NEUROMORPHIC VLSI DESIGN USING BAT ECHOLOCATION
1. ABSTRACT:
Birds and bats have long been the envy of engineers, demonstrating fast,
accurate sensing and agile flight control in complex, confined 3D
spaces, all in a tiny package. Their ability to fly rapidly through
cluttered forest environments in search of food far exceeds the
capabilities of any existing man-made system. The technology that is
developing and propose to bring to this application domain is
neuromorphic
VLSI. For more than a decade, a growing number of VLSI researchers worldwide have been developing a common toolbox of hybrid analog and digital VLSI techniques to mimic the signal processing of neural systems. This effort has spawned many projects in smart vision sensors and systems: silicon cochleae,
retinal and cochlear prosthetics, neural prosthetics, biologically realistic legged robotics, on-chip learning systems and many more. Using these design techniques, our laboratory recently has pursued the development of echolocation circuits that mimic the neural processing in the big brown bat, Eptesicus fuscus. One population of neurons that we have designed is ons that we have designed is tuned to detect the angle of echo arrival as determined by the relative loudness at two microphones placed on a model bat head. These biological algorithms are implemented in commercially available CMOS fabrication processes (e.g., the MOSIS service) and operate in real-time with power consumption in the range of milli watts. 2.INTRODUCTION: From a computational neuroscience perspective, bats are remarkable because of the very short timescale on which they operate. The barrage of returning sonar echoes from a bat's near-environment lasts approximately 30 milliseconds following a sonar emission with the echo from a specific target lasting, at most, a few milliseconds. At this timescale, a particular neuron has the opportunity to fire only one or two spikes to represent the echo. Unlike the “traditional” view of cortical processing where many spikes are integrated over time to compute an average rate, the bat must rely on populations of neurons that respond transiently but selectively to different objects in the environment. In these neural circuits, the details of spike timing, synaptic dynamics, and neuron biophysics become extremely important. Flying at speeds anywhere from 1m/s to 6 m/s, a bat’s sensory world jumps from pulse to pulse as it flies through the world. Sensory prediction is therefore likely to be very important in this animal. In spite of all this behavioral specialization, the bat brain is organized like most other mammalian brains suggesting that echolocation arises from only small modifications of the typical mammalian auditory system. 3.GOAL: Our goal is to construct a flying bat-sized creature that uses ultrasonic echolocation to both navigate and scrutinize its environment sufficiently to distinguish between obstacles and "insects". The bat's sensory and motor system will be constructed from neural models and implemented using "neuromorphic" VLSI techniques. Our intention is two-fold: 1) to test these neural algorithms in a real-time, closed-loop behavioral situation, and 2) to develop useful sonar sensors for use in miniature aircraft systems. BAT HEAD: We are working with two different hardware systems: a physically larger single-frequency sonar system ("narrowband") and a tiny broadband system. The narrowband system is being used to rapidly test concepts following initial software tests. Photos of these two systems are found below: In the photo to the left is our narrowband sonar system that operates only on a frequency of 40 kHz. The fixed arrangement of the microphones was chosen to produce a difference in echo amplitude with azimuthal direction. The current system roughly extracts direction and range and is capable of servoing the head (which is mounted on an model airplane servo) to track moving targets in real-time. On the right, we have a photo of our broadband system using a baked polymer clay bat head with a tiny Knowles (FG3329) microphone soldered to the end of a group of wires. This system has two broadband ultrasonic (and audio) microphones that will feed our silicon cochleae chips. Both of these physical heads produce intensity difference cues at each microphone that allows the system to determine the angle of the arriving echo. 4.AN ULTRASONIC COCHLEA: Echo locating bats specialize in high-frequency hearing using echolocation sounds that typically range in frequency from 20 kHz to 100 kHz. While some bats are specialized for specific frequencies with cochlear filtering at extremely high Q10dB values, we are studying bats that use a broadband vocalization and are ultrasonic frequency generalists (e.g., Myotis lucifugus) with Q10dB values in the range of 10 to 30. Good frequency resolution is important for vertical localization, discriminating close objects as well as for prey determination. To support our ongoing work in modeling bat echolocation, a binaural, ultrasonic cochlea-like filter bank has been designed with moderate quality (Q) factor (as high as 65) with spiking neurons that are driven by the filter