auditory cortex were recorded with a tungsten- wire microelectrode, and the effects of iontophoretic microinjections of strychnine (STR) and/or bicuculline methiodide (BMI) into the ALD w ere examined on the responses of these neurons. 2. STR(glycine recepto r antagonist) and/or BMI [gamma-aminobutyric acid-A (GABA(A)) receptor antagonist] injections into the ALD shortened the best delays of dela y-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 larg er the amount of shortening. 3. Inhibition mediated by glycine recepto rs plays a larger role in creating delay lines than that mediated by G ABA(A) receptors, because STR and BMI, respectively, shortened the bes t 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 tha t were tuned to echo delays shorter than 4.5 ms. 5. The present data s upport the hypothesis that long delay lines utilized by delay-tuned ne urons are created by inhibition occurring in the ALD of the inferior c olliculus. However, the amount of shortening in delay lines by STR and /or BMI was generally smaller than that predicted by a neural network model. Therefore the present study partially answers the questions of where and how long delay lines were created.