Processing of sinusoidally frequency modulated signals in the nuclei of the lateral lemniscus of the big brown bat, Eptesicus fuscus.

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. An obvious hypothesis would be that NLL neurons with sustained responses should discharge during the time when the frequency of a signal is within their response area, but that onset responders should discharge each time the frequency enters the excitatory portion of their response area. In this study we examined the responses of NLL neurons to sinusoidally frequency modulated (SFM) signals presented monaurally to awake, restrained bats. Extracellular recordings were obtained from single neurons in the multipolar and columnar divisions of the ventral nucleus (VNLLm and VNLLc), the intermediate nucleus (INLL) and the dorsal nucleus of the lateral lemniscus (DNLL). All NLL neurons responded synchronously to SFM signals under some conditions. The temporal precision of synchronization was quantified using a coefficient of synchronization (CS), where a value of I equals perfect synchrony. Maximum CS values ranged from 0.70 to >0.99, were generally highest at low modulation rates ( <200 Hz), and showed lowpass characteristics for modulation rate. The maximal modulation rates that elicited synchronous discharge ranged from 50 to 500 Hz. The highest maximal rates were found in the VNLLm and VNLLc, the lowest in DNLL. The ability of NLL neurons to synchronize their discharge to the pattern of an SFM signal is intermediate between that of neurons in the cochlear nucleus and in the inferior colliculus. 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.