VLSI analogs of neuronal visual processing: a synthesis of form and function

This thesis describes the development and testing of a simple visual system fabricated using complementary metal-oxide-semiconductor (CMOS) very large scale integration (VLSI) technology. This visual system is composed of three subsystems. A silicon retina, fabricated on a single chip, transduces light and performs signal processing in a manner similar to a simple vertebrate retina. A stereocorrespondence chip uses bilateral retinal input to estimate the location of objects in depth. A silicon optic nerve allows communication between chips by a method that preserves the idiom of action potential transmission in the nervous system. Each of these subsystems illuminates various aspects of the relationship between VLSI analogs and their neurobiological counterparts. The overall synthetic visual system demonstrates that analog VLSI can capture a significant portion of the function of neural structures at a systems level, and concomitantly, that incorporating neural architectures leads to new engineering approaches to computation in VLSI. The relationship between neural systems and VLSI is rooted in the shared limitations imposed by computing in similar physical media. The systems discussed in this text support the belief that the physical limitations imposed by the computational medium significantly affect the evolving algorithm. Since circuits are essentially physical structures, I advocate the use of analog VLSI as powerful medium of abstraction, suitable for understanding and expressing the function of real neural systems. The working chip elevates the circuit description to a kind of synthetic formalism. The behaving physical circuit provides a formal test of theories of function that can be expressed in the language of circuits

Sound Source Localization Mechanisms in Gerbil Medial Superior Olive

In this thesis we investigate how the medial superior olive in mammalian brainstem integrates inputs from both ears. We also address three dominant proposals for internal delay generation mechanism, which is responsible for precise sound source localization

Study, Design and Fabrication of an Analogue VLSI Ormia-Ochracea-Inspired Delay Magnification System

This Thesis entails the development of a low-power delay magnification system inspired by the mechanical structure of the ear of the parasitoid fly Ormia Ochracea (O2). The proposed system is suitable as a preprocessing unit for binaural sound localization processors equipped with miniature acoustic sensors. The core of the Thesis involves the study of a delay magnification system based on the O2 sound localization mechanism and the design and testing of a low-power analog integrated circuit based on a proposed, novel delay magnification system inspired by Ormia Ochracea. The study of the delay magnification system based on the O2 sound localization mechanism is divided into two main parts. The first part studies in detail the delay magnification mechanism of the O2 ears. This study sheds light and tries to comprehend what mechanical parameters of the O2 ears are involved in the delay magnification process and how these parameters contribute to the magnification of the delay. The study presents the signal-flow-graph of the O2 system which can be used as a generic delay magnification model for the O2 ears. We also explore the effects of the tuning of the O2 system parameters on the output interaural time difference (ITD). Inspired by the study of the O2 system, in the second part of our study, we modify the O2 system using simpler building blocks and structure which can provide a delay magnification comparable to the original O2 system. We present a new binaural sound localization system suitable for small ITDs which utilizes the new modified O2 system, cochlea filter banks, cross-correlograms and our re-mapping algorithm and show that it can be used to encode very small input delay values that could not be resolved by means of a conventional binaural processor based on the Jeffress’s coincidence detection model. We evaluate the sound localization performance of our new binaural sound localization system for a single sound source and a sound source in the presence of a competing sound source scenario through detailed simulation. The performance of the proposed system is also explored in the presence of filter bandwidth variation and cochlea filter mismatch. After the study of the O2 delay magnification system, we present an analog VLSI chip which morphs the O2 delay magnification system. To determine what topology is the best morphing platform for the O2 system, we present the design and comparative performance of the O2 system when log-domain and gm-C second order weak-inversion filters are employed. The design of the proposed low-power modified O2 system circuit based on translinear loops is detailed. Its performance is evaluated through detailed simulation. Subsequently the Thesis proceeds with the design, fabrication and testing of the new chip based on the modified O2 circuit. The synthesis and testing of the proposed circuit using 0.35μm AMS CMOS process technology parameters is discussed. Detailed measured results confirm the delay magnification ability of the modified O2 circuit and its compliance with theoretical analysis explained earlier in the Thesis. The fabricated system is tuned to operate in the 100Hz to 1kHz frequency range, is able to achieve a delay gain of approximately 3.5 to 9.5 when the input (physical) delay ranges from 0μs to 20μs, and consumes 13.1μW with a 2 V power supply

