John R. Hetling

Sensitivity and kinetics of mouse rod flash responses determined in vivo from paired‐flash electroretinograms

1 Electroretinograms (ERGs) were recorded corneally from C57BL/6J mice using a paired‐flash procedure in which a brief test flash at time zero was followed at time tprobe by a bright probe flash of fixed strength, and in which the probe response amplitude was determined at time t=tprobe+ 6 ms. Probe responses obtained in a series of paired‐flash trials were analysed to derive A(t), a family of amplitudes that putatively represents the massed response of the rod photoreceptors to the test flash. A central aim was to obtain a mathematical description of the normalized derived response A(t)/Amo as a function of Itest, the test flash strength. 2 With fixed tprobe (80 ≤tprobe≤ 1200 ms), A(t)/Amo was described by the saturating exponential function [1 ‐ exp(‐ktItest)], where kt is a time‐dependent sensitivity parameter. For t= 86 ms, a time near the peak of A(t), k86 was 7·0 ± 1·2 (scotopic cd s m−2)−1 (mean ± s.d.; n= 4). 3 A(t)/Amo data were analysed in relation to the equation below, a time‐generalized form of the above exponential function in which (k86Itest) is replaced by the product [k86Itestu(t)], and where u(t) is independent of the test flash strength. The function u(t) was modelled as the product of a scaling factor γ, an activation term 1 ‐ exp[‐α(t ‐ td)2]}, and a decay term exp(‐t/τω): where td is a brief delay, τω is an exponential time constant, and α characterizes the acceleration of the activation term. For Itest up to ∼2·57 scotopic cd s m−2, the overall time course of A(t) was well described by the above equation with γ= 2·21, td= 3·1 ms, τω= 132 ms and α= 2·32 × 10−4 ms−2. An approximate halving of α improved the fit of the above equation to ERG a‐wave and A(t)/Amo data obtained at t about 0‐20 ms. 4 Kinetic and sensitivity properties of A(t) suggest that it approximates the in vivo massed photocurrent response of the rods to a test flash, and imply that u(t) in the above equation is the approximate kinetic description of a unit, i.e. single photon, response.

Novel contact lens electrode array for multi-electrode electroretinography (meERG)

The multi-electrode electroretinogram (meERG) is a novel tool that may find application in early detection of retinal degenerative disease. A 33-channel contact lens electrode array has been prototyped, characterized, and tested on a human subject. The meERG was recorded with SNR well above minimum requirements for clinical electroretinography.

Neural prostheses for vision: designing a functional interface with retinal neurons.

Abstract A number of prevalent eye diseases exist which may lead to partial or total blindness, and for which there are currently no cures or means by which to restore lost sight. Based on recent progress, it has become apparent that artificial prosthetic devices, which would use electrical stimulation of neurons in the visual pathway to elicit visual percepts, are likely to some day become a viable treatment for patients blinded by these diseases. A number of recent scientific reviews have summarized general functional electrical stimulation (FES) approaches related to the visual system, and many of the technical considerations regarding fabrication, biocompatibility, stimulation thresholds and electrotoxicity. This review will address a principal outstanding question in retinal prosthesis development: the design and implementation of a functional interface with the retina. A functional interface between electrodes and retinal neurons will be stable, biocompatible, and will convey useful information to the visual system. Several parameters related to both the artificial and biological aspects of the interface must be considered; this paper will emphasize electrode design. Additional issues central to the development of prosthesis interface design, including retinal physiology, eye diseases, and existing animal models of retinal degeneration, are also summarized.

