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

Cone-driven strong flash electroretinograms in healthy adults: Prevalence of negative waveforms

PURPOSE

Both rod and cone-driven signals contribute to the electroretinogram (ERG) elicited by a standard strong flash in the dark. Negative ERGs usually reflect inner retinal dysfunction. However, in diseases where rod photoreceptor function is selectively lost, a negative waveform might represent the response of the dark-adapted cone system. To investigate the dark-adapted cone-driven waveform in healthy individuals, we delivered flashes on a dim blue background, designed to saturate the rods, but minimally adapt the cones.

METHODS

ERGs were recorded, using conductive fibre electrodes, in adults from the TwinsUK cohort. Responses to 13 cd m-2 s white xenon flashes (similar to the standard DA 10 flash), delivered on a blue background, were analysed. Photopic and scotopic strengths of the background were 1.3 and 30 cd m-2, respectively; through a dilated pupil, this is expected to largely saturate the rods, but adapt the cones much less than the standard ISCEV background.

RESULTS

Mean (SD) participant age was 62.5 (11.3) years (93% female). ERGs from 203 right and 204 left eyes were included, with mean (SD) b/a ratios of 1.22 (0.28) and 1.18 (0.28), respectively (medians, 1.19 and 1.17). Proportions with negative waveforms were 23 and 26%, respectively. Right and left eye b/a ratios were strongly correlated (correlation coefficient 0.74, p < 0.0001). We found no significant correlation of b/a ratio with age.

CONCLUSIONS

Over 20% of eyes showed b/a ratios less than 1, consistent with the notion that dark-adapted cone-driven responses to standard bright flashes can have negative waveforms. The majority had ratios greater than 1. Thus, whilst selective loss of rod function can yield a negative waveform (with reduced a-wave) in some, our findings also suggest that loss of rod function can occur without necessarily yielding a negative ERG. One potential limitation is possible mild cone system adaptation by the background.

Optical coherence tomography split-spectrum amplitude-decorrelation optoretinography

This pilot study reports the development of optical coherence tomography (OCT) split-spectrum amplitude-decorrelation optoretinography (SSADOR) that measures spatially resolved photoreceptor response to light stimuli. Using spectrally multiplexed narrowband OCT, SSADOR improves sensitivity to microscopic changes without the need for cellular resolution or optical phase detection. Therefore, a large field of view (up to 3 × 1 mm² demonstrated) using conventional OCT instrument design can be achieved, paving the way for clinical translation. SSADOR promises a fast, objective, and quantifiable functional biomarker for photoreceptor damage in the macula.

Visual electrophysiology and "the potential of the potentials"

Visual electrophysiology affords direct, quantitative, objective assessment of visual pathway function at different levels, and thus yields information complementary to, and not necessarily obtainable from, imaging or psychophysical testing. The tests available, and their indications, have evolved, with many advances, both in technology and in our understanding of the neural basis of the waveforms, now facilitating more precise evaluation of physiology and pathophysiology. After summarising the visual pathway and current standard clinical testing methods, this review discusses, non-exhaustively, several developments, focusing particularly on human electroretinogram recordings. These include new devices (portable, non-mydiatric, multimodal), novel testing protocols (including those aiming to separate rod-driven and cone-driven responses, and to monitor retinal adaptation), and developments in methods of analysis, including use of modelling and machine learning. It is likely that several tests will become more accessible and useful in both clinical and research settings. In future, these methods will further aid our understanding of common and rare eye disease, will help in assessing novel therapies, and will potentially yield information relevant to neurological and neuro-psychiatric conditions.

Protocol for isolation of melanopsin and rhodopsin in the human eye using silent substitution

Melanopsin-mediated visual and non-visual functions are difficult to study in vivo. To isolate melanopsin responses, non-standard light stimulation instruments are required, with at least as many primaries as photoreceptor classes in the eye. In this protocol, we describe the physical light calibrations of the display instrumentation, control of stimulus artefacts, and correction of individual between-eye differences in human observers. The protocol achieves complete photoreceptor silent substitution in psychophysical, pupillometry, and electroretinographic experiments for probing melanopsin, rod, and cone function. For complete details on the use and execution of this protocol, please refer to Uprety et al. (2022).1.

