NEURAL RESPONSES TO SIMPLE SIMULATED ECHOES IN THE AUDITORY BRAIN-STEM OF THE UNANESTHETIZED RABBIT

1. In most natural environments, sound waves from a single source will reach a listener through both direct and reflected paths. Sound trave ling the direct path arrives first, and determines the perceived locat ion of the source despite the presence of reflections from many differ ent locations. This phenomenon is called the ''law of the first wavefr ont'' or ''precedence effect.'' The time at which the reflection is fi rst perceived as a separately localizable sound defines the end of the precedence window and is called ''echo threshold.'' The precedence ef fect represents an important property of the auditory system, the neur al basis for which has only recently begun to be examined. Here we rep ort the responses of single neurons in the inferior colliculus (IC) an d superior olivary complex (SOC) of the unanesthetized rabbit to a sou nd and its simulated reflection. 2. Stimuli were pairs of monaural or binaural clicks delivered through earphones. The leading click, or con ditioner, simulated a direct sound, and the lagging click, or probe, s imulated a reflection. Interaural time differences (ITDs) were introdu ced in the binaural conditioners and probes to adjust their simulated locations. The probe was always set at the neuron's best ITD, whereas the conditioner was set at the neuron's best ITD or its worst ITD. To measure the time course of the effects of the conditioner on the probe , we examined the response to the probe as a function of the condition er-probe interval (CPI). 3. When IC neurons were tested with condition ers and probes set at the neuron's best ITD, the response to the probe as a function of CPI had one of two forms: early-low or early-high. I n early-low neurons the response to the probe was initially suppressed but recovered monotonically at longer CPIs. Early-high neurons showed a nonmonotonic recovery pattern. In these neurons the maximal suppres sion did not occur at the shortest CPIs, but rather after a period of less suppression. Beyond this point, recovery was similar to that of e arly-low neurons. The presence of early-high neurons meant that the ov erall population was never entirely suppressed, even at short CPIs. Ta ken as a whole, CPIs for 50% recovery of the response to the probe amo ng neurons ranged from 1 to 64 ms with a median of similar to 6 ms. 4. The above results are consistent with the time course of the preceden ce effect for the following reasons. 1) The lack of complete suppressi on at any CPI is compatible with behavioral results that show the pres ence of a probe can be detected even at short CPIs when it is not sepa rately localizable. 2) At a CPI corresponding to echo threshold for hu man listeners (similar to 4 ms CPI) there was a considerable response to the probe, consistent with it being heard as a separately localizab le sound at this CPI. 3) Full recovery for all neurons required a peri od much longer than that associated with the precedence effect. This i s consistent with the relatively long time required for conditioners a nd probes to be heard with equal loudness. 5. Conditioners with either the best ITD or worst ITD were used to determine the effect of ITD on the response to the probe. The relative amounts of suppression caused by the two ITDs varied among neurons. Some neurons were suppressed ab out equally by both types of conditioners, others were suppressed more by a conditioner with the best ITD, and still others by a conditioner with the worst ITD. Because the best ITD and worst ITD presumably act ivate different pathways, these results suggest that different neurons receive a different balance of inhibition from different sources. 6. The recovery functions of neurons not sensitive to ITDs were similar t o those of ITD-sensitive neurons. This suggests that the time course o f suppression may be common among different IC populations. 7. We also studied neurons in the SOC. Although many showed binaural interaction s, none were sensitive to ITDs. Thus the response of this population m ay not be directly comparable with the responses of ITD-sensitive neur ons in the IC. In general, the responses of SOC neurons to the probe r ecovered faster than those of IC neurons. The median CPI for 50% recov ery was similar to 2 ms for the SOC, compared with similar to 6 ms for the IC. These data imply that the mechanisms responsible for the long er suppression in the IC lie subsequent to the SOC.