BINAURAL PROCESSING IN THE DORSAL NUCLEUS OF THE LATERAL LEMNISCUS

We studied the binaural properties of 72 neurons in the dorsal nucleus of the lateral lemniscus (DNLL) of the mustache bat. There are six ma in findings: 1) Conventional EI neurons that were excited by stimulati on of the contralateral ear and inhibited by ipsilateral stimulation, comprise the majority (80%) of binaural DNLL cells. 2) For most EI neu rons the quantitative features of their interaural intensity disparity (IID) functions, maximum inhibition, dynamic range and 50% point IIDs , were largely unaffected by the absolute intensity at the contralater al ear. 3) Although the net effect of the inhibition evoked by ipsilat eral stimulation was to suppress discharges evoked by contralateral st imulation, our results indicate that the inhibitory inputs can act in three different ways. The first was a time-intensity trade, where incr easing the intensity at the ipsilateral ear evoked inhibitory effects with progressively shorter latencies. The second way was that the late ncy of inhibition did not appear to decrease with ipsilateral intensit y, but rather increasing ipsilateral intensity appeared only to increa se the strength of the inhibition. The third way was that the lowest e ffective ipsilateral intensity suppressed the first spikes evoked by t he contralateral stimulus and higher ipsilateral intensities then supp ressed the later discharges of the train. Each of these inhibitory pat terns was seen in about a third of the cells. 4) Neurons that had more complex binaural properties, such as the facilitated EI neurons (EI/F ) and neurons that were driven by sound to either ear (EE neurons), re presented about 20% of the binaural population. There were two types o f EE neurons; those in which there was a simple summation of discharge s evoked with certain IIDs, and those in which the spike-counts to bin aural stimulation at certain IIDs were greater than a summation of the monaural counts and thus were facilitated. 5) Air binaural neurons we re strongly inhibited with IIDs that favored the ipsilateral ear. Our findings indicate that the more complex binaural types, the facilitate d EI neurons (EI/F) as well as the two types of EE neurons, may be con structed from conventional EI neurons by adding inputs from several so urces that impart the more complex features to these neurons. We propo se four circuits that could account for the different binaural respons e properties that we observed. The circuits' are based on the known co nnections of the DNLL and the neurochemistry of those connections. Fin ally, we compared the binaural properties of neurons in the mustache b at DNLL with those of neurons in the mustache bat inferior colliculus and lateral superior olive. The comparative features suggest that the processing of the interaural intensity disparities undergoes at least four types of response transformations along the ascending auditory sy stem.