1. The biosonar pulse of the mustached bat, Pteronotus parnellii parnellii, consists of four harmonics of a constant-frequency component (CF1-4) followed by a frequency-modulated component (FM1-4). FM-FM combination-sensitive neurons in the auditory cortex and the medial geniculate body (MGB) show facilitative responses to certain combinations of FM components in a pulse-echo pair. They are tuned to particular delays of echo FM(n) (EFM(n)) (n = 2, 3, or 4) from pulse FM1 (PFM1). The neural mechanisms for creating their response properties involve delay lines, coincidence detection, and multiplication. Coincidence detection and multiplication take place in the MGB. It is not yet known where and how delay lines are created. The first aim of the present studies is to examine whether delay lines are created by subthalamic nuclei. FM-FM neurons are tuned to not only echo delays but also echo amplitudes. Therefore, the second aim of the present studies is to examine the extent to which amplitude selectivity is created by subthalamic nuclei. Responses of single nerve fibers to acoustic stimuli were recorded from the brachium of the inferior colliculus (BIC) using tungsten wire microelectrodes, and their response latencies and best amplitudes were measured. 2. All BIC fibers responded strongly to single tone bursts. No FM-FM combination-sensitive neurons were found in the BIC. The best frequencies of BIC fibers were predominantly within the frequency ranges of four harmonics of the species-specific biosonar pulse. 3. The response latencies of BIC fibers tuned to FM1 were more diverse (3.5-15.0 ms) than those of BIC fibers tuned to FM(n) (3.86. 5 ms). This difference in latency distribution was independent of stimulus amplitude. These data are consistent with the theory that delay lines utilized by FM-FM neurons are created by neurons tuned to the ''FM1 frequency,'' and indicate that the delay lines are mostly, if not all, created in a subthalamic nucleus or nuclei. 4. The best amplitudes of BIC fibers tuned to FM1 or CF1 were 63.2 +/- 4.5 (SE) dB SPL, and those of BIC fibers tuned to FM(n) or CF(n) were 48.2 +/- 10.7 dB SPL. The distributions of the best amplitudes of BIC fibers were very similar to those of FM-FM and CF/CF neurons in the MGB. These data indicate that the amplitude selectivity of thalamic FM-FM and CF/CF neurons is mainly a product of a subthalamic nucleus or nuclei. 5. Our data substantiate the theory that the neural processing of biosonar information is a combination of parallel and hierarchical processes. 6. In the BIC, there was a trend that low and high frequencies were respectively represented ventrolaterally and dorsomedially. This tonotopic organization reflects that of the inferior colliculus. However, the tonotopic organization in the BIC was complex, because the representation of the first harmonic interdigitated with that of the second harmonic, and because the third and fourth harmonics were represented in a few locations. This complex tonotopic organization is probably related to the creation of combination-sensitive neurons in the MGB and also to the multiple tonotopic representations in the different subdivisions of the MGB.
