Tetsuya Yagi

The role of retinal bipolar cell in early vision: an implication with analogue networks and regularization theory.

Abstract. A linear analogue network model is proposed to describe the neuronal circuit of the outer retina consisting of cones, horizontal cells, and bipolar cells. The model reflects previous physiological findings on the spatial response properties of these neurons to dim illumination and is expressed by physiological mechanisms, i.e., membrane conductances, gap-junctional conductances, and strengths of chemical synaptic interactions. Using the model, we characterized the spatial filtering properties of the bipolar cell receptive field with the standard regularization theory, in which the early vision problems are attributed to minimization of a cost function. The cost function accompanying the present characterization is derived from the linear analogue network model, and one can gain intuitive insights on how physiological mechanisms contribute to the spatial filtering properties of the bipolar cell receptive field. We also elucidated a quantitative relation between the Laplacian of Gaussian operator and the bipolar cell receptive field. From the computational point of view, the dopaminergic modulation of the gap-junctional conductance between horizontal cells is inferred to be a suitable neural adaptation mechanism for transition between photopic and mesopic vision.

Electrophysiological and histological studies of chronically implanted intrapapillary microelectrodes in rabbit eyes

PurposeTo determine the safety and efficacy of transsclerally placed intrapapillary wire microelectrodes implanted chronically into the optic nerve head of rabbit eyes.MethodsFour platinum wire microelectrodes were passed through the sclera and implanted into the optic nerve head of five rabbit eyes for 4–6 months. Color fundus photography, fluorescein angiography, electroretinograms (ERGs), and visually evoked potentials (VEPs) were used to monitor the retina. Electrically evoked potentials (EEPs) were elicited by bipolar electrical stimulation of the optic nerve axons by different combinations of the four electrodes immediately after the implantation and at 1-month intervals thereafter. The effects of the chronic implantation of the electrodes on the morphology of the optic nerve were evaluated by histological and immunohistochemical examinations at 4 and 6 months after the implantation.ResultsAll of the electrodes remained stable in the implanted sites throughout the post-implantation period, except for one electrode that had pulled out of the optic nerve head at 1 month after implantation. No intraocular infection, inflammation, or vitreoretinal proliferation was observed in any eye. EEPs could be elicited from each pair of electrodes at all testing times. The mean threshold currents (charge densities) to evoke EEPs increased from 19.3±9.2xa0μA (6.0±2.9xa0μC/cm2) on the implantation day to 78.8±31.9xa0μA (24.6±10.0xa0μC/cm2) at 1 month after implantation, but did not change significantly thereafter. The implicit time and amplitude of the a- and b-waves of the ERGs and of P1 of the VEPs did not change significantly throughout the post-implantation period. Histological evaluation of the optic nerve head revealed slight tissue encapsulations surrounding the electrode and increased expression of glial fibrillary acidic protein near the surface of the optic nerve.ConclusionsImplantation of transscleral intrapapillary microelectrodes appears to be safe and effective. These findings indicate that the implantation of microelectrodes in the optic nerve head should be considered for an optic nerve-based prosthesis.

Direct stimulation of optic nerve by electrodes implanted in optic disc of rabbit eyes.

PurposeTo determine whether wire microelectrodes implanted in the optic disc can be used to elicit cortical potentials.MethodsTwo or four platinum wire electrodes of two types, viz., the cut-end type and the exposed-tip type, were inserted through the vitreous and fixed in the optic disc of 16 rabbit eyes. Electrically evoked potentials (EEPs) were recorded after bipolar electrical stimulation with the two wire electrodes and by different combinations of the four-electrode system. The optic discs were examined histologically after the experiment.ResultsThe wire electrodes were successfully implanted and fixed into different positions of the optic disc without serious complications in all 16 eyes. EEPs could be elicited after bipolar electrical stimulation of the optic nerve using either the two-electrode system or different pairs of the four-electrode system. Threshold charge densities to elicit EEPs were 0.32–0.64xa0mC/cm2 in eyes using the cut-end type of electrodes and 0.93–6.21xa0μC/cm2 in eyes using the exposed-tip type. The amplitude of the EEPs increased with increasing electrical stimulus intensities. Histological evaluation revealed limited damage to the neural tissue adjacent to the electrode track.ConclusionsThe visual cortex can be activated by direct microelectrical stimulation of the optic nerve. The acute implantation of the wire microelectrodes into the optic disc by a transvitreal approach is feasible and results in only limited damage to the optic nerve.

Image processing regularization filters on layered architecture

Abstract Layered architecture is proposed for solving a class of regularization problems in image processing. There are two major hurdles in the implementation of regularization filters with second or higher order smoothness constraints: (a)Stability: With second or higher order constraints, a direct implementation of a regularization filter necessitates negative conductance which, in turn, gives rise to stability problems. (b)Wiring Complexity: A direct implementation of an N-th order regularization filter requires wiring between every pair of k-th nearest nodes for all k, 1 ≤ k ≤ N. Even though one of the authors managed to layout an N = 2 chip, the implementation of an N ≥ 3 chip would be an extremely difficult, if not impossible, task. The regularization filter architecture proposed here (a) requires no negative conductance; and (b) necessitates wiring only between nearest nodes. Smoothing-Contrast-Enhancement filter is given as an example of application. Since this filter is extremely fast, it will have a natural application to smart sensing, i.e., to the simultaneous achievement of sensing and processing. It is also explained how this architecture has been inspired by physiological findings on lower vertebrate retina by one of the authors.

