Collaborative Research: Mechanisms of Sound Source Localization Underlying an Ancestral Mode of Vertebrate Hearing

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The ability to locate the source of sounds enables animals to detect prey, avoid predators and communicate with others and is thus basic to survival in many species. While decades of behavioral, physiological and neuroanatomical research have revealed the physical cues and neural mechanisms that terrestrial animals use to localize sound, the mechanisms used by fish, the oldest living vertebrate group, remain a mystery. Collectively, these experiments will investigate the mechanisms of sound localization utilized by fish that likely formed the evolutionary foundation for more recent modes of vertebrate hearing and sound localization. Throughout the project, Drs. Sisneros and Forlano will train and mentor both graduate and undergraduate students and give annual public lectures regarding the supported research at the Friday Harbor Labs (FHL). As an integral part of this research program, Drs. Sisneros and Forlano will host GK-12 teachers every summer at FHL where they will participate in field and laboratory experiments. The researchers will also develop lesson plans, student projects and an educational website with the teachers at their home institutions. The investigation will take an integrated behavioral, anatomical, and brain activational approach to determine whether fish are fundamentally similar to other studied vertebrates, and use binaural information (information from both ears) to localize sound, or are fundamentally different, and achieve robust localization on the basis of monaural (single-ear) information alone. The central hypothesis to be tested is that binaural integration is essential for sound source localization in midshipman. To test this hypothesis, the investigators will 1) determine which inner ear endorgans are required for sound localization behavior by testing animals in an established sound playback paradigm before and after systematic unilateral or bilateral removal of each endorgan's otolith (saccule, lagena, utricle), 2) characterize the ipsilateral and contralateral projections of inner ear afferents from all three endorgans to known auditory processing regions in the hindbrain by bulk labeling each endorgan separately or in double or triple combination with different fluorescent-labeled dextran amine tracers, and 3) characterize the brain activation patterns resulting from controlled auditory directional stimulation in intact animals and in those that have undergone systematic endorgan removal, using c-Fos as a marker for neural activation. Duplicate digital files of all raw, processed and consolidated data will be stored locally and in the cloud by both researchers and will be made publically available within two years following publication.