outputs. The neuron addresses are reported off chip at the time of the spike in an un-arbitrated fashion and in current-mode to reduce the amount of capacitively- coupled feedback into the filters. This chip was fabricated in a commercially- .5 um CMOS process and consumes 0.425 milli watts at 5 volts. When echoes arrive from different directions, the number of spikes generated in the auditory nerve and the cochlear nucleus varies with the intensity at each ear. Using this information, the first binaural nucleus in the mammalian auditory system, the lateral superior olive (or LSO) becomes selective to the direction of arrival. These cells are excited by the intensity from one ear and inhibited by the intensity from the other ear. The binaural LSO response and the monaural response from the cochlear nucleus are projected to the inferior colliculus (IC) via the doral nucleus of the lateral lemniscus (or DNLL), resulting in very similar responses in both DNLL and IC. With similar responses in the LSO as in the IC, one can ask the question, "What kind of computation is going on here?" In the figure above is a set of tuning curves for three LSO cells that have different synaptic weightings from the left and right ears. By comparing the responses of the population of LSO cells, each of which have different synaptic weightings, we can determine which direction an echo is arriving from. 5.DELAY TUNED CELLS (RANGE TUNING): Information about target range has many uses for bats during both prey-capture and navigation tasks. Beyond the extraction of distance and velocity, it may be important for less obvious tasks, such as optimizing the parameters of the echolocation process. For example, as a bat approaches a target, it alters the repetition rate, duration, spectral content, and amplitude of it vocalizations. Not only is echolocation used for insect capture, it provides to the bat information about obstacles, roosts, altitude, and other flying creatures. In the bat’s brainstem and midbrain exist neural circuits that are sensitive to the specific difference in time between the outgoing sonar vocalization and the returning echo. While some of the details of the neural mechanisms are known to be species-specific, a basic model of reference-triggered, post-inhibitory rebound timing is reasonably well supported by available data. Neurons have been found in bats that show a ‘facilitated’ response to paired sounds (a simulated vocalization and an echo) presented at particular delays. The cells’ responses to sounds presented at the appropriate delays are much greater than the sum of the responses to the individual sounds presented alone. These cells are part of a larger class of neurons called ‘combination-sensitive’ neurons, and are specifically referred to as delay-tuned cells. Delay-tuned cells are found at many levels in the bat auditory system. They have been found in the inferior colliculus (IC), the medial geniculate body (MGB), and the auditory cortex. Disruption of cortical delay-tuned cells has been shown to impair a bat’s ability to discriminate artificial pulse-echo pair delays. It is likely that delay- tuned neurons play a role in forming the bat’s perception of range, although delay-tuned cells have also been shown to respond to the social calls of other bats. 6.COMMERCIAL APPLICATIONS: There are many obvious commercial and industrial applications of integrated sensory systems implemented in low-power VLSI. The development of a small, sophisticated, power-efficient, low-cost echolocation system has many potential applications beyond neural modeling. In the biomedical realm, such devices are beginning to be used as another option for collision avoidance and spatial sensing for blind or low vision patients. These devices when properly scaled down could also be used to guide endoscopic instruments or provide additional information about distance to monocular, visually guided surgical tools. Air-coupled sonar, las a basic sensor module for; mobile robotics, has not advanced significantly beyond a narrow-beam, closest-target sensor, despite decades of use, with robotic vacuum cleaners finally hitting the market, a low power module with significantly more sensing capability at low cost could facilitate a new range of commercial products and toys that have the ability to sense objects in the near-field like a full set of whiskers. From a micro- aerial vehicle (MAV) perspective, while GPS have successfully enabled long-range navigation, the final leg of many desirable machines occurs in locations where the lack of GPS signals and unmapped obstacles make Navigation untenable; such locations include inside building, under the forest canopy, in canyons, and in caves. Obtaining the range to objects directly, while computing azimuth, sonar systems are a natural complement to vision systems for these challenging environments. When combined with an ornithopter airframe, a nearly silent device (to humans), the ability to fly in darkness seems to be within reach. 7.CONCLUSION: Overall this paper proves to be wonderful framework of in which to pursue different types of scientific engineering-oriented research and education. Understanding bat echolocation involves many interesting problems of signal processing within the context of biological data representations and neural hardware. BIBILOGRAPHY: 1.signal processing journal (IEEE)
