Baseband version of the bat-inspired spectrogram correlation and transformation receiver

Echolocating bats have evolved an excellent ability to detect and discriminate targets in highly challenging environments. They have had more than 50 million years of evolution to optimise their echolocation system with respect to their surrounding environment. Behavioural experiments have shown their exceptional ability to detect and classify targets even in highly cluttered surroundings. The way bats process signals is not exactly the same as in radar and hence it can be useful to investigate the differences. The Spectrogram Correlation And Transformation receiver (SCAT) is an existing model of the bat auditory system that takes into account the physiology and underlying neural organisation in bats which emit chirped signals. In this paper, we propose a baseband receiver equivalent to the SCAT. This will allow biologically inspired signal processing to be applied to radar baseband signals. It will also enable further theoretical analysis of the key concepts, advantages and limitations of the "bat signal processing" for the purpose of target detection, localisation and resolution. The equivalence is demonstrated by comparing the output of the original SCAT to that of our proposed baseband version using both simulated and experimental target echoes. Results show that the baseband receiver provides compatible frequency interference pattern for two closely located scatterers

Dynamic Echo Information Guides Flight in the Big Brown Bat

Animals rely on sensory feedback from their environment to guide locomotion. For instance, visually guided animals use patterns of optic flow to control their velocity and to estimate their distance to objects (e.g. Srinivasan et al. 1991, 1996). In this study, we investigated how acoustic information guides locomotion of animals that use hearing as a primary sensory modality to orient and navigate in the dark, where visual information is unavailable. We studied flight and echolocation behaviors of big brown bats as they flew under infrared illumination through a corridor with walls constructed from a series of individual vertical wooden poles. The spacing between poles on opposite walls of the corridor was experimentally manipulated to create dense/sparse and balanced/imbalanced spatial structure. The bats’ flight trajectories and echolocation signals were recorded with high-speed infrared motion-capture cameras and ultrasound microphones, respectively. As bats flew through the corridor, successive biosonar emissions returned cascades of echoes from the walls of the corridor. The bats flew through the center of the corridor when the pole spacing on opposite walls was balanced and closer to the side with wider pole spacing when opposite walls had an imbalanced density. Moreover, bats produced shorter duration echolocation calls when they flew through corridors with smaller spacing between poles, suggesting that clutter density influences features of the bat’s sonar signals. Flight speed and echolocation call rate did not, however, vary with dense and sparse spacing between the poles forming the corridor walls. Overall, these data demonstrate that bats adapt their flight and echolocation behavior dynamically when flying through acoustically complex environments

Bio-inspired two target resolution at radio frequencies

Echolocating bats show a unique ability to detect, resolve and discriminate targets. The Spectrogram Correlation and Transformation (SCAT) receiver is a model of the Eptesicus fuscus auditory system that presents key signal processing differences compared to radar which may offer useful lessons for improvement. A baseband version of the SCAT is used to investigate advantages and disadvantages of bat-like signal processing against the task of target resolution. The baseband receiver is applied to RF experimental data and results show higher range resolution than the reciprocal of the transmitted bandwidth can be achieved for two closely spaced scatterers