Features of visual function in the naked mole-rat Heterocephalus glaber

The eyes and visual capacity of the naked mole-rat, Heterocephalus glaber, a subterranean rodent, were evaluated using anatomical, biochemical, and functional assays, and compared to other rodents of similar body size (mouse and gerbil). The eye is small compared to mouse, yet possesses cornea, lens, and retina with typical mammalian organization. The optic nerve cross-sectional area and fiber density are ~10% and ~50% that of gerbil, respectively. Levels per unit retinal area of 11-cis and all-trans retinal, derivatives of vitamin A associated with the visual cycle, are comparable to mouse. The corneal electroretinogram (ERG) exhibits early and late negative components that scale with flash strength; raising the body temperature of this poikilothermic animal from 30°C (normal for H. glaber ) to 37°C (normal for mouse) revealed an ERG response with typically mammalian features, but greatly attenuated and with slower kinetics. Leaving the nest chamber was a behavior correlated with light onset displayed preferentially by breeding females. Optical models of five mole-rat eyes suggest reasonable, but variable, image formation at the retina, possibly related to age. Results are consistent with amorphous light detection, possibly useful for circadian entrainment or escape behavior in the event of tunnel breeches.

Dynamic and steady‐state light adaptation of mouse rod photoreceptors in vivo

1 Electroretinographic (ERG) methods were used to investigate the effects of background illumination on the responses of mouse rod photoreceptors in vivo. A paired‐flash procedure, involving the recording and analysis of the ERG a‐wave response to a bright probe flash presented after a brief test flash, was used to derive the rod response to the test flash in steady background light. A related, step‐plus‐probe procedure was used to derive the step response of the rods to backgrounds of defined strength. 2 Steady background light produced a maintained derived response that was graded with background strength. Determinations of the full time course of the derived weak‐flash response in steady background light, and of the effect of background strength on the flash response at fixed post‐test‐flash times, showed that moderate backgrounds reduce the peak amplitude and duration of the flash response. 3 The response to stepped onset of an approximately half‐saturating background (1.2 sc cd m−2) exhibited a gradual rise over the first 200‐300 ms, and an apparent subsequent relaxation to plateau amplitude within 1 s after background onset. Determinations of normalized amplitudes of the derived response to a test flash presented at 50 or 700 ms after background onset indicated substantial development of background‐induced shortening of the test flash response within this 1 s period. These findings indicate a time scale of ≈1 s or less for the near‐completion of light adaptation at this background strength. 4 Properties of the derived response to a stepped background and to test flashes presented in steady background light are in general agreement with photocurrent data obtained from mammalian rods in vitro and suggest that the present results describe, to good approximation, the in vivo desensitization of mouse rods by background light.

Real-time odor discrimination using a bioelectronic sensor array based on the insect electroantennogram.

Current trends in artificial nose research are strongly influenced by knowledge of biological olfactory systems. Insects have evolved over millions of years to detect and maneuver toward a food source or mate, or away from predators. The insect olfactory system is able to identify volatiles on a time scale that matches their ability to maneuver. Here, biological olfactory sense organs, insect antennae, have been exploited in a hybrid-device biosensor, demonstrating the ability to identify individual strands of odor in a plume passing over the sensor on a sub-second time scale. A portable system was designed to utilize the electrophysiological responses recorded from a sensor array composed of male or female antennae from four or eight different species of insects (a multi-channel electroantennogram, EAG). A computational analysis strategy that allows discrimination between odors in real time is described in detail. Following a training period, both semi-parametric and k-nearest neighbor (k-NN) classifiers with the ability to discard ambiguous responses are applied toward the classification of up to eight odors. EAG responses to individual strands in an odor plume are classified or discarded as ambiguous with a delay (sensor response to classification report) on the order of 1 s. The dependence of classification error rate on several parameters is described. Finally, the performance of the approach is compared to that of a minimal conditional risk classifier.

Chronic delivery of low-level exogenous current preserves retinal function in pigmented P23H rat.

