Sound-localization experiments with barn owls in virtual space: Influence of interaural time difference on head-turning behavior

Specific cues in a sound signal are naturally linked to certain parameters in acoustic space. In the barn owl, interaural time difference (ITD) varies mainly with azimuth, while interaural level difference (ILD) varies mainly with elevation. Previous data suggested that ITD is indeed the main cue fo r azimuthal sound localization in this species, while ILD is an important c ue for elevational sound localization. The exact contributions of these par ameters could be tested only indirectly because it was not possible to gene rate a stimulus that contained all relevant spatial information on the one hand, and allowed for a clean separation of these parameters on the other h and. Virtual auditory worlds offer this opportunity: Here we show that barn owls responded to azimuthal variations in virtual spare in the same way as to variations in free-field stimuli. We interpret the increase of turning angle with sound-source azimuths (up to +/- 140 degrees) such that the owls did not experience front/back confusions with virtual stimuli. We then sep arated the influence of ITD from the influence of all other stimulus parame ters by fixing the overall ITD in virtual stimuli to a constant value (+100 mus or +/- 100 mus) while leaving all other sound characteristics unchange d. This manipulation influenced both azimuthal and elevational components o f head arms. Since the owls' azimuthal head-turn amplitude always resembled the value signified by the ITD, these data demonstrated that azimuthal sou nd localization is influenced only: by ITD both in the frontal hemisphere a nd in large parts of the rear hemisphere. ILDs did not have an influence on azimuthal components of head turns. While response latency to normal virtu al stimuli was found to be largely independent of stimulus position, respon se delays of the head turns became longer if tile ITD information pointed i nto a different hemisphere as the other cues of the sounds.