The acoustic system of the barn owl

03 July 2014
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The barn owl is a model system for sound localization. It hunts small vertebrates typically at dawn and dusk. This bird has developed a number of specialisations that prove to be very exciting both for basic research and in terms of bionic applications.
The specializations may be understood in the context of evolution.  Evolution is an optimisation process. Particularly good solutions are found in animals that are specialised in specific tasks. This makes the study of such "specialists" interesting.
The barn owl mainly uses its highly specialised hearing during hunting. The name-giving ruff (in German “Schleier”) acts as an amplifier. This allows the bird to hear sounds that are about ten times fainter than the faintest sounds perceptible to humans. The sound from a sound source arrives later at the ear facing away from the source than it does at the ear facing towards the source. This results in propagation-time differences between the ears. Differences in loudness occur between the ears because the head attenuates sounds at the ear facing away from the source. Due to its smaller head, the range of the interaural time differences in the barn owl is smaller than that in humans. Consequently, the barn owl should localise sound sources less accurately than humans. But that is not the case as may be shown by behavioural tests in the laboratory. In the horizontal plane, humans and the barn owl localise sound sources almost equally, whereas in the vertical plane the barn owl actually achieves a higher accuracy than humans. This is due to another special feature: the ear flaps of the barn owl are not arranged symmetrically with respect to the interocular axis, but the left ear flap is located higher than the right, and faces downwards while the right one faces upwards. The ruff and the aural asymmetry interact to produce differences in loudness not only in the horizontal plane, but also in the vertical plane. While humans use differences in both interaural time and level to determine the horizontal angle, the owl uses the interaural time differences to determine the horizontal angle, and level differences to determine the vertical angle.
If the barn owl, with such a small head, achieves equally good or even better localisation accuracy than a human being, it must be better able to evaluate the directional information contained in the sound than humans are. The brain of the barn owl is highly specialised for processing acoustic signals. Its hearing organ is greatly enlarged in comparison to other birds. The structures in the auditory pathway involved in the processing of interaural time difference are also enlarged. The very small time differences which the sound takes to travel the distance between the ears are only a few hundred millionths of a second, and are in fact too small to be detected by unspecialized nerve cells. We suppose that it is further specialisations in the form of the molecules which generate the neural signals, and the interaction of many sensory and nerve cells what make such a precise temporal resolution possible. A deeper understanding of the optimization at the molecular, cellular and network level is of particular interest, because it may prove useful in improving cochlear implants and hearing aids. In addition, we are working on sound-localising "autonomous agents“ based on the kinds of computational operations that also take place in the brain of the owl.