REPRESENTATION OF INTERAURAL LEVEL DIFFERENCE IN THE VLVP, THE FIRST SITE OF BINAURAL COMPARISON IN THE BARN OWLS AUDITORY-SYSTEM

In the avian auditory system, the posterior division of the ventral nu cleus of the lateral lemniscus (VLVp) is the first site where the leve ls of sound arriving at the two ears art compared. VLVp units are exci ted by sound at the contralateral ear and are inhibited by sound at th e ipsilateral ear, and, as a result, are sensitive to interaural level differences (ILD). In this study, we investigate the functional prope rties of VLVp units and describe the topography of ILD sensitivity alo ng the dorsoventral dimension of this nucleus. The responses of VLVp u nits were tested with monaural and binaural noise delivered through ea rphones. Excitatory and inhibitory responsiveness was quantified using several measures that assessed the effect of contra-ear stimulation a nd the effect of ipsi-ear stimulation on the contra-ear response. On t he basis of these measures, we characterize the map of ILD sensitivity in the VLVp. The temporal pattern of unit responses were also analyze d. The discharges of VLVp units were regular and time-locked to the on set of a stimulus, a pattern of discharge reminiscent of the 'chopper pattern' observed in the lateral superior olive (LSO) of mammals. The temporal discharge patterns of a single VLVp neuron often distinguishe d between equivalent ILDs, resulting from different combinations of co ntra- and ipsi-ear levels, that were not distinguished by spike count alone. However, the temporal response pattern did not distinguish betw een all such combinations of contra- and ipsi-ear levels. The addition al information was encoded by the pattern of activity across the entir e population of VLVp neurons. This study describes similarities in the functional properties of VLVp and LSO units that suggest similar phys iological mechanisms in avians and mammals for encoding similar acoust ic information.