Coding of spatial and temporal frequency in bat biosonar

Fledermäuse senden Ultraschallrufe aus und lauschen auf Echos um sich in ihrer Umgebung zu orientieren und Beute zu jagen. Dank dieser Fähigkeit zur Echoortung sowie zum aktiven Flug haben sich Fledermäuse eine überaus ergiebige ökologische Nische erschlossen, den nächtlichen Luftraum. Ihr "sechster Sinn" hat Fledermäusen also Unabhängigkeit vom Sonnenlicht beschert. Aber inwiefern kann Hören Sehen ersetzen? Die vorliegende Arbeit beschäftigt sich mit der Frage wie Echoortung bestimmte räumliche und zeitliche Parameter der Umgebung verarbeitet. Wenn es um die Wahrnehmung räumlicher Strukturen geht, stehen echoortende Tiere vor einer speziellen Herausforderung. Die Cochlea, das Sinnesepithel des Hör-systems, kann Rauminformation nicht direkt kodieren. Stattdessen muss Rauminformation errechnet werden, über den Vergleich der Signale an beiden Ohren. Im ersten Kapitel dieser Dissertation teste ich die Hypothese, dass Echoortung dennoch Raumfrequenzen heranzieht um ein Bild der Umgebung zu formen. Das Konzept der Raumfrequenz spielte eine entscheidende Rolle in unserem Verständnis von visueller Wahrnehmung. In der vorliegenden Arbeit zeige ich, dass trotz grundlegender mechanistischer Unterschiede zwischen Seh- und und Hörvermögen beide Sinnessysteme Zugang zu Raumfrequenzinformation haben. Sechs Fledermäuse (Phyllostomus discolor) wurden darauf andressiert, eine Oberfläche mit Wellen unter-schiedlicher Raumfrequenz und Tiefe von einer glatten Oberfläche zu unterscheiden. Meine Messungen zeigen dass Fledermäuse viel empfindlicher gegenüber hohen Raumfrequenzen sind als gegenüber niedrigen Raumfrequenzen, d.h. einen sensorischen Hochpassfilter für Raumfrequenz besitzen. Zusätzlich untersuchte ich welche sensorischen Reize den Fledermäusen zur Verfügung stehen um Raumfrequenz zu bewerten. Ich fand heraus, dass diese Reize sich grundlegend von solchen unterscheiden, welche die visuelle Wahrnehmung von Raumfrequenz vermitteln. Während visuelle Raumfrequenzwahrnehmung das Ergebnis feinabge¬stimmter räumlicher Empfindlichkeit der Retinazellen ist, wird Raumfrequenz¬wahrnehmung mit Echoortung durch objektspezifische Reflektionseigenschaften erreicht. Der Nachweis eines Hochpassfilters in der Echoortung von Fledermäusen offenbart funktionelle Gemeinsamkeiten zwischen Sehen und Echoortung, die beiden Systemen Zugang zum Raumprofil der Umgebung ermöglichen und damit der Figur-Grund-Wahrnehmung zugrunde liegen. Diese funktionellen Gemeinsamkeiten, aber mechanistischen Unterschiede machen deutlich, dass ein Sinnessystem-übergreifender Bedarf an räumlicher Umgebungsinformation besteht. Das Gehör brilliert in der Messung winziger Laufzeitunterschiede. Doch wenn es darum geht zeitlichen Änderungen von Echoparametern zu folgen, scheint das Echo-ortungssystem einer typischen Fledermaus im Nachteil. Der Ortungsruf einer frequenzmodulierenden Fledermaus ist zu kurz um einen kompletten Bewegungs¬zyklus abzubilden. Um Bewegung nachzuverfolgen müssen Fledermäuse die Laufzeit-unterschiede ganzer Sequenzen von Ruf-Echo-Paaren vergleichen. Im zweiten und dritten Kapitel der vorliegenden Arbeit quantifizierte ich die Empfindlichkeit von Fledermaus¬echoortung für zeitliche Modulationen verschiedener Echoparameter. Schlagende Insektenflügel erzeugen natürliche Echomodulationen, und zwar gleichzeitig Modulationen von Laufzeit und Lautstärke. Im zweiten Kapitel führe ich eine Methode ein, mit deren Hilfe sich Laufzeit und Lautstärke der Echos unabhängig voneinander manipulieren lassen. Eine akustische virtuelle Realität ermöglicht die separate Untersuchung der Effekte des jeweiligen Parameters auf die Wahrnehmung der Fledermaus. Ich zeige, dass bei der frequenz-modulierenden Fledermaus P. discolor die Empfindlichkeit für Modulationen der Echolaufzeit stark von der Modulationsrate abhängt. Am empfindlichsten waren die Tiere bei Modulationsraten unter 20 Hz und über 50 Hz. Ich zeige, dass Echoortung für Wechselwirkungen zwischen Modulationsrate und Rufrate anfällig ist, ein Phänomen, das ich als einen echoakustischen Wagenradeffekt bezeichne. Weiter zeige ich, dass bei hohen Modulationsraten Doppler-Verzerrungen zusätzliche spektrale und zeitliche Reize herbeiführen, was den Wiederanstieg der Empfindlichkeit bei hohen Modulationsraten erklären kann. Die bedeutet, dass für die weltweit hunderten Arten frequenzmodulierender Fledermäuse Doppler-Verzerrungen eine wichtige Rolle bei der Flügelschlagwahrnehmung spielen könnten. Im dritten Kapitel vertiefe ich meine Untersuchungen zum Thema Empfindlichkeit von Echoortung gegenüber Echomodulationen. Mit Hilfe der virtuellen Realität moduliere ich die Echolautstärke unabhängig von der Echolaufzeit. Ich kann zeigen, dass P. discolor diese Lautstärkemodulationen wahrnehmen kann und dass die Detektionsleistung der Tiere mit der Modulationsrate ansteigt. Ich führe an, dass sich die Wahrnehmung von Lautstärkemodulationen mit Echoortung grundlegend von der Wahrnehmung von Laufzeitmodulationen unterscheidet. Weiter spekuliere ich, dass der Wahrnehmung schneller Lautstärkemodulationen spektrale Reize zu Grunde liegen. In ihrer Gesamtheit liefert die vorliegende Arbeit experimentelle Nachweise zu wichtigen perzeptorischen Prozessen in der Echoortung frequenzmodulierender Fledermäuse. Meine Erkenntnisse zeigen eine Möglichkeit auf, wie Fledermäuse dem vermeintlich unumgänglichen Kompromiss zwischen räumlichem und zeitlichem Auflösungsvermögen entgehen könnten. Damit stelle ich eine Alternative zur