VLSI. For more than a decade, a growing number of VLSI researchers worldwide have been developing a common toolbox of hybrid analog and digital VLSI techniques to mimic the signal processing of neural systems. This effort has spawned many projects in smart vision sensors and systems: silicon cochleae,
retinal and cochlear prosthetics, neural prosthetics, biologically realistic legged robotics, on-chip learning systems and many more. Using these design techniques, our laboratory recently has pursued the development of echolocation circuits that mimic the neural processing in the big brown bat, Eptesicus fuscus. One population of neurons that we have designed is ons that we have designed is tuned to detect the angle of echo arrival as determined by the relative loudness at two microphones placed on a model bat head. These biological algorithms are implemented in commercially available CMOS fabrication processes (e.g., the MOSIS service) and operate in real-time with power consumption in the range of milli watts. 2.INTRODUCTION: From a computational neuroscience perspective, bats are remarkable because of the very short timescale on which they operate. The barrage of returning sonar echoes from a bat's near-environment lasts approximately 30 milliseconds following a sonar emission with the echo from a specific target lasting, at most, a few milliseconds. At this timescale, a particular neuron has the opportunity to fire only one or two spikes to represent the echo. Unlike the “traditional” view of cortical processing where many spikes are integrated over time to compute an average rate, the bat must rely on populations of neurons that respond transiently but selectively to different objects in the environment. In these neural circuits, the details of spike timing, synaptic dynamics, and neuron biophysics become extremely important. Flying at speeds anywhere from 1m/s to 6 m/s, a bat’s sensory world jumps from pulse to pulse as it flies through the world. Sensory prediction is therefore likely to be very important in this animal. In spite of all this behavioral specialization, the bat brain is organized like most other mammalian brains suggesting that echolocation arises from only small modifications of the typical mammalian auditory system. 3.GOAL: Our goal is to construct a flying bat-sized creature that uses ultrasonic echolocation to both navigate and scrutinize its environment sufficiently to distinguish between obstacles and "insects". The bat's sensory and motor system will be constructed from neural models and implemented using "neuromorphic" VLSI techniques. Our intention is two-fold: 1) to test these neural algorithms in a real-time, closed-loop behavioral situation, and 2) to develop useful sonar sensors for use in miniature aircraft systems. BAT HEAD: We are working with two different hardware systems: a physically larger single-frequency sonar system ("narrowband") and a tiny broadband system. The narrowband system is being used to rapidly test concepts following initial software tests. Photos of these two systems are found below: In the photo to the left is our narrowband sonar system that operates only on a frequency of 40 kHz. The fixed arrangement of the microphones was chosen to produce a difference in echo amplitude with azimuthal direction. The current system roughly extracts direction and range and is capable of servoing the head (which is mounted on an model airplane servo) to track moving targets in real-time. On the right, we have a photo of our broadband system using a baked polymer clay bat head with a tiny Knowles (FG3329) microphone soldered to the end of a group of wires. This system has two broadband ultrasonic (and audio) microphones that will feed our silicon cochleae chips. Both of these physical heads produce intensity difference cues at each microphone that allows the system to determine the angle of the arriving echo. 4.AN ULTRASONIC COCHLEA: Echo locating bats specialize in high-frequency hearing using echolocation sounds that typically range in frequency from 20 kHz to 100 kHz. While some bats are specialized for specific frequencies with cochlear filtering at extremely high Q10dB values, we are studying bats that use a broadband vocalization and are ultrasonic frequency generalists (e.g., Myotis lucifugus) with Q10dB values in the range of 10 to 30. Good frequency resolution is important for vertical localization, discriminating close objects as well as for prey determination. To support our ongoing work in modeling bat echolocation, a binaural, ultrasonic cochlea-like filter bank has been designed with moderate quality (Q) factor (as high as 65) with spiking neurons that are driven by the filter outputs. The neuron addresses are reported off chip at the time of the spike in an un-arbitrated fashion and in current-mode to reduce the amount of capacitively- coupled feedback into the filters. This chip was fabricated in a commercially- .5 um CMOS process and