Emmanuel M. Drakakis

Multi-site optical excitation using ChR2 and micro-LED array.

Studying neuronal processes such as synaptic summation, dendritic physiology and neural network dynamics requires complex spatiotemporal control over neuronal activities. The recent development of neural photosensitization tools, such as channelrhodopsin-2 (ChR2), offers new opportunities for non-invasive, flexible and cell-specific neuronal stimulation. Previously, complex spatiotemporal control of photosensitized neurons has been limited by the lack of appropriate optical devices which can provide 2D stimulation with sufficient irradiance. Here we present a simple and powerful solution that is based on an array of high-power micro light-emitting diodes (micro-LEDs) that can generate arbitrary optical excitation patterns on a neuronal sample with micrometre and millisecond resolution. We first describe the design and fabrication of the system and characterize its capabilities. We then demonstrate its capacity to elicit precise electrophysiological responses in cultured and slice neurons expressing ChR2.

Optobionic vision?a new genetically enhanced light on retinal prosthesis

The recent discovery that neurons can be photostimulated via genetic incorporation of artificial opsins is creating a revolution in the field of neural stimulation. In this paper we show its potential in the field of retinal prosthesis. We show that we need typically 100 mW cm(-2) in instantaneous light intensity on the neuron in order to stimulate action potentials. We also show how this can be reduced down to safe levels in order to negate thermal and photochromic damage to the eye. We also describe a gallium nitride LED light source which is also able to generate patterns of the required intensity in order to transfer reliable images.

Micro-LED arrays: a tool for two-dimensional neuron stimulation

Stimulating neuron cells with light is an exciting new technology that is revolutionizing the neurosciences. To date, due to the optical complexity that is involved, photostimulation has only been achieved at a single site using high power light sources. Here we present a GaN based micro-light emitting diode (LED) array that can open the way to multi-site photostimulation of neuron cells. The device is a two-dimensional array of micrometre size LED emitters. Each emitter has the required wavelength, optical power and modulation bandwidth to trigger almost any photosensitizer and is individually addressable. We demonstrate micrometre resolution photoactivation of a caged fluorophore and photostimulation of sensitized living neuron cells. In addition, a complete system that combines the micro-LED array with multi-site electrophysiological recording based on microelectrode array technology and/or fluorescence imaging is presented.

History and future of auditory filter models

Auditory filter models have a history of over a hundred years, with explicit bio-mimetic inspiration at many stages along the way. From passive analogue electric delay line models, through digital filter models, active analogue VLSI models, and abstract filter shape models, these filters have both represented and driven the state of progress in auditory research. Today, we are able to represent a wide range of linear and nonlinear aspects of the psychophysics and physiology of hearing with a rather simple and elegant set of circuits or computations that have a clear connection to underlying hydrodynamics and with parameters calibrated to human performance data. A key part of the progress in getting to this stage has been the experimental clarification of the nature of cochlear nonlinearities, and the modelling work to map these experimental results into the domain of circuits and systems. No matter how these models are built into machine-hearing systems, their bio-mimetic roots will remain key to their performance. In this paper we review some of these models, explain their advantages and disadvantages and present possible ways of implementing them. As an example, a continuous-time analogue CMOS implementation of the One Zero Gammatone Filter (OZGF) is presented together with its automatic gain control that models its level-dependent nonlinear behaviour.

MEMRISTOR MODEL AND ITS APPLICATION FOR CHAOS GENERATION

This paper contributes to the understanding of memristor operation and its possible application fields through: (a) derivation of a complete mathematical model for the HP memristor which takes into consideration the inter-dependence between memristance, charge and flux along with the boundary and initial conditions of operation; (b) an introduction of detailed charge- and flux-controlled SPICE memristor models realizing the proposed mathematical memristor model; (c) The incorporation of the memristor model in the SPICE realization of a third-order chaotic system where a single HP memristor acts as the nonlinear part of the system. Simulation results are provided to validate the mathematical model and the synthesis and operation of the third-order chaotic system.