traditionellen Sichtweise, dass die sensorischen Einschränkungen des Gehörs automatisch zu geringerer Leistungsfähigkeit führen. Ich lege dar wie divers die Selektionsfaktoren sind, die auf das Echoortungssystem von Fledermäusen einwirken. Diese Dissertation nimmt daher Einfluss auf die Forschungsbereiche Neuroethologie, Verhaltensökologie, Tierphysiologie und Evolution, und kann zur Weiterentwicklung technischen Sonars beitragen.Bats emit ultrasonic cries and listen to the reflected sounds to orient and forage in their environment. The rich ecological niche of nocturnal air space became accessible through bats’ capability of sustained flight and echolocation. Their “sixth sense” gained them autonomy from sunlight, but to what extent can hearing replace vision? This thesis addresses the question how echolocation encodes certain spatial and temporal parameters of the environment. Echolocation poses a challenge to the perception of spatial layouts because the auditory sensory epithelium, the cochlea, does not explicitly encode space like the eye’s retina does; space must be computed by comparing echo cues at both ears. In the first chapter of this thesis, I test the hypothesis that despite this challenge, bat echolocation utilizes the concept of spatial frequency to form perceptual representations of bats’ habitat. Spatial frequency has been crucial to understand visual perception. I show that both sensory systems, echolocation and vision, have access to spatial frequency information despite their fundamental mechanistic differences. I trained six bats (Phyllostomus discolor) to discriminate ripples of different spatial frequencies from a smooth surface and measured echo-acoustic depth-contrast-sensitivity functions. I show that bats are much more sensitive to high spatial frequencies, exemplifying a spatial high-pass filter. Additionally, I evaluated the perceptual cues available to the bats to assess spatial frequency and found them fundamentally different from those in vision. While spatial frequency perception in vision is a result of spatial tuning, starting already in the retina, spatial frequency perception in echolocation is achieved by object-specific reflection properties that determine the perceived echo-acoustic object signature. The demonstration of a high-pass filter in bat echolocation reveals a functional similarity between vision and echolocation, which underlies figure-ground-separation and allows both systems access to the spatial contours in the environment. The functional similarities, yet mechanistic differences, highlight the need for spatial environmental information, independent of sensory system. The auditory system excels in measuring minute differences in echo arrival times. But when it comes to the tracking of changes of echo properties over time, the echolocation system of a typical bat seems to be at a disadvantage. The echolocation call of frequency-modulating bats is too short to track an entire movement cycle. In order to track movement, bats have to compare memorised sequences of call-echo pairs. In the second and third chapters, I quantified the sensitivity of bat echolocation to the temporal modulation of echo parameters. In nature, fluttering insect wings cause echo modulations; the echoes carry modulations in echo delay and in echo amplitude simultaneously. In the second chapter, I introduce an auditory virtual reality where I can manipulate delay independently from amplitude and tease apart the effects of both parameters on perception. I demonstrate that in the frequency-modulating bat Phyllostomus discolor the sensitivity for modulations in echo delay depends on the rate of the modulation, with bats being most sensitive at modulation rates below 20 Hz and above 50 Hz. I show that echolocation is susceptible to interference between call repetition rate and modulation rate. I propose that this phenomenon constitutes an echo-acoustic wagon-wheel effect. I further demonstrate how at high modulation rates sensitivity could be rescued by using spectral and temporal cues introduced by Doppler-distortions. Thus, I present evidence that Doppler distortions may play a crucial role in flutter sensitivity in the hundreds of frequency-modulating bat species worldwide. In the third chapter, I deepen my investigations into the sensitivity of bat echolocation to temporal echo modulations. I use the virtual reality approach to generate modulations in echo amplitude independent from echo delay. I show that Phyllostomus discolor successfully detected these modulations in echo amplitude and that their performance increased with the rate of the modulation. I suggest that amplitude-modulation detection with echolocation differs fundamentally from delay-modulation detection and speculate that the mechanism to detect fast amplitude modulations relies on spectral cues. In summary, this thesis provides experimental evidence on important perceptual processes in the echolocation of frequency-modulating bats. I give a proof-of-principle demonstration offering release from the supposed trade-off between temporal and spatial acuity and challenging the view that the auditory system’s sensory constraints inevitably lead to detrimental echo-acoustic performance. Thereby, my findings highlight the diversity of selective pressures working on the echolocation system of bats. This thesis therefore has implications on the fields of neuroethology, behavioural ecology, animal physiology and evolution, and may contribute to the further development of technical sonar

