A fully flexible stimulator using 65 nm CMOS process for 1024-electrode epi-retinal prosthesis

Crosstalk in Polymer Microelectrode Arrays

Thin-film polymer microelectrode arrays (MEAs) facilitate the high-resolution neural recording with its superior mechanical compliance. However, the densely packed electrodes and interconnects along with the ultra-thin polymeric encapsulation/substrate layers give rise to non-negligible crosstalk, which could result in severe interference in the neural signal recording. Due to the lack of standardized characterization or modeling of crosstalk in neural electrode arrays, to date, crosstalk in polymer MEAs remains poorly understood. In this work, the crosstalk between two adjacent polymer microelectrodes is measured experimentally and modeled using equivalent circuits. Importantly, this study demonstrated a two-well measuring platform and systematically characterized the crosstalk in polymer microelectrodes with true isolation of the victim channel and precise control of its grounding condition. A simple, unified equation from detailed circuit modeling was proposed to calculate the crosstalk in different environments. Finite element analysis (FEA) analysis was conducted further to explore the crosstalk in more aggressively scaled polymer electrode threads. In addition to standardizing neural electrode array crosstalk characterization, this study not only reveals the dependence of the crosstalk in polymer MEAs on a variety of key device parameters but also provides general guidelines for the design of thin polymer MEAs for high-quality neural signal recording.

Multidimensional gain control in image representation and processing in vision

A generic model of automatic gain control (AGC) is proposed as a general framework for multidimensional automatic contrast sensitivity adjustment in vision, as well as in other sensory modalities. We show that a generic feedback AGC mechanism, incorporating a nonlinear synaptic interaction into the feedback loop of a neural network, can enhance and emphasize important image attributes, such as curvature, size, depth, convexity/concavity and more, similar to its role in the adjustment of photoreceptors and retinal network sensitivity over the extremely high dynamic range of environmental light intensities, while enhancing the contrast. We further propose that visual illusions, well established by psychophysical experiments, are a by-product of the multidimensional AGC. This hypothesis is supported by simulations implementing AGC, which reproduce psychophysical data regarding size contrast effects known as the Ebbinghaus illusion, and depth contrast effects. Processing of curvature by an AGC network illustrates that it is an important mechanism of image structure pre-emphasis, which thereby enhances saliency. It is argued that the generic neural network of AGC constitutes a universal, parsimonious, unified mechanism of neurobiological automatic contrast sensitivity control. This mechanism/model can account for a wide range of physiological and psychophysical phenomena, such as visual illusions and contour completion, in cases of occlusion, by a basic neural network. Likewise, and as important, biologically motivated AGC provides attractive new means for the development of intelligent computer vision systems.

Safety ensuring retinal prosthesis with precise charge balance and low power consumption

Ensuring safe operation of stimulators is the most important issue in neural stimulation. Safety, in terms of stimulators' electrical performances, can be related mainly to two factors; the zero-net charge transfer to tissue and the heat generated by power dissipation at tissue. This paper presents a safety ensuring neuro-stimulator for retinal vision prostheses, featuring precise charge balancing capability and low power consumption, using a 0.35 μm HV (high voltage) CMOS process. Also, the required matching accuracy of the biphasic current pulse for safe stimulation is mathematically derived. Accurate charge balance is achieved by employing a dynamic current mirror at the output of a stimulator. In experiments, using a simple electrode model (a resistor (R) and a capacitor (C) in parallel), the proposed stimulator ensures less than 30 nA DC current flowing into tissue over all stimulation current ranges (32 μA-1 mA), without shorting. With shorting enabled, further reduction is achieved down to 1.5 nA. Low power consumption was achieved by utilising small bias current, sharing of key biasing blocks, and utilising a short duty cycle for stimulation. Less than 30 μW was consumed during stand-by mode, mostly by bias circuitry.

Crosstalk current measurements using multi-electrode arrays in saline

This paper investigates how the configuration of return electrodes in an electrode array affects the amount of current crosstalk when electrodes are driven simultaneously in saline. Two pairs of electrodes in different return configurations were stimulated with different-amplitude biphasic currents. Stimulating electrodes were controlled by current sinks and current sources while return electrodes were connected to supply voltage or ground. Measurement results show that no matter what return configuration was used, the return current was almost equally distributed amongst the return electrodes, which is problematic in bipolar concurrent stimulation, at least in saline. This result is due to the fact that the spreading impedance of saline solution is small compared to the electrode-electrolyte impedance, which makes the saline solution have almost the same potential. This result suggests that monopolar stimulation using a common remote return electrode be used in simultaneous stimulation to avoid crosstalk.