title: Improvements of sound localization abilities by the facial ruff of the barn owl (Tyto alba) as demonstrated by virtual ruff removal.
abstract: BACKGROUND
  When sound arrives at the eardrum it has already been filtered by the body, head, and outer ear. This process is mathematically described by the head-related transfer functions (HRTFs), which are characteristic for the spatial position of a sound source and for the individual ear. HRTFs in the barn owl (Tyto alba) are also shaped by the facial ruff, a specialization that alters interaural time differences (ITD), interaural intensity differences (ILD), and the frequency spectrum of the incoming sound to improve sound localization. Here we created novel stimuli to simulate the removal of the barn owl's ruff in a virtual acoustic environment, thus creating a situation similar to passive listening in other animals, and used these stimuli in behavioral tests.
  
  
  RESULTS
  HRTFs were recorded from an owl before and after removal of the ruff feathers. Normal and ruff-removed conditions were created by filtering broadband noise with the HRTFs. Under normal virtual conditions, no differences in azimuthal head-turning behavior between individualized and non-individualized HRTFs were observed. The owls were able to respond differently to stimuli from the back than to stimuli from the front having the same ITD. By contrast, such a discrimination was not possible after the virtual removal of the ruff. Elevational head-turn angles were (slightly) smaller with non-individualized than with individualized HRTFs. The removal of the ruff resulted in a large decrease in elevational head-turning amplitudes.
  
  
  CONCLUSIONS
  The facial ruff a) improves azimuthal sound localization by increasing the ITD range and b) improves elevational sound localization in the frontal field by introducing a shift of iso-ILD lines out of the midsagittal plane, which causes ILDs to increase with increasing stimulus elevation. The changes at the behavioral level could be related to the changes in the binaural physical parameters that occurred after the virtual removal of the ruff. These data provide new insights into the function of external hearing structures and open up the possibility to apply the results on autonomous agents, creation of virtual auditory environments for humans, or in hearing aids.
authors.affiliation: Institute for Biology II, RWTH Aachen, Aachen, Germany. laura@bio2.rwth-aachen.de
authors.forename: Laura
authors.initials: L
authors.lastname: Hausmann
authors.forename: Mark
authors.initials: M
authors.lastname: von Campenhausen
authors.forename: Frank
authors.initials: F
authors.lastname: Endler
authors.forename: Martin
authors.initials: M
authors.lastname: Singheiser
authors.forename: Hermann
authors.initials: H
authors.lastname: Wagner
mesh_terms: D000161:Acoustic Stimulation; D000818:Animals; D001306:Auditory Pathways; D001307:Auditory Perception; D001522:Behavior, Animal; D004423:Ear; D005241:Feathers / Q000502:physiology*; D006257:Head; D006309:Hearing; D013016:Sound; D013017:Sound Localization; D020305:Strigiformes / Q000502:physiology*; D013997:Time Factors