A Biomimetic Model of the Outer Plexiform Layer by Incorporating Memristive Devices

In this paper we present a biorealistic model for the first part of the early vision processing by incorporating memristive nanodevices. The architecture of the proposed network is based on the organisation and functioning of the outer plexiform layer (OPL) in the vertebrate retina. We demonstrate that memristive devices are indeed a valuable building block for neuromorphic architectures, as their highly non-linear and adaptive response could be exploited for establishing ultra-dense networks with similar dynamics to their biological counterparts. We particularly show that hexagonal memristive grids can be employed for faithfully emulating the smoothing-effect occurring at the OPL for enhancing the dynamic range of the system. In addition, we employ a memristor-based thresholding scheme for detecting the edges of grayscale images, while the proposed system is also evaluated for its adaptation and fault tolerance capacity against different light or noise conditions as well as distinct device yields

A Time Decoding Realization with a CNN

Time encoding is a novel real-time asynchronous mechanism for encoding amplitude information into a time sequence. The analog bandlimited input is fed into a simple nonlinear neuron-like circuit that generates a strictly increasing time sequence based on which the signal can be reconstructed. The heart of the reconstruction is solving a system of illconditioned linear equations. This contribution shows that the equations can be manipulated so that the reconstruction becomes feasible using a Cellular Neural Network (CNN) with a banded system matrix. In particular, the system is first transformed into a well-conditioned smaller system; and then, the Lanczos process is used to lay it out into a set of even smaller systems characterized by a set of tridiagonal matrices. Each of these systems can directly be solved by CNNs, whereas the preprocessing (transformation and Lanczos algorithm) and simple postprocessing phases can be partly or fully implemented by using the digital capabilities of the CNN Universal Machine (CNN-UM). Each step of the proposed formulation is confirmed by numerical (digital) simulations

Restoring light sensitivity using tunable near-infrared sensors

Enabling near-infrared light sensitivity in a blind human retina may supplement or restore visual function in patients with regional retinal degeneration. We induced near-infrared light sensitivity using gold nanorods bound to temperature-sensitive engineered transient receptor potential (TRP) channels. We expressed mammalian or snake TRP channels in light-insensitive retinal cones in a mouse model of retinal degeneration. Near-infrared stimulation increased activity in cones, ganglion cell layer neurons, and cortical neurons, and enabled mice to perform a learned light-driven behavior. We tuned responses to different wavelengths, by using nanorods of different lengths, and to different radiant powers, by using engineered channels with different temperature thresholds. We targeted TRP channels to human retinas, which allowed the postmortem activation of different cell types by near-infrared light

Rods in daylight act as relay cells for cone-driven horizontal cell mediated surround inhibition

Vertebrate vision relies on two types of photoreceptors, rods and cones, which signal increments in light intensity with graded hyperpolarizations. Rods operate in the lower range of light intensities while cones operate at brighter intensities. The receptive fields of both photoreceptors exhibit antagonistic center-surround organization. Here we show that at bright light levels, mouse rods act as relay cells for cone-driven horizontal cell-mediated surround inhibition. In response to large, bright stimuli that activate their surrounds, rods depolarize. Rod depolarization increases with stimulus size, and its action spectrum matches that of cones. Rod responses at high light levels are abolished in mice with nonfunctional cones and when horizontal cells are reversibly inactivated. Rod depolarization is conveyed to the inner retina via postsynaptic circuit elements, namely the rod bipolar cells. Our results show that the retinal circuitry repurposes rods, when they are not directly sensing light, to relay cone-driven surround inhibition.status: publishe

Cell types of the human retina and its organoids at single cell resolution: developmental convergence, transcriptional identity, and disease map

Organoids are stem cell-derived artificial organs that mimic aspects of organ development, function and disease. How closely cell type diversity and cell type maturation in human organoids recapitulate that of their target organ is not well understood. We performed histochemistry and sequenced the RNA of 163,971 single cells from improved human retinal organoids at different developmental stages and from donated healthy adult human retinas and choroid. Cell types in mature organoids had morphologies and transcriptomes that resembled their adult equivalents. Remarkably, organoids developed at a similar rate to the developing human retina and the transcriptome trajectory of cell types contained a progression of key developmental markers. Mapping disease-associated genes to cell types revealed cellular targets for studying disease mechanism in organoids and performing targeted repair in adult retinas