Coding of spatial and temporal frequency in bat biosonar

Fledermäuse senden Ultraschallrufe aus und lauschen auf Echos um sich in ihrer Umgebung zu orientieren und Beute zu jagen. Dank dieser Fähigkeit zur Echoortung sowie zum aktiven Flug haben sich Fledermäuse eine überaus ergiebige ökologische Nische erschlossen, den nächtlichen Luftraum. Ihr "sechster Sinn" hat Fledermäusen also Unabhängigkeit vom Sonnenlicht beschert. Aber inwiefern kann Hören Sehen ersetzen? Die vorliegende Arbeit beschäftigt sich mit der Frage wie Echoortung bestimmte räumliche und zeitliche Parameter der Umgebung verarbeitet. Wenn es um die Wahrnehmung räumlicher Strukturen geht, stehen echoortende Tiere vor einer speziellen Herausforderung. Die Cochlea, das Sinnesepithel des Hör-systems, kann Rauminformation nicht direkt kodieren. Stattdessen muss Rauminformation errechnet werden, über den Vergleich der Signale an beiden Ohren. Im ersten Kapitel dieser Dissertation teste ich die Hypothese, dass Echoortung dennoch Raumfrequenzen heranzieht um ein Bild der Umgebung zu formen. Das Konzept der Raumfrequenz spielte eine entscheidende Rolle in unserem Verständnis von visueller Wahrnehmung. In der vorliegenden Arbeit zeige ich, dass trotz grundlegender mechanistischer Unterschiede zwischen Seh- und und Hörvermögen beide Sinnessysteme Zugang zu Raumfrequenzinformation haben. Sechs Fledermäuse (Phyllostomus discolor) wurden darauf andressiert, eine Oberfläche mit Wellen unter-schiedlicher Raumfrequenz und Tiefe von einer glatten Oberfläche zu unterscheiden. Meine Messungen zeigen dass Fledermäuse viel empfindlicher gegenüber hohen Raumfrequenzen sind als gegenüber niedrigen Raumfrequenzen, d.h. einen sensorischen Hochpassfilter für Raumfrequenz besitzen. Zusätzlich untersuchte ich welche sensorischen Reize den Fledermäusen zur Verfügung stehen um Raumfrequenz zu bewerten. Ich fand heraus, dass diese Reize sich grundlegend von solchen unterscheiden, welche die visuelle Wahrnehmung von Raumfrequenz vermitteln. Während visuelle Raumfrequenzwahrnehmung das Ergebnis feinabge¬stimmter räumlicher Empfindlichkeit der Retinazellen ist, wird Raumfrequenz¬wahrnehmung mit Echoortung durch objektspezifische Reflektionseigenschaften erreicht. Der Nachweis eines Hochpassfilters in der Echoortung von Fledermäusen offenbart funktionelle Gemeinsamkeiten zwischen Sehen und Echoortung, die beiden Systemen Zugang zum Raumprofil der Umgebung ermöglichen und damit der Figur-Grund-Wahrnehmung zugrunde liegen. Diese funktionellen Gemeinsamkeiten, aber mechanistischen Unterschiede machen deutlich, dass ein Sinnessystem-übergreifender Bedarf an räumlicher Umgebungsinformation besteht. Das Gehör brilliert in der Messung winziger Laufzeitunterschiede. Doch wenn es darum geht zeitlichen Änderungen von Echoparametern zu folgen, scheint das Echo-ortungssystem einer typischen Fledermaus im Nachteil. Der Ortungsruf einer frequenzmodulierenden Fledermaus ist zu kurz um einen kompletten Bewegungs¬zyklus abzubilden. Um Bewegung nachzuverfolgen müssen Fledermäuse die Laufzeit-unterschiede ganzer Sequenzen von Ruf-Echo-Paaren vergleichen. Im zweiten und dritten Kapitel der vorliegenden Arbeit quantifizierte ich die Empfindlichkeit von Fledermaus¬echoortung für zeitliche Modulationen verschiedener Echoparameter. Schlagende Insektenflügel erzeugen natürliche Echomodulationen, und zwar gleichzeitig Modulationen von Laufzeit und Lautstärke. Im zweiten Kapitel führe ich eine Methode ein, mit deren Hilfe sich Laufzeit und Lautstärke der Echos unabhängig voneinander manipulieren lassen. Eine akustische virtuelle Realität ermöglicht die separate Untersuchung der Effekte des jeweiligen Parameters auf die Wahrnehmung der