The role of spectral cues in sound localization by the barn owl

The barn owl (Tyto alba) is a nocturnal predator with excellent spatial hearing. Evolutionary pressure on the auditory system of the barn owl has produced numerous adaptations for processing spatial information; my dissertation addresses two new aspects of sound localization in this auditory specialist. (I) Barn owls have been shown to use interaural time differences (ITDs) and interaural intensity differences (IIDs) to localize the sources of sounds. Such binaural difference cues are also used by a variety of mammals, including humans. Mammals also exploit a monaural sound localization cue. When sound arrives at the eardrum it has been filtered by the external ear in a location- and frequency-dependent manner; this spectral cue underlies our ability to localize in the vertical plane. Recent measurements in barn owls have shown that spectral features similar to those thought to encode vertical position in humans also exist in barn owls. I show that radical variations in the monaural spectra which do not produce concomitant changes in the binaural difference spectrum have no effect on barn owl sound localization behavior. In contrast to humans, barn owls do make some use of the frequency-specific IID information in the binaural difference spectrum. (II) In order to use a binaural difference cue, the sound from a source that enters the left ear must be compared with sound from the same source that enters the right ear. This results in the perception of a single auditory image; this process is known as binaural fusion. One feature of sound which is important to fusion is the degree to which the sounds arriving at the two ears are correlated. The ability of both barn owls and humans to localize based on ITD is eliminated if binaural correlation is reduced to zero. However, I show that barn owls can extract the IID of a binaurally uncorrelated signal, and use that IID to control an auditory saccade. In addition I show that neurons in the thalamic auditory pathway, in contrast to the more well understood collicular auditory pathway, encode the IID of binaurally uncorrelated sounds.