Diffuse electrical currents delivered to the eye were investigated in a rat model of retinitis pigmentosa for potentially therapeutic effects. Low-level currents were passed between electrodes placed on the cornea and in the mouth during 30-min sessions two times per week from 4 to 16 weeks of age. Single-flash electroretinograms (ERG) were recorded and analyzed for amplitude and measures of sensitivity, and basic histology was performed. ERG a-wave amplitudes were slightly greater in treated vs. age-matched controls at 16 weeks of age, but the combined thicknesses of the outer nuclear layer and outer segment layer were similar at this age. Treated animals exhibited a significant preservation of b-wave amplitudes, and a striking preservation of rod sensitivity, measured as the stimulus strength required to reach half-saturation of the a-wave. Analysis of the leading edge of the a-wave using a delayed Gaussian function revealed a decrease in the parameter reflecting gain of the phototransduction cascade over the 12-week course of treatment, and no significant change in control animals over the same period. These results suggest that while the exogenous currents failed to preserve the number or gross structure of rods, the responsivity of individual photoreceptors was relatively preserved, perhaps via an increase in efficiency of photon capture (R/photon). This preserved functionality may delay the retraction of bipolar cell dendrites from degenerating photoreceptors.

A preparation for studying electrical stimulation of the retina in vivo in rat

A remaining challenge to the development of electronic prostheses for vision is improving the effectiveness of retinal stimulation. Electrode design and stimulus parameters need to be optimized such that the neural output from the retina conveys information to the minds eye that aids the patient in interpreting his or her environment. This optimization will require a detailed understanding of the response of the retina to electrical stimulation. The identity and response characteristics of the cellular targets of stimulation need to be defined and evaluated. Described here is an in vivo preparation for studying electrical stimulation of the retina in rat at the cellular level. The use of rat makes available a number of well-described models of retinal disease that motivate prosthesis development. Artificial stimulation can be investigated by adapting techniques traditionally employed to study the response of the retina to photic stimuli, such as recording at the cornea, single-cell recording, and pharmacological dissection of the response. Pilot studies include amplitude-intensity response data for subretinal and transretinal stimulation paradigms recorded in wild-type rats and a transgenic rat model of autosomal dominant retinitis pigmentosa. The ability to record single-unit ganglion cell activity in vivo is also demonstrated.

Neuroprotective Effects of Low Level Electrical Stimulation Therapy on Retinal Degeneration

Low-level electrical stimulation applied to the eye has been shown to have neuroprotective effects on photoreceptors and retinal ganglion cells. In this review, we compare the effects of Subretinal Electrical Stimulation (SES), Transcorneal Electrical Stimulation (TES), and Whole Eye Stimulation (WES) on preserving retinal structure and function, and visual acuity, in retinal degeneration. Similarities and differences in stimulus parameters, targeted cells and growth factor expression will be discussed with emphasis on studies that have translated laboratory findings into clinical trials.

Corneal Potential Maps Measured With Multi-Electrode Electroretinography in Rat Eyes With Experimental Lesions

Purpose Conventional full-field flash electroretinography (ERG) yields a single response waveform that can be useful in the early detection and diagnosis of many diseases affecting the retina. It is an objective measurement that probes the entire retina. However, localized areas of dysfunction have relatively small influence on ERG amplitudes compared to normal ranges. Here we evaluate the use of corneal potential maps obtained in response to full-field flash stimuli for sensitivity to local areas of retinal damage. Methods A contact lens electrode array was used to record 25 ERG waveforms simultaneously following saturating full-field flash stimuli (multi-electrode electroretinography, meERG) in rats. Waveforms were evaluated for a-wave and b-wave amplitudes; these values were normalized and further evaluated for spatial differences across the corneal surface. Cluster analysis and a support vector machine approach were used to classify meERG responses from healthy eyes and eyes with central (photocoagulation) or peripheral (cryocoagulation) experimental lesions. Results A normative normalized corneal potential map was obtained from healthy eyes (n = 26). Corneal potential maps from eyes with experimental lesions (n = 13) could be classified with sensitivity and specificity of approximately 80% based solely on the normalized spatial distribution of corneal potentials, that is, with no knowledge of absolute amplitudes. Conclusions Corneal potential maps obtained in response to full-field flash stimuli are altered in eyes with scotomas in the central and far-peripheral retina. The meERG approach yields useful spatial information following a single brief flash, analogous to body-surface potential maps used to evaluate heart and brain.