Interaural time differences are an important cue for azimuthal sound localization. It is still unclear whether the same neuronal mechanisms underlie the representation in the brain of interaural time difference in different vertebrates and whether these mechanisms are driven by common constraints, such as optimal coding. Current sound localization models may be discriminated by studying the spectral distribution of response peaks in tuning curves that measure the sensitivity to interaural time difference. The sound localization system of the barn owl has been studied intensively, but data that would allow discrimination between currently discussed models are missing from this animal. We have therefore obtained extracellular recordings from the time-sensitive subnuclei of the barn owl's inferior colliculus. Response peaks were broadly scattered over the physiological range of interaural time differences. A change in the representation of the interaural phase differences with frequency was not observed. In some neurons, response peaks fell outside the physiological range of interaural time differences. For a considerable number of neurons, the peak closest to zero interaural time difference was not the behaviorally relevant peak. The data are in best accordance with models suggesting that a place code underlies the representation of interaural time difference. The data from the high-frequency range, but not from the low-frequency range, are consistent with predictions of optimal coding. We speculate that the deviation of the representation of interaural time difference from optimal-coding models in the low-frequency range is attributable to the diminished importance of low frequencies for catching prey in this species.

BackgroundWhen 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.Methodology/Principal FindingsHRTFs 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/SignificanceThe 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.

The barn owl, a nocturnal predator, derives its German name ("Schleiereule", direct English translation "veil owl") from the conspicuous ruff that covers the ear openings and gives the head a face-like appearance. The ruff is a specialization for the perception of sound. The densely-ramified reflector feathers forming the border of the ruff direct sound to the ear-openings. We studied the influence of the ruff on the behaviorally relevant sound-localization parameters interaural time difference (ITD) and interaural level difference (ILD). The directionality of the ear was much greater when the ruff was intact than when the reflector feathers were removed. With ruff intact, the distribution of ILDs was oblique and the maximum ITD occurred around 110 degrees of azimuth. When all head feathers were removed, the steepest ILD gradient was much closer to the horizontal axis and ITD was maximal at 90 degrees . Many effects were frequency specific. Thus, the ruff reflects some properties of the human pinna. However, by shifting the point where ITD becomes maximal beyond 90 degrees , the ruff also introduces a break of the front-back symmetry of ITD.

Excessive foaming causes instabilities in fermentation processes, particularly when producing biosurfactants, which can be overcome by intensifying the fermentation via insitu product recovery. A reductive, multi-step approach for selecting organic...