MEDIAL SUPERIOR OLIVE IN THE FREE-TAILED BAT - RESPONSE TO PURE-TONESAND AMPLITUDE-MODULATED TONES

In mammals with good low-frequency hearing and a moderate to large int erear distance, neurons in the medial superior olive (MSG) are sensiti ve to interaural time differences (ITDs). Most small mammals, however, do not hear low frequencies and do not experience significant ITDs, s uggesting that their MSOs participate in functions other than ITD codi ng. In one bat species, the mustached bat, the MSO is a functionally m onaural nucleus, acting as a low-pass filter for the rate of sinusoida lly amplitude-modulated (SAM) stimuli. We investigated whether the mor e typical binaural MSG of the Mexican free-tailed bat also acts as an SAM filter. We recorded from 60 MSO neurons with their best frequencie s covering the entire audiogram of this bat. The majority revealed bil ateral excitation and indirect evidence for inhibition (EI/EI; 55%). T he remaining neurons exhibited reduced inputs, mostly lacking ipsilate ral inputs (28% I/EI; 12% O/EI; 5% EI/O). Most neurons (64%) responded with a phasic discharge to pure tones; the remaining neurons exhibite d an additional sustained component. For stimulation with pure tones, two thirds of the cells exhibited monotonic rate-level functions for i psilateral, contralateral, or binaural stimulation. In contrast, nearl y all neurons exhibited nonmonotonic rate-level functions when tested with SAM stimuli. Eighty-eight percent of the neurons responded with a phase-locked discharge to SAM stimuli at low modulation rates and exh ibited low-pass filter characteristics in the modulation transfer func tion (MTF) for ipsilateral, contralateral, and binaural stimulation. T he MTF for ipsilateral stimulation usually did not match that for cont ralateral stimulation. Introducing interaural intensity differences (I IDs) changed the MTF in unpredictable ways. We also found that respons es to SAMs depended on the carrier frequency. In some neurons we measu red the time course of the ipsilaterally and contralaterally evoked in hibition by presenting brief frequency-modulated sweeps at different I TDs. The duration and timing of inhibition could be related to the SAM cutoff for binaural stimulation. We conclude that the response of the MSG in the foe-tailed bat is created by a complex interaction of inhi bition and excitation. The different time constants of inputs create a low-pass filter for SAM stimuli. However, the MSO output is an integr ated response to the temporal structure of a stimulus as well as its a zimuthal position, i.e., IIDs. There are no in vivo results concerning filter characteristics in a ''classical'' MSG, but our data confirm a n earlier speculation about this interdependence based on data accesse d from a gerbil brain slice preparation.