This project will address the organization of neurons into parallel pathways within the central auditory system. While the anatomical evidence for parallel circuitry in the auditory system is compelling, the functional consequences of this circuitry are not known. This study will elucidate the role of this circuitry in central auditory processing. Neurons in the auditory system can be classified according to their binaural response properties. Monaural neurons respond only to sounds presented in one ear. Binaural neurons can be effected by sounds delivered to both ears. Both types of neuron may be involved in a variety of auditory functions, but some behaviors, like sound localization, may emphasize one class of neuron over another. Monaural and binaural neurons can be found at all levels of the auditory system, but, the anatomical cell types and circuits related to these responses are not certain. We hypothesize that cells which respond only to monaural stimulation are synaptically connected to form one or more monaural circuits. These monaural circuits would be distinct from circuits concerned with binaural processing. In the present proposal, we will focus on the neurons with monaural response features in the lower brain stem. The cochlear nucleus (CN) and ventral nucleus of the lateral lemniscus (VNLL) are major centers with monaural neurons, and both project to the inferior colliculus (IC). Despite the importance of these structures for understanding monaural responses in the IC and in higher centers, critical data are still lacking on the cells involved in these projections. Therefore, AIM 1 is to determine which cell types participate in the CN-VNLL-IC circuit and compare them to the cells involved in the CN-IC circuit. Axonal transport labeling in vivo and intracellular injections in fixed brain slices will be used. We predict that different cell types will participate in each circuit. Double-labeling experiments will determine if the same neurons in the CN project to both the VNLL and IC. Finally, anterograde transport and intracellular experiments will determine the nature of the synaptic connections made in the VNLL by axons from the CN. We anticipate that specific synaptic arrangements will characterize the neuropil of the VNLL. The monaural responses in VNLL are attributed to strong inputs from the contralateral CN and a dearth of inputs from the superior olivary complex. There is little physiological data on the VNLL to confirm its monaural properties in terrestrial mammals. Therefore, AIM 2 is to characterize the response properties of units in the VNLL of the awake rabbit. We anticipate that most cells in VNLL will be monaural; however, their responses may be different from those in the CN.