Bat azimuthal echolocation using interaural level differences: modeling and implementation by a VLSI-based hardware system

Bats have long fascinated both scientists and engineers due to their superb ability to use echolocation to fly with speed and agility through complex natural environments in complete darkness. This dissertation presents a neuromorphic VLSI circuit model of bat azimuthal echolocation. Interaural level differences (ILDs) are the cues for bat azimuthal echolocation and are also the primary cues used by other mammals to localize high frequency sounds. The fact that neurons in bats respond to short echoes by one or two spikes strongly suggests that the conventionally used firing rate is an unlikely code. The operation of first spike latency in ILD computation and transformation is investigated in a network of spiking neurons linking the lateral superior olive (LSO), dorsal nucleus of the lateral lemniscus (DNLL), and inferior colliculus (IC). The results of the investigation suggest that spatially distributed first spike latencies can serve as a fast code for azimuth that can be ``read-out'' by ascending stages. With the hardware echolocation model that uses spike timing representation, we study how multiple echoes can affect bat echolocation and demonstrate that the response to multiple sounds is not a simple linear addition of the response to single sounds. By developing functional models of the bat echolocation system, we can study the efficient implementation demonstrated by nature. For example, variations among analog VLSI circuit units due to the unavoidable transistor mismatch - traditionally thought of as a hurdle to overcome - have been found beneficial in generating the desired diversity of response that is similar to their neural counterparts. This work advocates the use and design of summating and exponentially decaying synapses. A compact and easily controllable synapse circuit has found an application in achieving a linear temporal spike summation by operating with a very short time constants. It has also been applied in modeling a nonlinear intensity-latency trading by working with a long synaptic time constant. We propose a new synapse circuit model that is compatible with those used in computational models and implementable by CMOS transistors operating in the subthreshold region

Acoustic modelling of bat pinnae utilising the TLM method

This thesis describes the numerical modelling of bioacoustic structures, the focus being the outer ear or pinnae of the Rufous Horseshoe bat (Rhinolophus rouxii). There have been several novel developments derived from this work including: • A method of calculating directionality based on the sphere with a distribution of measuring points such that each lies in an equal area segment. • Performance estimation of the pinna by considering the directionality of an equivalent radiating aperture. • A simple synthetic geometry that appears to give similar performance to a bat pinna. The outcome of applying the methods have yielded results that agree with measurements, indeed, this work is the first time TLM has been applied to a structure of this kind. It paves the way towards a greater understanding of bioacoustics and ultimately towards generating synthetic structures that can perform as well as those found in the natural world

NEUROMORPHIC VLSI REALIZATION OF THE HIPPOCAMPAL FORMATION AND THE LATERAL SUPERIOR OLIVE

In this work, the focus is on realizing the function of the hippocampal formation (HF) and the lateral superior olive (LSO) in electronic circuits. The first major contribution of this dissertation is to realize the function of the HF in silicon. This was based on the GRIDSmap model and the Bayesian integration. For this, two novel circuits were designed and integrated with others. The first circuit was that of a Bayesian integration synapse which can perform Bayesian integration at the single neuron level. The second circuit was that of a velocity integrator which is so compact that it can enable integration of the entire system on a single chip compared to its predecessors which would have needed 27 chips! However, since the computational neuroscience models of the hippocampal place cells do not explain all the characteristics observed empirically, a novel model for the place cells, based on the sensori-motor integration of inputs is proposed. This is the second major contribution of this thesis. The third major contribution is to demonstrate a VLSI system which can perform azimuthal localization based on population response of the LSO. This system was based on the Reed and Blum's model of the LSO. For this, a novel circuit of a second order synapse and that of a conductance neuron was designed and integrated with other circuits. This synapse circuit can produce an output current whose peak is delayed and is proportional to the number of inputs it receives. The HF is thought to aid in spatial navigation and the LSO is thought to be involved in azimuthal localization of sounds both of which are useful for autonomous robotic spatial navigation. Hence, silicon realization of these two will be useful in robotics which is an area of interest for the neuromorphic engineers