Some aspects of the pupil response in relation to stimulus movement and colour

Recent investigations have shown that the activity of the visual system in the processing of some stimulus attributes such as spatial structure and stimulus colour are reflected in the pupil response in the form of a small transient constriction at the stimulus onset. The response amplitude appears to be proportional to the level of activity generated and varies systematically with the properties of the visual stimulus. In this report, it has been shown that the processing of coherent motion information can also produce pupillary changes in the form of pupil motion responses (PMRs). The results show that PMRs are present for both foveal and peripheral stimulus presentations and are not affected significantly by degradation of retinal image quality. It has been demonstrated that PMRs are also elicited to sudden changes in stimulus speed and direction, and that the response amplitudes vary systematically with the percentage change involved. There are extensive psychophysical data showing that blindsight subjects are sensitive to movement information when the stimulus is presented in their blind field. In this study, pupillometric parallels for the psychophysical findings have been demonstrated. Data which reveal the existence of residual chromatic discrimination in the absence of VI have also been obtained in two blindsight subjects. It has been shown that their psychophysical performance improves with the level of chromatic saturation of the stimulus. The pupil colour responses have been investigated under similar conditions. Results show that the pupil responses to coloured stimuli can also be elicited in the blind field and that these responses parallel the psychophysical findings. The results obtained have been discussed in relation to the properties of the neuronal structures and visual pathways which are likely to mediate the observed pupillometric and psychophysical findings

Algorithms for VLSI stereo vision circuits applied to autonomous robots

Since the inception of Robotics, visual information has been incorporated in order to allow the robots to perform tasks that require an interaction with their environment, particularly when it is a changing environment. Depth perception is a most useful information for a mobile robot to navigate in its environment and interact with its surroundings. Among the different methods capable of measuring the distance to the objects in the scene, stereo vision is the most advantageous for a small, mobile robot with limited energy and computational power. Stereoscopy implies a low power consumption because it uses passive sensors and it does not require the robot to move. Furthermore, it is more robust, because it does not require a complex optic system with moving elements. On the other hand, stereo vision is computationally intensive. Objects in the scene have to be detected and matched across images. Biological sensory systems are based on simple computational elements that process information in parallel and communicate among them. Analog VLSI chips are an ideal substrate to mimic the massive parallelism and collective computation present in biological nervous systems. For mobile robotics they have the added advantage of low power consumption and high computational power, thus freeing the CPU for other tasks. This dissertation discusses two stereoscopic methods that are based on simple, parallel cal- culations requiring communication only among neighboring processing units (local communication). Algorithms with these properties are easy to implement in analog VLSI and they are also very convenient for digital systems. The first algorithm is phase-based. Disparity, i.e., the spatial shift between left and right images, is recovered as a phase shift in the spatial-frequency domain. Gábor functions are used to recover the frequency spectrum of the image because of their optimum joint spatial and spatial-frequency properties. The Gábor-based algorithm is discussed and tested on a Khepera miniature mobile robot. Two further approximations are introduced to ease the analog VLSI and digital implementations. The second stereoscopic algorithm is difference-based. Disparity is recovered by a simple calculation using the image differences and their spatial derivatives. The algorithm is simulated on a digital system and an analog VLSI implementation is proposed and discussed. The thesis concludes with the description of some tools used in this research project. A stereo vision system has been developed for the Webots mobile robotics simulator, to simplify the testing of different stereo algorithms. Similarly, two stereo vision turrets have been built for the Khepera robot