DELAY-LINES AND AMPLITUDE SELECTIVITY ARE CREATED IN SUBTHALAMIC AUDITORY NUCLEI - THE BRACHIUM OF THE INFERIOR COLLICULUS OF THE MOUSTACHED BAT

1. The biosonar pulse of the mustached bat, Pteronotus parnellii parne llii, consists of four harmonics of a constant-frequency component (CF 1-4) followed by a frequency-modulated component (FM1-4). FM-FM combin ation-sensitive neurons in the auditory cortex and the medial genicula te body (MGB) show facilitative responses to certain combinations of F M 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 ne ural mechanisms for creating their response properties involve delay l ines, coincidence detection, and multiplication. Coincidence detection and multiplication take place in the MGB. It is not yet known where a nd how delay lines are created. The first aim of the present studies i s 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 e xtent to which amplitude selectivity is created by subthalamic nuclei. Responses of single nerve fibers to acoustic stimuli were recorded fr om the brachium of the inferior colliculus (BIC) using tungsten wire m icroelectrodes, 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 ran ges of four harmonics of the species-specific biosonar pulse. 3. The r esponse 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 diff erence 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 t uned to FM1 or CF1 were 63.2 +/- 4.5 (SE) dB SPL, and those of BIC fib ers tuned to FM(n) or CF(n) were 48.2 +/- 10.7 dB SPL. The distributio ns 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 ampli tude selectivity of thalamic FM-FM and CF/CF neurons is mainly a produ ct of a subthalamic nucleus or nuclei. 5. Our data substantiate the th eory that the neural processing of biosonar information is a combinati on of parallel and hierarchical processes. 6. In the BIC, there was a trend that low and high frequencies were respectively represented vent rolaterally and dorsomedially. This tonotopic organization reflects th at 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 comple x tonotopic organization is probably related to the creation of combin ation-sensitive neurons in the MGB and also to the multiple tonotopic representations in the different subdivisions of the MGB.