Fledermaus. Ich zeige, dass bei der frequenz-modulierenden Fledermaus P. discolor die Empfindlichkeit für Modulationen der Echolaufzeit stark von der Modulationsrate abhängt. Am empfindlichsten waren die Tiere bei Modulationsraten unter 20 Hz und über 50 Hz. Ich zeige, dass Echoortung für Wechselwirkungen zwischen Modulationsrate und Rufrate anfällig ist, ein Phänomen, das ich als einen echoakustischen Wagenradeffekt bezeichne. Weiter zeige ich, dass bei hohen Modulationsraten Doppler-Verzerrungen zusätzliche spektrale und zeitliche Reize herbeiführen, was den Wiederanstieg der Empfindlichkeit bei hohen Modulationsraten erklären kann. Die bedeutet, dass für die weltweit hunderten Arten frequenzmodulierender Fledermäuse Doppler-Verzerrungen eine wichtige Rolle bei der Flügelschlagwahrnehmung spielen könnten. Im dritten Kapitel vertiefe ich meine Untersuchungen zum Thema Empfindlichkeit von Echoortung gegenüber Echomodulationen. Mit Hilfe der virtuellen Realität moduliere ich die Echolautstärke unabhängig von der Echolaufzeit. Ich kann zeigen, dass P. discolor diese Lautstärkemodulationen wahrnehmen kann und dass die Detektionsleistung der Tiere mit der Modulationsrate ansteigt. Ich führe an, dass sich die Wahrnehmung von Lautstärkemodulationen mit Echoortung grundlegend von der Wahrnehmung von Laufzeitmodulationen unterscheidet. Weiter spekuliere ich, dass der Wahrnehmung schneller Lautstärkemodulationen spektrale Reize zu Grunde liegen. In ihrer Gesamtheit liefert die vorliegende Arbeit experimentelle Nachweise zu wichtigen perzeptorischen Prozessen in der Echoortung frequenzmodulierender Fledermäuse. Meine Erkenntnisse zeigen eine Möglichkeit auf, wie Fledermäuse dem vermeintlich unumgänglichen Kompromiss zwischen räumlichem und zeitlichem Auflösungsvermögen entgehen könnten. Damit stelle ich eine Alternative zur traditionellen Sichtweise, dass die sensorischen Einschränkungen des Gehörs automatisch zu geringerer Leistungsfähigkeit führen. Ich lege dar wie divers die Selektionsfaktoren sind, die auf das Echoortungssystem von Fledermäusen einwirken. Diese Dissertation nimmt daher Einfluss auf die Forschungsbereiche Neuroethologie, Verhaltensökologie, Tierphysiologie und Evolution, und kann zur Weiterentwicklung technischen Sonars beitragen.Bats emit ultrasonic cries and listen to the reflected sounds to orient and forage in their environment. The rich ecological niche of nocturnal air space became accessible through bats’ capability of sustained flight and echolocation. Their “sixth sense” gained them autonomy from sunlight, but to what extent can hearing replace vision? This thesis addresses the question how echolocation encodes certain spatial and temporal parameters of the environment. Echolocation poses a challenge to the perception of spatial layouts because the auditory sensory epithelium, the cochlea, does not explicitly encode space like the eye’s retina does; space must be computed by comparing echo cues at both ears. In the first chapter of this thesis, I test the hypothesis that despite this challenge, bat echolocation utilizes the concept of spatial frequency to form perceptual representations of bats’ habitat. Spatial frequency has been crucial to understand visual perception. I show that both sensory systems, echolocation and vision, have access to spatial frequency information despite their fundamental mechanistic differences. I trained six bats (Phyllostomus discolor) to discriminate ripples of different spatial frequencies from a smooth surface and measured echo-acoustic depth-contrast-sensitivity functions. I show that bats are much more sensitive to high spatial frequencies, exemplifying a spatial high-pass filter. Additionally, I