Title: Spatial Tuning to Sound Source Azimuth in the Inferior Colliculus of the Unanesthetized Rabbit. 2

41 Despite decades of research devoted to the study of inferior colliculus (IC) neurons’ tuning to 42 sound-source azimuth, there remain many unanswered questions because no previous study examined 43 azimuth tuning over a full range of 360° azimuths at a wide range of stimulus levels in an unanesthetized 44 preparation. Furthermore, a comparison of azimuth tuning to binaural and contralateral-ear stimulation 45 over ranges of full azimuths and widely varying stimulus levels was not previously reported. To fill this 46 void, we conducted a study of azimuth tuning in the IC of the unanesthetized rabbit over a 300° range of 47 azimuths at stimulus levels of 10-50 dB re: neural threshold to both binaural and contralateral-ear 48 stimulation using virtual auditory space VAS stimuli. This study provides systematic evidence for neural 49 coding of azimuth. We found the following: 1. Level tolerant azimuth tuning was observed in top 35% 50 regarding vector strength and in the top 15% regarding vector angle of IC neurons. 2. Preserved 51 azimuth tuning to binaural stimulation at high stimulus levels was created as a consequence of binaural 52 facilitation in the contralateral sound field and binaural suppression in the ipsilateral sound field. 3. The 53 direction of azimuth tuning to binaural stimulation was primarily in the contralateral sound field and its 54 center shifted laterally towards -90° with increasing stimulus level. 4. At 10 dB, azimuth tuning to 55 binaural and contralateral stimulation was similar indicating that it was mediated by monaural 56 mechanisms. 5. At higher stimulus levels azimuth tuning to contralateral-ear stimulation was severely 57 degraded. These findings form a foundation for understanding neural mechanisms of localizing sound58 source azimuth. 59 60 6