Representation of auditory space in the avian midbrain: integration without a map?

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While spatial vision in birds is well understood, spatial auditory processing remains puzzling for most avian species. Even though most bird species move in all three dimensions in a highly structured world, auditory localization is believed to be restricted to the azimuth in most birds, that is, only along the horizontal dimension. Elevational cues to determine the height of a sound source are usually introduced by outer ear structures that most bird species do not possess; only specialists such as the barn owl have anatomical features (such as asymmetric ear openings) that allow localization in elevation. However, recent data have shown that the spherical avian head induces acoustic nonlinearities that might allow sound localization in elevation by head movements. We will study this behavior in chicken in a controlled paradigm to determine whether non-specialists species are indeed able to locate sound in all dimensions. In addition, we will investigate how the auditory information is integrated into the map of visual space in the midbrain optic tectum. If the auditory system is indeed incapable of locating sounds in elevation, how can auditory information be integrated into the precise map of visual space in the tectum? We will record the responses of midbrain neurons under auditory, visual, and audio-visual stimulation to determine the receptive fields in each modality and delineate the integration rules. Preliminary data indicate that audio-visual integration is very different from the previous models developed in barn owls. We expect to find computational mechanisms that allow sufficient behavioral precision even without the peripheral structures that were previously thought to be indispensable.