evaluated the perceptual cues available to the bats to assess spatial frequency and found them fundamentally different from those in vision. While spatial frequency perception in vision is a result of spatial tuning, starting already in the retina, spatial frequency perception in echolocation is achieved by object-specific reflection properties that determine the perceived echo-acoustic object signature. The demonstration of a high-pass filter in bat echolocation reveals a functional similarity between vision and echolocation, which underlies figure-ground-separation and allows both systems access to the spatial contours in the environment. The functional similarities, yet mechanistic differences, highlight the need for spatial environmental information, independent of sensory system. The auditory system excels in measuring minute differences in echo arrival times. But when it comes to the tracking of changes of echo properties over time, the echolocation system of a typical bat seems to be at a disadvantage. The echolocation call of frequency-modulating bats is too short to track an entire movement cycle. In order to track movement, bats have to compare memorised sequences of call-echo pairs. In the second and third chapters, I quantified the sensitivity of bat echolocation to the temporal modulation of echo parameters. In nature, fluttering insect wings cause echo modulations; the echoes carry modulations in echo delay and in echo amplitude simultaneously. In the second chapter, I introduce an auditory virtual reality where I can manipulate delay independently from amplitude and tease apart the effects of both parameters on perception. I demonstrate that in the frequency-modulating bat Phyllostomus discolor the sensitivity for modulations in echo delay depends on the rate of the modulation, with bats being most sensitive at modulation rates below 20 Hz and above 50 Hz. I show that echolocation is susceptible to interference between call repetition rate and modulation rate. I propose that this phenomenon constitutes an echo-acoustic wagon-wheel effect. I further demonstrate how at high modulation rates sensitivity could be rescued by using spectral and temporal cues introduced by Doppler-distortions. Thus, I present evidence that Doppler distortions may play a crucial role in flutter sensitivity in the hundreds of frequency-modulating bat species worldwide. In the third chapter, I deepen my investigations into the sensitivity of bat echolocation to temporal echo modulations. I use the virtual reality approach to generate modulations in echo amplitude independent from echo delay. I show that Phyllostomus discolor successfully detected these modulations in echo amplitude and that their performance increased with the rate of the modulation. I suggest that amplitude-modulation detection with echolocation differs fundamentally from delay-modulation detection and speculate that the mechanism to detect fast amplitude modulations relies on spectral cues. In summary, this thesis provides experimental evidence on important perceptual processes in the echolocation of frequency-modulating bats. I give a proof-of-principle demonstration offering release from the supposed trade-off between temporal and spatial acuity and challenging the view that the auditory system’s sensory constraints inevitably lead to detrimental echo-acoustic performance. Thereby, my findings highlight the diversity of selective pressures working on the echolocation system of bats. This thesis therefore has implications on the fields of neuroethology, behavioural ecology, animal physiology and evolution, and may contribute to the further development of technical sonar

