Photoresponses of human rods in vivo derived from paired-flash electroretinograms

The interpretation of multifocal binary kernels

The interpretation of binary kernels derived by means of the mulifocal ERG has frequently been the source of confusion and controversy. The aim of this paper is to provide an intuitive understanding of the concept of kernels and to demonstrate with models and examples, the relationship between the kernels and individual responses. The sample data used to illustrate the various aspects of the technique were derived and analyzed by means of the multifocal VERIS instrument using bipolar Burian-Allen contact lens electrodes. From the kernel data, response waveforms are synthesized that can be readily compared with those obtained with conventional flash, double flash and multi-flash ERG techniques. From the kernels of a single multifocal record, waveforms are synthesized that describe responses at various times after a preceding focal flash. It is shown that the observed adaptive changes in the waveforms depend not only on the flash interval but also on retinal eccentricity. Features on the response waveform exhibit independent adaptive behavior suggesting that they originate from different retinal sources. On one hand, the examples presented here illustrate the rich information on retinal dynamics contained in the kernels. On the other hand, they demonstrate how the interpretation of multifocal data can be facilitated when the kernel series is transformed into representations with more direct physiological appeal.

Spatial and temporal properties of light adaptation in the rod system

Little is known about the mechanism that regulates the sensitivity of rod system at its normal operating light levels. Two experiments are reported in this paper. First, we searched for nonlinear distortion products in rod vision that could be generated from any local adaptation process, using a sensitive experimental procedure that has demonstrated local adaptation in cone vision. No local adaptation was evident in the rod system, even at near saturating light levels. Second, to investigate the dynamics of light adaptation in the rod system we presented a uniform flickering background, sinusoidally modulated in time, and measured increment thresholds for brief test flashes that were superimposed on this background at different times during the sinusoidal flicker cycle. At frequencies less than 5-6 Hz, the rod increment threshold follows the background modulation, with a slight phase advance. When the background is modulated faster than 5-6 Hz, the increment threshold remains the same regardless of when the test flash occurred during the background cycle. Thus the rod system sensitivity, unlike that of the cone system, can only change slowly, and is set by a space-integrated signal rather than independently for different rods.

Human cone photoreceptor responses measured by the electroretinogram [correction of electoretinogram] a-wave during and after exposure to intense illumination

We recorded the a-wave of the electroretinogram from human subjects with normal vision, using a corneal fibre electrode and ganzfeld stimulation under photopic conditions, so as to extract the parameters of cone phototransduction. The amplitude of bright flash responses provided a measure of the massed circulating current of the cones, while the amplitude of dim flash responses provided a measure of the product of the fraction of cone photopigment present, and the amplification constant of transduction within the cones. In the presence of steady background illumination, the cone circulating current declined to half at 3000 photopic trolands, and to a quarter at 20 000 photopic trolands. At very early times after the delivery of a near-total bleach, we could not determine the level of circulating current as our bright flashes did not appear to saturate the a-wave (presumably because so little pigment was present). However, by 20-30 s after a total bleach, the cone circulating current had returned to its dark-adapted level. Following smaller bleaches (when ca 50 % of the pigment remained present) the bright flashes were able to saturate the a-wave even at very early times. Within 3 s of extinction of the illumination, the cone circulating current had returned to its dark-adapted level. This is at least a factor of 300 times faster than the period of ca 15 min required for full recovery of rods exposed to the same level of bleach, and indicates a major difference between rods and cones in the way that they cope with the photoproducts of bleaching. Despite the very rapid recovery of circulating current after bleaches, the recovery of dim-flash sensitivity was much slower, with a time constant of ca 1.5 min after a near-total bleach. This time course is very similar to previous measurements of the regeneration of cone photopigment, and it seems highly probable that the reduction in dim-flash sensitivity results from pigment depletion.