A New Individually Addressable Micro-LED Array for Photogenetic Neural Stimulation

Here, we demonstrate the use of a micro light emitting diode (LED) array as a powerful tool for complex spatiotemporal control of photosensitized neurons. The array can generate arbitrary, 2-D, excitation patterns with millisecond and micrometer resolution. In particular, we describe an active matrix control address system to allow simultaneous control of 256 individual micro LEDs. We present the system optically integrated into a microscope environment and patch clamp electrophysiology. The results show that the emitters have sufficient radiance at the required wavelength to stimulate neurons expressing channelrhodopsin-2 (ChR2).

A Biomimetic, 4.5

This paper deals with the design and performance evaluation of a new analog CMOS cochlea channel of increased biorealism. The design implements a recently proposed transfer function, namely the One-Zero Gammatone filter (or OZGF), which provides a robust foundation for modeling a variety of auditory data such as realistic passband asymmetry, linear low-frequency tail and level-dependent gain. Moreover, the OZGF is attractive because it can be implemented efficiently in any technological medium-analog or digital-using standard building blocks. The channel was synthesized using novel, low-power, class-AB, log-domain, biquadratic filters employing MOS transistors operating in their weak inversion regime. Furthermore, the paper details the design of a new low-power automatic gain control circuit that adapts the gain of the channel according to the input signal strength, thereby extending significantly its input dynamic range. We evaluate the performance of a fourth-order OZGF channel (equivalent to an 8th-order cascaded filter structure) through both detailed simulations and measurements from a fabricated chip using the commercially available 0.35 mum AMS CMOS process. The whole system is tuned at 3 kHz, dissipates a mere 4.46 muW of static power, accommodates 124 dB (at < 5% THD) of input dynamic range at the center frequency and is set to provide up to 70 dB of amplification for small signals.

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The scope of this paper is to present certain insights and advances towards the synthesis and transistor-level implementation of high dynamic range (> 120 dB), micropower, CMOS Sinh companding filters. In particular, we present detailed technical insights on a recently proposed Sinh integrator which may serve as the basic building block for higher-order filter structures. The particular integrator exhibits a promising simulated linearity performance mainly because it does not rely on the complementarity of both N- and P-type MOS transistors to achieve its Class-AB operation. Rather, it is designed with single-type devices in its signal processing path. The integrator is evaluated through detailed simulation results obtained by performing both large-signal transient and periodic steady-state (PSS) analyses in Cadence IC Design Frameworkreg with the parameters of the commercially available AMS 0.35-mum CMOS process. SNR, SNDR, IP3 and mismatch are some of the performance figures reported in this work. A detailed head-to-head comparison with a typical pseudo-differential Class-AB Log-domain integrator, designed in the same technology and with identical specifications, is also performed in order to reveal any potential benefits of the Sinh circuit paradigm.

W, 120+ dB, Log-Domain Cochlea Channel With AGC

This paper introduces a novel theoretical framework suitable for the study of memristors. The articulation of the framework relies upon the identification of a certain type of dynamics which comply with Bernoullis differential equation and are thus termed Bernoulli dynamics. The paper explains how the Bernoulli dynamics: a) govern the dynamic behaviour of the ideal Williams memristors and other memelements, and b) how they can be exploited for the derivation and subsequent study of analytic expressions of the form Imres = f(Vmres) or Vmres = g(Imres) which define the relation between the memristor current Imres and the memristor voltage Vmres Implications of the adoption of the new theoretical framework for future memristor-based circuit research are also discussed.

Insights and Advances on the Design of CMOS Sinh Companding Filters

We introduce a mathematical framework for the analysis of the input–output dynamics of externally driven memristors. We show that, under general assumptions, their dynamics comply with a Bernoulli differential equation and hence can be nonlinearly transformed into a formally solvable linear equation. The Bernoulli formalism, which applies to both charge- and flux-controlled memristors when either current or voltage driven, can, in some cases, lead to expressions of the output of the device as an explicit function of the input. We apply our framework to obtain analytical solutions of the i–v characteristics of the recently proposed model of the Hewlett–Packard memristor under three different drives without the need for numerical simulations. Our explicit solutions allow us to identify a dimensionless lumped parameter that combines device-specific parameters with properties of the input drive. This parameter governs the memristive behaviour of the device and, consequently, the amount of hysteresis in the i–v. We proceed further by defining formally a quantitative measure for the hysteresis of the device, for which we obtain explicit formulas in terms of the aforementioned parameter, and we discuss the applicability of the analysis for the design and analysis of memristor devices.