Bio-inspired processing of radar target echoes

Echolocating bats have evolved the ability to detect, resolve and discriminate targets in highly challenging environments using biological sonar. The way bats process signals in the receiving auditory system is not the same as that of radar and sonar and hence investigating differences and similarities might provide useful lessons to improve synthetic sensors. The Spectrogram Correlation And Transformation (SCAT) receiver is an existing model of the bat auditory system that takes into account the physiology and the neural organisation of bats that emit broadband signals. In this study, the authors present a baseband receiver equivalent to the SCAT that allows an analysis of target echoes at baseband. The baseband SCAT (BSCT) is used to investigate the output of the bat-auditory model for two closely spaced scatterers and to carry out an analysis of range resolution performance and a comparison with the conventional matched filter. Results firstly show that the BSCT provides improved resolution performance. It is then demonstrated that the output of the BSCT can be obtained with an equivalent matched-filter based receiver. The results are verified with a set of laboratory experiments at radio frequencies in a high signal-to-noise ratio

Acoustic scattering of broadband echolocation signals from prey of Blainville's beaked whales : modeling and analysis

Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2006Blainville's beaked whales (Mesoplodon densirostris) use broadband, ultrasonic echolocation signals (27 to 57 kHz) to search for, localize, and approach prey that generally consist of mid-water and deep-water fishes and squid. Although it is well known that the spectral characteristics of broadband echoes from marine organisms are a strong function of size, shape, orientation and anatomical group, little is known as to whether or not these or other toothed whales use spectral cues in discriminating between prey and non-prey. In order to study the prey-classification process, a stereo acoustic tag was mounted on a Blainville's beaked whale so that emitted clicks and corresponding echoes from prey could be recorded. A comparison of echoes from prey selected by the whale and those from randomly chosen scatterers suggests that the whale may have, indeed, discriminated between echoes using spectral features and target strengths. Specifically, the whale appears to have favored prey with one or more deep nulls in the echo spectra as well as ones with higher target strength. A three-dimensional, acoustic scattering model is also developed to simulate broadband scattering from squid, a likely prey of the beaked whale. This model applies the distorted wave Born approximation (DWBA) to a weakly-scattering, inhomogeneous body using a combined ray trace and volume integration approach. Scatterer features are represented with volume elements that are small (less than 1/12th of the wavelength) for the frequency range of interest (0 to 120 kHz). Ranges of validity with respect to material properties and numerical considerations are explored using benchmark computations with simpler geometries such as fluid-filled spherical and cylindrical fluid shells. Modeling predictions are compared with published data from live, freely swimming squid. These results, as well as previously published studies, are used in the analysis of the echo spectra of the whale's ensonified targets

Insight on how fishing bats discern prey and adjust their mechanic and sensorial features during the attack sequence

Several insectivorous bats have included fish in their diet, yet little is known about the processes underlying this trophic shift. We performed three field experiments with wild fishing bats to address how they manage to discern fish from insects and adapt their hunting technique to capture fish. We show that bats react only to targets protruding above the water and discern fish from insects based on prey disappearance patterns. Stationary fish trigger short and shallow dips and a terminal echolocation pattern with an important component of the narrowband and low frequency calls. When the fish disappears during the attack process, bats regulate their attack increasing the number of broadband and high frequency calls in the last phase of the echolocation as well as by lengthening and deepening their dips. These adjustments may allow bats to obtain more valuable sensorial information and to perform dips adjusted to the level of uncertainty on the location of the submerged prey. The observed ultrafast regulation may be essential for enabling fishing to become cost-effective in bats, and demonstrates the ability of bats to rapidly modify and synchronise their sensorial and motor features as a response to last minute stimulus variations.This study was part of the Ministerio de Ciencia e Innovacion (MICINN) project CGL2009-12393. The University of The Basque Country (UPV/EHU) (INF09/15) and the Basque Government (IT385-07 and IT301-10) funded this study and provided grant support to O.A. and A.A (BFI-2009-252, BFI-2010-190, Doktore berriak eta Ikertzaile doktoreak espezializatzeko kontratatzeko laguntzak)