ORIENTING IN 3D SPACE: BEHAVIORAL AND NEUROPHYSIOLOGICAL STUDIES IN BIG BROWN BATS

In their natural environment, animals engage in a wide range of behavioral tasks that require them to orient to stimuli in three-dimensional space, such as navigating around obstacles, reaching for objects and escaping from predators. Echolocating bats, for example, have evolved a high-resolution 3D acoustic orienting system that allows them to localize and track small moving targets in azimuth, elevation and range. The bat’s active control over the features of its echolocation signals contributes directly to the information represented in its sonar receiver, and its adaptive adjustments in sonar signal design provide a window into the acoustic features that are important for different behavioral tasks. When bats inspect sonar objects and require accurate 3D localization of targets, they produce sonar sound groups (SSGs), which are clusters of sonar calls produced at short intervals and flanked by long interval calls. SSGs are hypothesized to enhance the bat’s range resolution, but this hypothesis has not been directly tested. We first, in Chapter 2, provide a comprehensive comparison of SSG production of bats flying in the field and in the lab under different environmental conditions. Further, in Chapter 3, we devise an experiment to specifically compare SSG production under conditions when target motion is predictable and unpredictable, with the latter mimicking natural conditions where bats chase erratically moving prey. Data from both of these studies are consistent with the hypothesis that SSGs improve the bat’s spatio-temporal resolution of target range, and provide a behavioral foundation for the analysis and interpretation of neural recording data in chapters 4 and 6. The complex orienting behaviors exhibited by animals can be understood as a feedback loop between sensing and action. A primary brain structure involved in sensorimotor integration is the midbrain superior colliculus (SC). The SC is a widely studied brain region and has been implicated in species-specific orienting behaviors. However, most studies of the SC have investigated its functional organization using synthetic 2D (azimuth and elevation) stimuli in restrained animals, leaving gaps in our knowledge of how 3D space (azimuth, elevation and distance) is represented in the CNS. In contrast, the representation of stimulus distance in the auditory systems of bats has been widely studied. Almost all of these studies have been conducted in passively listening bats, thus severing the loop between sensing and action and leaving gaps in our knowledge regarding how target distance is represented in the auditory system of actively echolocating bats. In chapters 4, 5 and 6, we attempt to fill gaps in our knowledge by recording from the SC of free flying echolocating bats engaged in a naturalistic navigation task where bats produce SSGs. In chapter 4, we provide a framework to compute time-of-arrival and direction of the instantaneous echo stimuli received at the bats ears. In chapters 5 and 6, we provide an algorithm to classify neural activity in the SC as sensory, sensorimotor and premotor and then compute spatial receptive fields of SC neurons. Our results show that neurons in the SC of the free-flying echolocating bat respond selectively to stimulus azimuth, elevation and range. Importantly, we find that SC neuron response profiles are modulated by the bat’s behavioral state, indicated by the production of SSG. Broadly, we use both behavior and electrophysiology to understand the action-perception loop that supports spatial orientation by echolocation. We believe that the results and methodological advances presented here will open doors to further studies of sensorimotor integration in freely behaving animals

Sonar Beam Direction and Flight Control in an Echolocating Bat

Echolocating insectivorous bats are nocturnal mammals that capture fast, erratically moving insects in flight. Bats emit short ultrasonic pulses that form beams of sound and use the returning echoes to guide behavior. The frequency, duration and timing of the sonar pulses, along with the spatial direction of the sonar beam restrict the information returning to the bat, and can be considered a component of the acoustic gaze of bats. A great deal is known about the time-frequency structure of bat echolocation calls and their relationship to the stages of foraging flight in bats. It is however not known how bats direct their sonar beam in flight or how beam direction is related to flight control. This is the first study of the sonar beam direction in freely flying bats as they chase and capture insects. An apparatus and method to measure the sonar beam pattern of echolocating bats (<it>Eptesicus fuscus</it>, big brown bats) as they fly in a laboratory flight room is described. It is shown that the bat locks its sonar beam tightly onto a target during pursuit (Chapter 2). The flying bat's sonar beam consists of two lobes directed apart in the vertical plane (Chapter 3). There is a coupling between acoustic gaze (sonar beam axis) direction and flight turn rate that can be expressed as a delayed linear control law. The gain of this law (steepness of the relationship) varies with the bat's behavioral state (Chapter 4). The bat, when pursuing erratically flying insects, adopts a strategy that keeps the absolute direction to the target a constant. This strategy is shown, under some assumptions, to minimize time-to-intercept of erratically maneuvering targets and is similar to parallel navigation implemented in guided missiles (Chapter 5). The bat is not helpless against ultrasound-triggered evasive dives evolved by some hearing insects. The bat adopts flight strategies to counter such dives (Chapter 6). This work allows us to compare spatial behaviors well studied in visual animals, with similar behaviors in an animal that is guided by hearing and make inferences about common computational strategies employed by nervous systems

Sonar Beam Direction and Flight Control in an Echolocating Bat

Echolocating insectivorous bats are nocturnal mammals that capture fast, erratically moving insects in flight. Bats emit short ultrasonic pulses that form beams of sound and use the returning echoes to guide behavior. The frequency, duration and timing of the sonar pulses, along with the spatial direction of the sonar beam restrict the information returning to the bat, and can be considered a component of the acoustic gaze of bats. A great deal is known about the time-frequency structure of bat echolocation calls and their relationship to the stages of foraging flight in bats. It is however not known how bats direct their sonar beam in flight or how beam direction is related to flight control. This is the first study of the sonar beam direction in freely flying bats as they chase and capture insects. An apparatus and method to measure the sonar beam pattern of echolocating bats (<it>Eptesicus fuscus</it>, big brown bats) as they fly in a laboratory flight room is described. It is shown that the bat locks its sonar beam tightly onto a target during pursuit (Chapter 2). The flying bat's sonar beam consists of two lobes directed apart in the vertical plane (Chapter 3). There is a coupling between acoustic gaze (sonar beam axis) direction and flight turn rate that can be expressed as a delayed linear control law. The gain of this law (steepness of the relationship) varies with the bat's behavioral state (Chapter 4). The bat, when pursuing erratically flying insects, adopts a strategy that keeps the absolute direction to the target a constant. This strategy is shown, under some assumptions, to minimize time-to-intercept of erratically maneuvering targets and is similar to parallel navigation implemented in guided missiles (Chapter 5). The bat is not helpless against ultrasound-triggered evasive dives evolved by some hearing insects. The bat adopts flight strategies to counter such dives (Chapter 6). This work allows us to compare spatial behaviors well studied in visual animals, with similar behaviors in an animal that is guided by hearing and make inferences about common computational strategies employed by nervous systems

Towards a bionic bat: A biomimetic investigation of active sensing, Doppler-shift estimation, and ear morphology design for mobile robots.