Dissecting the dark-adapted electroretinogram

Although gross recordings of the ganzfeld flash-evoked electroretinogram (ERG) can potentially provide information about the activity of many, if not all, retinal cell types, it is necessary to dissect the ERG into its components to realize this potential fully. Here we describe various procedures that have been used in intact mammalian eyes to identify and characterize the contributions to the dark-adapted ERG of different cells in the retinal rod pathway. These include (1) examination of the very early part of the response to a flash (believed to reflect directly the photocurrent of rods), (2) application of high-energy probe flashes to provide information about the underlying rod photoreceptor response even when this component is obscured by the responses of other cells, (3) pharmacological suppression of responses of amacrine and ganglion cells to identify the contribution of these cells and to reveal the weaker responses of bipolar cells, (4) use of pharmacological agents that block transmission of signals from rods to more proximal neurons to separate responses of rods from those of later neurons, (5) examination of the ERG changes produced by ganglion-cell degeneration or pharmacological block of nerve-spike generation to identify the contribution of spiking neurons, (6) modeling measured amplitude-energy functions and timecourse of flash responses and (7) using steady backgrounds to obtain differential reductions in sensitivity of different cell types. While some of these procedures can be applied to humans, the results described here have all been obtained in studies of the ERG of anaesthetized cats, or macaque monkeys whose retinas are very similar to those of humans.

Light adaptation and dark adaptation of human rod photoreceptors measured from the a-wave of the electroretinogram

1. We recorded the a-wave of the human electroretinogram from subjects with normal vision, using a corneal electrode and ganzfeld (full-field) light stimulation. From analysis of the rising phase of rod-isolated flash responses we determined the maximum size (amax) of the a-wave, a measure of the massed circulating current of the rods, and the amplification constant (A) of transduction within the rod photoreceptors. 2. During light adaptation by steady backgrounds the maximal response was reduced, as reported previously. amax declined approximately as I0/(I0 + IB), where IB is retinal illuminance and I0 is a constant. In different subjects I0 ranged from 40 to 100 trolands, with a mean of 70 trolands, corresponding to about 600 photoisomerizations s-1 per rod. (1 troland is the retinal illuminance that results when a surface luminance of 1 cd m-2 is viewed through a pupil area of 1 mm2.) The amplification constant A decreased only slightly in the presence of steady backgrounds. 3. Following a full bleach amax recovered along an S-shaped curve over a period of 30 min. There was no detectable response for the first 5 min, and half-maximal recovery took 13-17 min. 4. The apparent amplification constant decreased at early times after large bleaches. However, upon correction for reduced light absorption due to loss of pigment, with regeneration of rhodopsin occurring with a time constant of 9-15 min in different subjects, it appeared that the true value of A was probably unchanged by bleaching. 5. The recovery of amax following a bleach could be converted into recovery of equivalent background intensity, using a 'Crawford transformation' derived from the light adaptation results. Following bleaches ranging from 10 to > 99 %, the equivalent background intensity decayed approximately exponentially, with a time constant of about 3 min. 6. The time taken for amax to recover to a fixed proportion of its original level increased approximately linearly (rather than logarithmically) with fractional bleach, with a slope of about 12 min per 100 % bleach. Similar behaviour has previously been seen in psychophysical dark adaptation experiments, for the dependence of the 'second component' of recovery on the level of bleaching.

Insights into the function of Rim protein in photoreceptors and etiology of Stargardt's disease from the phenotype in abcr knockout mice

Rim protein (RmP) is an ABC transporter of unknown function in rod outer segment discs. The human gene for RmP (ABCR) is affected in several recessive retinal degenerations. Here, we characterize the ocular phenotype in abcr knockout mice. Mice lacking RmP show delayed dark adaptation, increased all-trans-retinaldehyde (all-trans-RAL) following light exposure, elevated phosphatidylethanolamine (PE) in outer segments, accumulation of the protonated Schiff base complex of all-trans-RAL and PE (N-retinylidene-PE), and striking deposition of a major lipofuscin fluorophore (A2-E) in retinal pigment epithelium (RPE). These data suggest that RmP functions as an outwardly directed flippase for N-retinylidene-PE. Delayed dark adaptation is likely due to accumulation in discs of the noncovalent complex between opsin and all-trans-RAL. Finally, ABCR-mediated retinal degeneration may result from "poisoning" of the RPE due to A2-E accumulation, with secondary photoreceptor degeneration due to loss of the RPE support role.

