Hearing research
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Changes in amplitude are a characteristic feature of most natural sounds, including the biosonar signals used by bats for echolocation. Previous evidence suggests that the nuclei of the lateral lemniscus play an important role in processing timing information that is essential for target range determination in echolocation. Neurons that respond to unmodulated tones with a sustained discharge are found in the dorsal nucleus (DNLL), intermediate nucleus (INLL) and multipolar cell division of the ventral nucleus (VNLLm). ⋯ The maximal modulation rates that elicited synchronous responses were similar for the different cell groups, ranging from 320 Hz in VNLLm to 230 Hz in DNLL. The range of best modulation rates was greater for SAM than for SFM; this was also true of the range of maximal modulation rates at which synchronous discharge occurred. There was little correlation between a neuron's best modulation rate or maximal modulation rate for SAM signals and those for SFM signals, suggesting that responsiveness to amplitude and frequency modulations depends on different neural processing mechanisms.
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Neurons in the nuclei of the lateral lemniscus (NLL) of the big brown bat, Eptesicus fuscus, show several distinctive patterns of response to unmodulated tones. Previous work suggests that sustained responders are specialized to transmit information about sound level and duration while onset responders transmit precise timing information. The biosonar signals of E. fuscus consist of multiple, downward frequency modulated sweeps that change in slope and repetition rate as the bat approaches a target. ⋯ For the majority of neurons in VNLLm, INLL and DNLL, the precision of synchronization was approximately equal for the downward and upward components of the SFM signal; in contrast, 69% of VNLLc neurons responded selectively to the downward component of the SFM signal. All VNLLc neurons and a subset of those in VNLLm, INLL, and DNLL responded synchronously to SFM signals only if the frequency excursions included a border of the excitatory frequency bandwidth, suggesting that the synchronous discharge was due primarily to the repeated passage of the stimulus frequency into and out of the excitatory portion of the response area. In the case of VNLLc neurons, only the high frequency border was effective; Other neurons, especially those in DNLL, responded synchronously to SFM signals with frequency excursions that were confined entirely within the excitatory response area.