Institute of Perception, Action and BehaviourSo-called CF-FM bats are highly mobile creatures who emit long calls in which much of the energy is concentrated in a single frequency. These bats face sensor interpretation problems very similar to those of mobile robots provided with ultrasonic sensors, while navigating in cluttered environments. This dissertation presents biologically inspired engineering on the use of narrowband Sonar in mobile robotics. It replicates, using robotics as a modelling medium, how CF-FM bats process and use the constant frequency part of their emitted call for several tasks, aiming to improve the design and use of narrowband ultrasonic sensors for mobile robot navigation. The experimental platform for the work is RoBat, the biomimetic sonarhead designed by Peremans and Hallam, mounted on a commercial mobile platform as part of the work reported in this dissertation. System integration, including signal processing capabilities inspired by the bat’s auditory system and closed loop control of both sonarhead and mobile base movements, was designed and implemented. The result is a versatile tool for studying the relationship between environmental features, their acoustic correlates and the cues computable from them, in the context of both static, and dynamic real-time closed loop, behaviour. Two models of the signal processing performed by the bat’s cochlea were implemented, based on sets of bandpass filters followed by full-wave rectification and low-pass filtering. One filterbank uses Butterworth filters whose centre frequencies vary linearly across the set. The alternative filterbank uses gammatone filters, with centre frequencies varying non-linearly across the set. Two methods of estimating Doppler-shift from the return echoes after cochlear signal processing were implemented. The first was a simple energy-weighted average of filter centre frequencies. The second was a novel neural network-based technique. Each method was tested with each of the cochlear models, and evaluated in the context of several dynamic tasks in which RoBat was moved at different velocities towards stationary echo sources such as walls and posts. Overall, the performance of the linear filterbank was more consistent than the gammatone. The same applies to the ANN, with consistently better noise performance than the weighted average. The effect of multiple reflectors contained in a single echo was also analysed in terms of error in Doppler-shift estimation assuming a single wider reflector. Inspired by the Doppler-shift compensation and obstacle avoidance behaviours found in CF-FM bats, a Doppler-based controller suitable for collision detection and convoy navigation in robots was devised and implemented in RoBat. The performance of the controller is satisfactory despite low Doppler-shift resolution caused by lower velocity of the robot when compared to real bats. Barshan’s and Kuc’s 2D object localisation method was implemented and adapted to the geometry of RoBat’s sonarhead. Different TOF estimation methods were tested, the parabola fitting being the most accurate. Arc scanning, the ear movement technique to recover elevation cues proposed by Walker, and tested in simulation by her, Peremans and Hallam, was here implemented on RoBat, and integrated with Barshan’s and Kuc’s method in a preliminary narrowband 3D tracker. Finally, joint work with Kim, K¨ampchen and Hallam on designing optimal reflector surfaces inspired by the CF-FM bat’s large pinnae is presented. Genetic algorithms are used for improving the current echolocating capabilities of the sonarhead for both arc scanning and IID behaviours. Multiple reflectors around the transducer using a simple ray light-like model of sound propagation are evolved. Results show phase cancellation problems and the need of a more complete model of wave propagation. Inspired by a physical model of sound diffraction and reflections in the human concha a new model is devised and used to evolve pinnae surfaces made of finite elements. Some interesting paraboloid shapes are obtained, improving performance significantly with respect to the bare transducer

Adaptive map alignment in the superior colliculus of the barn owl: a neuromorphic implementation

Adaptation is one of the basic phenomena of biology, while adaptability is an important feature for neural network. Young barn owl can well adapt its visual and auditory integration to the environmental change, such as prism wearing. At first, a mathematical model is introduced by the related study in biological experiment. The model well explained the mechanism of the sensory map realignment through axongenesis and synaptogenesis. Simulation results of this model are consistent with the biological data. Thereafter, to test the model’s application in hardware, the model is implemented into a robot. Visual and auditory signals are acquired by the sensors of the robot and transferred back to PC through bluetooth. Results of the robot experiment are presented, which shows the SC model allowing the robot to adjust visual and auditory integration to counteract the effects of a prism. Finally, based on the model, a silicon Superior Colliculus is designed in VLSI circuit and fabricated. Performance of the fabricated chip has shown the synaptogenesis and axogenesis can be emulated in VLSI circuit. The circuit of neural model provides a new method to update signals and reconfigure the switch network (the chip has an automatic reconfigurable network which is used to correct the disparity between signals). The chip is also the first Superior Colliculus VLSI circuit to emulate the sensory map realignment