Showing papers by "Hermann Wagner published in 2011"

The three-dimensional shape of serrations at barn owl wings: towards a typical natural serration as a role model for biomimetic applications

Abstract: Barn owl feathers at the leading edge of the wing are equipped with comb-like structures termed serrations on their outer vanes. Each serration is formed by one barb ending that separates and bends upwards. This structure is considered to play a role in air-flow control and noise reduction during flight. Hence, it has considerable potential for engineering applications, particularly in the aviation industry. Several publications have reported possible functions of serrations at artificial airfoils. However, only crude approximations of natural serrations have so far been investigated. We refer to these attempts as zero-order approximations of serrations. It was the goal of this study to present a quantitative three-dimensional characterization of natural serrations as first-order approximations (mean values) and second-order approximations (listed differences depending on the position of the serration along the leading edge). Confocal laser scanning microscopy was used for a three-dimensional reconstruction and investigation with high spatial resolution. Each serration was defined by its length, profile geometry and curvature. Furthermore, the orientation of the serrations at the leading edge was characterized by the inclination angle, the tilt angle and the separation distance of neighboring serrations. These data are discussed with respect to possible applications of serration-like structures for noise suppression and air-flow control.

Disparity sensitivity in man and owl: Psychophysical evidence for equivalent perception of shape-from-stereo.

Abstract: The perception of shape-from-stereo is best characterized by the spatial disparity-contrast sensitivity function (DSF). This is the stereo analogue of the well-known luminance-contrast sensitivity function (CSF). In principle, the DSF and CSF portray a visual system's ability to detect spatial modulation as specified by changes in binocular disparity and luminance, respectively. In humans, less fine detail is visible in the stereo domain than is possible in the luminance domain. Here, we characterize for the first time the DSF in a non-human species, viz. the barn owl. At the same time, we re-examined the human DSF with identical apparatus and methods to directly compare between two vertebrate species that evolved stereovision independently. We discovered a close relationship between the owl and human ability to detect shape-from-stereo. In particular, the shift in absolute position between the human and owl DSF, as measured here, closely corresponds to the shift in absolute position between their respective CSFs, as known from the literature. In conclusion, our study establishes unprecedented experimental proof of a striking similarity in the prowess of humans and owls to achieve shape-from-stereo.

From optics to attention: visual perception in barn owls.

Abstract: Barn owls are nocturnal predators which have evolved specific sensory and morphological adaptations to a life in dim light. Here, some of the most fundamental properties of spatial vision in barn owls are reviewed. The eye with its tubular shape is rigidly integrated in the skull so that eye movements are very much restricted. The eyes are oriented frontally, allowing for a large binocular overlap. Accommodation, but not pupil dilation, is coupled between the two eyes. The retina is rod dominated and lacks a visible fovea. Retinal ganglion cells form a marked region of highest density that extends to a horizontally oriented visual streak. Behavioural visual acuity and contrast sensitivity are poor, although the optical quality of the ocular media is excellent. A low f-number allows high image quality at low light levels. Vernier acuity was found to be a hyperacute percept. Owls have global stereopsis with hyperacute stereo acuity thresholds. Neurons of the visual Wulst are sensitive to binocular disparities. Orientation based saliency was demonstrated in a visual-search experiment, and higher cognitive abilities were shown when the owl's were able to use illusory contours for object discrimination.

The barn owl wing: an inspiration for silent flight in the aviation industry?

Abstract: Barn owls are specialists in prey detection using acoustic information. The flight apparatus of this bird of prey is most efficiently adapted to the hunting behavior by reducing flight noise. An understanding of the underlying mechanisms owls make use of could help minimize the noise disturbances in airport or wind power plant neighborhood. Here, we characterize wings of barn owls in terms of an airfoil as a role model for studying silent flight. This characterization includes surface and edge specialization (serrations, fringes) evolved by the owl. Furthermore, we point towards possible adaptations of either noise suppression or air flow control that might be an inspiration for the construction of modern aircraft. Three-dimensional imaging techniques such as surface digitizing, computed tomography and confocal laser scanning microscopy were used to investigate the wings and feathers in high spatial resolution. We show that wings of barn owls are huge in relation to their body mass resulting in a very low wing loading which in turn enables a slow flight and an increased maneuverability. Profiles of the wing are highly cambered and anteriorly thickened, especially at the proximal wing, leading to high lift production during flight. However, wind tunnel experiments showed that the air flow tends to separate at such wing configurations, especially at low-speed flight. Barn owls compensated this problem by evolving surface and edge modifications that stabilize the air flow. A quantitative three-dimensionally characterization of some of these structures is presented.

Using Virtual Acoustic Space to Investigate Sound Localisation

Abstract: It is an important task for the future to further close the gap between basic and applied science, in other words to make our understanding of the basic principles of auditory processing available for applications in medicine or information technology. Current examples are hearing aids (Dietz et al., 2009) or sound-localising robots (Calmes et al., 2007). This effort will be helped by better quantitative data resulting from more and more sophisticated experimental approaches. Despite new methodologies and techniques, the complex human auditory system is only accessible in a restricted way to many experimental approaches. This gap is closed by animal model systems that allow a more focused analysis of single aspects of auditory processing than human studies. The most commonly used animals in auditory research are birds (barn owls, chicken) and mammals (monkeys, cats, bats, ferrets, guinea pigs, rats and gerbils). When these animals are tested with various auditory stimuli in behavioural experiments, the accuracy (distance of a measured value to the true value) and precision (repeatability of a given measured value) of the animal’s behavioural response allows to draw conclusions on the difficulty with which the animal can use the stimulus to locate sound sources. An example is the measurement of minimum audible angles (MAA) to reveal the resolution threshold of the auditory system for the horizontal displacement of a sound source (Bala et al., 2007). Similarly, one can exploit the head-turn amplitude of humans or animals in response to narrowband or broadband sounds as a measure for the relevance of specific frequency bands, as well as binaural and monaural cues or perception thresholds (e.g. May & Huang, 1995; Poganiatz et al., 2001; Populin, 2006). The barn owl (Tyto alba) is an auditory specialist, depending to a large extent on listening while localising potential prey. In the course of evolution, the barn owl has developed several morphological and neuronal adaptations, which may be regarded as more optimal solutions to problems than the structures and circuits found in generalists. The owl has a characteristic facial ruff, which amplifies sound and is directionally sensitive for frequencies above 4 kHz (Coles & Guppy, 1988). Additionally, the left and right ear openings and flaps are asymmetrically with the left ear lying slightly higher than the right one. This asymmetry creates a steep gradient of interaural level differences (ILDs) in the owl’s frontal field (Campenhausen & Wagner, 2006). These adaptations to sound localisation are one of the reasons why barn owl hearing was established as an important model system during the last decades.