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 gamma, an activation term 1 - exp[-alpha(t - td)2]), and a decay term exp(-t/tauomega): A(t)/Amo = 1 - exp[-k86Itestu(t)]; u(t) = gamma(1 - exp[-alpha(t - td)2](exp(-t/tauomega) where td is a brief delay, tauomega is an exponential time constant, and alpha characterizes the acceleration of the activation term. For Itest up to approximately 2.57 scotopic cd s m-2, the overall time course of A(t) was well described by the above equation with gamma = 2.21, td = 3.1 ms, tauomega = 132 ms and alpha = 2.32 x 10-4 ms-2. An approximate halving of alpha 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.

Lower-level visual processing and models of light adaptation

Before there was a formal discipline of psychology, there were attempts to understand the relationship between visual perception and retinal physiology. Today, there is still uncertainty about the extent to which even very basic behavioral data (called here candidates for lower-level processing) can be predicted based upon retinal processing. Here, a general framework is proposed for developing models of lower-level processing. It is argued that our knowledge of ganglion cell function and retinal mechanisms has advanced to the point where a model of lower-level processing should include a testable model of ganglion cell function. This model of ganglion cell function, combined with minimal assumptions about the role of the visual cortex, forms a model of lower-level processing. Basic behavioral and physiological descriptions of light adaptation are reviewed, and recent attempts to model lower-level processing are discussed.

Rod photoresponses in 6-week and 4-month-old human infants

Rod-only electroretinograms (ERGs) were recorded from 6-week and 4-month-old normal human infants. The leading edge of the rod a-wave was fitted with a model of the activation phase of phototransduction to provide estimates of S (a sensitivity parameter) and RmP3 (the maximum saturated photoreceptor response) at each of the investigated ages. Both S and RmP3 increased over the first postnatal months but followed different developmental time courses with S approaching adult-like values sooner than RmP3. The changes in S and RmP3 can be interpreted within the context of a model incorporating the combined effects of increased levels of rhodopsin and the changing structure of the rod outer segment during development.

Assessing abnormal rod photoreceptor activity with the a-wave of the electroretinogram: applications and methods

The impact of a disease on phototransduction can be assessed by fitting the leading edge of the rod a-wave to high-energy flashes with a quantitative expression. Two parameters of rod receptor activity are obtained, S (sensitivity) and Rm (maximum response). In this study, the meaning of these parameters and examples of conditions that change them were examined. In addition, a new protocol was developed for obtaining these parameters. A set of three to five white flashes were first presented in the dark and then on an adapting field (30 cd/m2). Subtracting the light-adapted responses from the dark-adapted responses yielded isolated rod a-wave responses. A clinical protocol was developed based on a single white flash energy. It is possible to determine whether a disease is producing a change in S and/or Rm with this single flash energy without the use of any equations.

Null mutation in the rhodopsin kinase gene slows recovery kinetics of rod and cone phototransduction in man

Rhodopsin kinase (RK), a specialized G-protein-coupled receptor kinase expressed in retina, is involved in quenching of light-induced signal transduction in photoreceptors. The role of RK in recovery after photoactivation has been explored in vitro and in vivo experimentally but has not been specifically defined in humans. We investigated the effects on human vision of a mutation in the RK gene causing Oguchi disease, a recessively inherited retinopathy. In vitro experiments demonstrated that the mutation, a deletion of exon 5, abolishes the enzymatic activity of RK and is likely a null. Both a homozygote and heterozygote with this RK mutation had recovery phase abnormalities of rod-isolated photoresponses by electroretinography (ERG); photoactivation was normal. Kinetics of rod bleaching adaptation by psychophysics were dramatically slowed in the homozygote but normal final thresholds were attained. Light adaptation was normal at low backgrounds but became abnormal at higher backgrounds. A slight slowing of cone deactivation kinetics in the homozygote was detected by ERG. Cone bleaching adaptation and background adaptation were normal. In this human in vivo condition without a functional RK and probable lack of phosphorylation and arrestin binding to activated rhodopsin, reduction of photolyzed chromophore and regeneration processes with 11-cis-retinal probably constitute the sole pathway for recovery of rod sensitivity. The role of RK in rods would thus be to accelerate inactivation of activated rhodopsin molecules that in concert with regeneration leads to the normal rate of recovery of sensitivity. Cones may rely mainly on regeneration for the inactivation of photolyzed visual pigment, but RK also contributes to cone recovery.