Temporal properties of retinal ganglion cell responses to local transretinal current stimuli in the frog retina.

Extracellular current stimuli have been used in both electrophysiological and clinical studies. The present study elucidates the temporal properties of the frog retinal ganglion cell response induced by local transretinal current stimuli. Two classes of spike response were recorded from the ganglion cell. One had a constant latency ranging from 1.5 to 4.5 ms after the onset of the stimulus regardless of differences in stimulus parameters. Another class had a latency that varied from trial to trial between 3.5 and 71.5 ms at the threshold even when stimulus parameters were identical. The latency became shorter and the number of spike responses increased as the charge applied via the stimulus pulse was increased by increasing the amplitude (from 50 to 200 microA) or the pulse duration (from 100 to 1000 micros). In both classes, the current stimuli with the same amount of charge induced responses of a similar latency for amplitudes between 50 and 200 microA and for pulse durations between 100 and 1000 micros.

Computational Studies on the Interaction Between Red Cone and H1 Horizontal Cell

We propose an equivalent circuit model of a discrete formulation to describe the interaction between the red cone syncytium and the H1 horizontal cell syncytium in lower vertebrate retinas. Analytical solutions of the model provide intuitive understandings of spatio-temporal properties of light-induced responses in reference to membrane impedance, strength of chemical synapse and coupling resistance connecting neighbouring cells. Physiologically plausible values of these parameters are estimated using the solutions. Quantitative studies are made to elucidate the function of (1) the negative feedback from the H1 horizontal cell to the red cone, and (2) the resistance increase of H1 horizontal cell coupling by dopamine.

Light-adaptive architectures for regularization vision chips

Abstract Light-adaptive algorithms/architectures are proposed for regularization vision chips. The adaptation mechanisms allow the regularization parameters to change in an adaptive manner in accordance with the light intensity of given images. This is achieved by adaptively changing the conductance values associated with massively parallel resistive networks. The algorithms/architectures are inspired by the adaptation mechanisms of the horizontal cells in the lower vertebrate retina.

A parallel analog intelligent vision sensor with a variable receptive field

The retina is an intelligent vision sensor indispensable in real-time image information processing of animals. The parallel image processing function of the retina was realized in an analog CMOS integrated circuit (vision chip). The chip was implemented with a one-dimensional Laplacian–Gaussian receptive field the size of which can be controlled by external signals. Active pixel sensors were used in the sensor part of the chip where the light sensitivity can be controlled by varying the accumulation time. Circuits compensating for noise due to the mismatch of transistor characteristics were incorporated, and the experimental results verified that sufficiently accurate outputs can be obtained under natural illumination. The chip can effectively extract the spatial frequency bandwidth of the signal in the presence of noise.

A Parallel Analog CMOS Signal Processor for Image Contrast Enhancement

A 2D analog CMOS network with a 53 × 52 array of embedded photosensors implements a convolution function approximating the Laplacian of a Gaussian. Experimental results verify that high quality contrast enhancement of images of simple objects may be obtained in analog circuits with careful circuit design. The 7.9 × 9.2 mm IC dissipates 350 mW from a 5 V power supply.

Ca2+ regulation by the Na+-Ca2+ exchanger in retinal horizontal cells depolarized by l-glutamate

This study is concerned with regulation of the intracellular Ca2+ concentration ([Ca2+]i) of horizontal cells isolated from cyprinid fish retinae, with the main emphasis on the role of the (Na+)-Ca2+ exchanger. An inward current was blocked by Ca2+ (4 mM) during prolonged (> 1 h) depolarization by L-glutamate (100 microM) in the whole-cell voltage-clamp configuration, suggesting the persistent activation of voltage-gated Ca2+ channels. This (Co2+)-sensitive current was absent when extracellular Na+ was replaced by Li+ to suppress (Na+)-Ca2+ exchange. Measurement of [Ca2+]i using the Fura-2 ratiometric method gave the following results. (1) L-Glutamate (100 microM) caused [Ca2+]i to increase from the resting level of 75.4+/-36.8 nM (mean +/-S.D., n = 11) to the maximum level (2.2+/-1.4 microM, n = 11) within 15 s and then to decrease to a steady level of 0.59+/-0.23 microM (n = 11). (2) Nifedipine (100 microM) lowered the L-glutamate-induced steady [Ca2+]i level, which was still higher than the resting level. (3) L-Glutamate caused [Ca2+]i to increase even after blockading the voltage-gated Ca2+ channels by nifedipine or by clamping the membrane voltage at -55 mV. (4) (Na+)-free superfusate elevated the L-glutamate-induced steady [Ca2+]i level. (5) The time course of the [Ca2+]i decrease from the L-glutamate-induced steady level to the resting level was prolonged in the (Na+)-free superfusate. These results suggest that the (Na+)-Ca2+ exchanger extrudes intracellular Ca2+ to maintain a low [Ca2+]i level by counteracting the continuous Ca2+ influx through the voltage-gated Ca2+ channels and glutamate-gated channels when horizontal cells in situ are tonically depolarized by L-glutamate released from the photoreceptors. The (Na+)-Ca2+ exchange current isolated by a voltage-clamp experiment depends exponentially on the membrane potential.