Sound use, sequential behavior and ecology of foraging bottlenose dolphins, Tursiops truncatus

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1999Odontocetes are assumed to use echolocation for navigation and foraging, but neither of these uses of biosonar has been conclusively demonstrated in free-ranging animals. Many bats are known to use echolocation throughout foraging sequences, changing the structure and timing of clicks as they progress towards prey capture. For odontocetes, however, we do not know enough about their foraging behavior to describe such sequences. To conduct detailed behavioral observations of any subject animal, the observer must be able to maintain continuous visual contact with the subject for a period commensurate with the duration of the behavior(s) of interest. Behavioral studies of cetaceans, which spend approximately 95% of their time below the water's surface, have been limited to sampling surface behavior except in special circumstances, e.g. clear-water environments, or with the use of technological tools. I addressed this limitation through development of an observation platform consisting of a remote controlled video camera suspended from a tethered airship with boat-based monitoring, adjustment, and recording of video. The system was used successfully to conduct continuous behavioral observations of bottlenose dolphins in the Sarasota Bay, FL area. This system allowed me to describe previously unreported foraging behaviors and elucidate functions for behaviors already defined but poorly understood. Dolphin foraging was modeled as a stage-structured sequence of behaviors, with the goal-directed feeding event occurring at the end of a series of search, encounter, and pursuit behaviors. The behaviors preceding a feeding event do not occur in a deterministic sequence, but are adaptive and plastic. A single-step transition analysis beginning with prey capture and receding in time has identified significant links between observed behaviors and demonstrated the stage-structured nature of dolphin foraging. Factors affecting the occurrence of specific behaviors and behavioral transitions include mesoscale habitat variation and individual preferences. The role of sound in foraging, especially echolocation, is less well understood than the behavioral component. Recent studies have explored the use of echolocation in captive odontocete foraging and presumed feeding in wild animals, but simultaneous, detailed behavioral and acoustic observations have eluded researchers. The current study used two methods to obtain acoustic data. The overhead video system includes two towed hydrophones used to record 'ambient' sounds of dolphin foraging. The recordings are of the 'ambient' sounds because the source of the sounds, i.e. animal, could not be localized. Many focal follows, however, were conducted with single animals, and from these records the timing of echolocation and other sounds relative to the foraging sequence could be examined. The 'ambient' recordings revealed that single animals are much more vocal than animals in groups, both overall and during foraging. When not foraging, single animals vocalized at a rate similar to the per animal rate in groups of ≥2 animals. For single foraging animals, the use of different sound types varies significantly by the habitat in which the animal is foraging. These patterns of use coupled with the characteristics of the different sound types suggest specific functions for each. The presence of multiple animals in a foraging group apparently reduces the need to vocalize, and potential reasons for this pattern are discussed. In addition, the increased vocal activity of single foraging animals lends support to specific hypotheses of sound use in bottlenose dolphins and odontocetes in general. The second acoustic data collection method records sounds known to be from a specific animal. An acoustic recording tag was developed that records all sounds produced by an animal including every echolocation click. The tag also includes an acoustic sampling interval controller and a sensor suite that measures pitch, roll, heading, and surfacing events. While no foraging events occurred while an animal was wearing an acoustic data logger, the rates of echolocation and whistling during different activities, e.g. traveling, were measured.This work was supported by the Education Office of the Woods Hole Oceanographic Institution, two grants from the Rinehart Coastal Research Center, the Ocean Ventures Fund; WHOI Sea Grant, ONR Grant #N00014-94-1-0692 to P. Tyack, and a Graduate Fellowship from the Office of Naval Research