Not just a pretty face: The facial ruff of barn owls and sound localisation
Barn owls are the subject of many studies on auditory neuroscience because of their exquisite ability to localize sound. The auditory system is interesting from a neuronal computation point of view because the inner ear, where sounds are detected, relays no information to the brain as to the location of the sound source in space. It is then up to the neurons in the brain to extract other information transmitted from the ear to build an auditory space map which can be used for sound localization. The basic model is that the auditory system does this by comparing the differences in intensities of the sound at the two ears (interaural level differences) and the differences in the time of arrival of the sound to each of the ears (interaural time differences), and that this information is sufficient to sound localize.
But the sound that reaches the tympanum is not an exact replica of the sound emanating from the source, to a great extent due to the way that sound interacts with and is modified by the animal’s own head structures (for example, the external ear or pinna). These changes in the structure of the sound are described in the Head Related Transfer Function (HRTF). Because we are not identical clones of each other, each one of us has a slightly different head related transfer function.
In a recent study published in PLoS One, Laura Hausmann, Mark von Campenhausen, Frank Endler, Martin Singheiser, and Hermann Wagner examined whether the contribution of the facial ruff to the barn owl’s HRTF affected the owl’s ability to sound localize. They recorded the HRTF of each of the owls in the study, as well as the HRTF of a barn owl in which the facial ruff had been cut off. They used these functions to build a virtual auditory stimulus that simulated the presence or absence of the ruff, and assessed the ability of the barn owls to sound localize. (They can do this behaviourally, because barn owls turn their heads to the source of the stimulus with great precision.)
Their results show that the facial ruff contributes to the cues used for sound localization by increasing the effective interaural time difference range that barn owls use to localise sound in the horizon and by contributing to the differences in interaural level differences that barn owls use to localize sound in elevation. But they also showed that there were no differences in the localization ability when when barn owls used interaural time differences whether the virtual acoustic stimulus was built with their own HRTF or one of another owl. This was not true when the owls were using interaural level differences, where the owls did not localize equally to the different HRTFs. Removal of the ruff had, as expected, several effects on sound localisation, one being that they lost the ability to discriminate between sounds coming from the front or from the back.
Barn owls learn how to associate interaural time and level differences with the location of the sound source during their first two months of life, when their head is growing and the facial ruff develops. They do this by instructive signals derived from the visual system, by which they attribute specific combinations of interaural time and level differences to particular sound source locations, and that leads to the development of a ‘space map’ that is custom built for each particular owl.
- Disclaimer: Hermann Wagner is a former collaborator of mine.
Hausmann, L., von Campenhausen, M., Endler, F., Singheiser, M., & Wagner, H. (2009). Improvements of Sound Localization Abilities by the Facial Ruff of the Barn Owl (Tyto alba) as Demonstrated by VirtualRuff Removal PLoS ONE, 4 (11) DOI: 10.1371/journal.pone.0007721