consumes 0.425 milli watts at 5 volts. When echoes arrive from different directions, the number of spikes generated in the auditory nerve and the cochlear nucleus varies with the intensity at each ear. Using this information, the first binaural nucleus in the mammalian auditory system, the lateral superior olive (or LSO) becomes selective to the direction of arrival. These cells are excited by the intensity from one ear and inhibited by the intensity from the other ear. The binaural LSO response and the monaural response from the cochlear nucleus are projected to the inferior colliculus (IC) via the doral nucleus of the lateral lemniscus (or DNLL), resulting in very similar responses in both DNLL and IC. With similar responses in the LSO as in the IC, one can ask the question, "What kind of computation is going on here?" In the figure above is a set of tuning curves for three LSO cells that have different synaptic weightings from the left and right ears. By comparing the responses of the population of LSO cells, each of which have different synaptic weightings, we can determine which direction an echo is arriving from. 5.DELAY TUNED CELLS (RANGE TUNING): Information about target range has many uses for bats during both prey-capture and navigation tasks. Beyond the extraction of distance and velocity, it may be important for less obvious tasks, such as optimizing the parameters of the echolocation process. For example, as a bat approaches a target, it alters the repetition rate, duration, spectral content, and amplitude of it vocalizations. Not only is echolocation used for insect capture, it provides to the bat information about obstacles, roosts, altitude, and other flying creatures. In the bat’s brainstem and midbrain exist neural circuits that are sensitive to the specific difference in time between the outgoing sonar vocalization and the returning echo. While some of the details of the neural mechanisms are known to be species-specific, a basic model of reference-triggered, post-inhibitory rebound timing is reasonably well supported by available data. Neurons have been found in bats that show a ‘facilitated’ response to paired sounds (a simulated vocalization and an echo) presented at particular delays. The cells’ responses to sounds presented at the appropriate delays are much greater than the sum of the responses to the individual sounds presented alone. These cells are part of a larger class of neurons called ‘combination-sensitive’ neurons, and are specifically referred to as delay-tuned cells. Delay-tuned cells are found at many levels in the bat auditory system. They have been found in the inferior colliculus (IC), the medial geniculate body (MGB), and the auditory cortex. Disruption of cortical delay-tuned cells has been shown to impair a bat’s ability to discriminate artificial pulse-echo pair delays. It is likely that delay- tuned neurons play a role in forming the bat’s perception of range, although delay-tuned cells have also been shown to respond to the social calls of other bats. 6.COMMERCIAL APPLICATIONS: There are many obvious commercial and industrial applications of integrated sensory systems implemented in low-power VLSI. The development of a small, sophisticated, power-efficient, low-cost echolocation system has many potential applications beyond neural modeling. In the biomedical realm, such devices are beginning to be used as another option for collision avoidance and spatial sensing for blind or low vision patients. These devices when properly scaled down could also be used to guide endoscopic instruments or provide additional information about distance to monocular, visually guided surgical tools. Air-coupled sonar, las a basic sensor module for; mobile robotics, has not advanced significantly beyond a narrow-beam, closest-target sensor, despite decades of use, with robotic vacuum cleaners finally hitting the market, a low power module with significantly more sensing capability at low cost could facilitate a new range of commercial products and toys that have the ability to sense objects in the near-field like a full set of whiskers. From a micro- aerial vehicle (MAV) perspective, while GPS have successfully enabled long-range navigation, the final leg of many desirable machines occurs in locations where the lack of GPS signals and unmapped obstacles make Navigation untenable; such locations include inside building, under the forest canopy, in canyons, and in caves. Obtaining the range to objects directly, while computing azimuth, sonar systems are a natural complement to vision systems for these challenging environments. When combined with an ornithopter airframe, a nearly silent device (to humans), the ability to fly in darkness seems to be within reach. 7.CONCLUSION: Overall this paper proves to be wonderful framework of in which to pursue different types of scientific engineering-oriented research and education. Understanding bat echolocation involves many interesting problems of signal processing within the context of biological data representations and neural hardware. BIBILOGRAPHY: 1.signal processing journal (IEEE)
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