Human retinal dark adaptation tracked in vivo with the electroretinogram: insights into processes underlying recovery of cone- and rod-mediated vision

The substantial time taken for regaining visual sensitivity (dark adaptation) following bleaching exposures has been investigated for over a century. Psychophysical studies yielded the classic biphasic curve representing recovery of cone-driven and rod-driven vision. The electroretinogram (ERG) permits direct assessment of recovery at the level of the retina (photoreceptors, bipolar cells), with the first report over 70 years ago. Over the last two decades, ERG studies of dark adaptation have generated insights into underlying physiological processes. After large bleaches, rod photoreceptor circulating current, estimated from the rod-isolated bright-flash ERG a-wave, takes 30 min to recover, indicating that products of bleaching, thought to be free opsin (unbound to 11-cis-retinal), continue to activate phototransduction, shutting off rod circulating current. In contrast, cone current, assessed with cone-driven bright-flash ERG a-waves, recovers within 100 ms following similar exposures, suggesting that free opsin is less able to shut off cone current. The cone-driven dim-flash a-wave can be used to track recovery of cone photopigment, showing regeneration is 'rate-limited' rather than first order. Recoveries of the dim-flash ERG b-wave are consistent also with rate-limited rod photopigment regeneration (where free opsin, desensitising the visual system as an 'equivalent background', is removed by rate-limited delivery of 11-cis-retinal). These findings agree with psychophysical and retinal densitometry studies, although there are unexplained points of divergence. Post-bleach ERG recovery has been explored in age-related macular degeneration and in trials of visual cycle inhibitors for retinal diseases. ERG tracking of dark adaptation may prove useful in future clinical contexts.

Electrical responses from human retinal cone pathways associate with a common genetic polymorphism implicated in myopia

Myopia is the commonest visual impairment. Several genetic loci confer risk, but mechanisms by which they do this are unknown. Retinal signals drive eye growth, and myopia usually results from an excessively long eye. The common variant most strongly associated with myopia is near the GJD2 gene, encoding connexin-36, which forms retinal gap junctions. Light-evoked responses of retinal neurons can be recorded noninvasively as the electroretinogram (ERG). We analyzed these responses from 186 adult twin volunteers who had been genotyped at this locus. Participants underwent detailed ERG recordings incorporating international standard stimuli as well as experimental protocols aiming to separate dark-adapted rod- and cone-driven responses. A mixed linear model was used to explore association between allelic dosage at the locus and international standard ERG parameters after adjustment for age, sex, and family structure. Significant associations were found for parameters of light-adapted, but not dark-adapted, responses. Further investigation of isolated rod- and cone-driven ERGs confirmed associations with cone-driven, but not rod-driven, a-wave amplitudes. Comparison with responses to similar experimental stimuli from a patient with a prior central retinal artery occlusion, and from two patients with selective loss of ON-bipolar cell signals, was consistent with the associated parameters being derived from signals from cone-driven OFF-bipolar cells. Analysis of single-cell transcriptome data revealed strongest GJD2 expression in cone photoreceptors; bipolar cell expression appeared strongest in OFF-bipolar cells and weakest in rod-driven ON-bipolar cells. Our findings support a potential role for altered signaling in cone-driven OFF pathways in myopia development.

Light adaptation characteristics of melanopsin

Following photopigment bleaching, the rhodopsin and cone-opsins show a characteristic exponential regeneration in the dark with a photocycle dependent on the retinal pigment epithelium. Melanopsin pigment regeneration in animal models requires different pathways to rods and cones. To quantify melanopsin-mediated light adaptation in humans, we first estimated its photopigment regeneration kinetics through the photo-bleach recovery of the intrinsic melanopsin pupil light response (PLR). An intense broadband light (~120,000 Td) bleached 43% of melanopsin compared to 86% of the cone-opsins. Recovery from a 43% bleach was 3.4X slower for the melanopsin than cone-opsin. Post-bleach melanopsin regeneration followed an exponential growth with a 2.5 min time-constant (τ) that required 11.2 min for complete recovery; the half-bleaching level (I_p) was ~ 4.47 log melanopic Td (16.10 log melanopsin effective photons.cm^-2.s^-1; 8.25 log photoisomerisations.photoreceptor^-1.s^-1). The effect on the cone-directed PLR of the level of the melanopsin excitation during continuous light adaptation was then determined. We observed that cone-directed pupil constriction amplitudes increased by ~ 10% when adapting lights had a higher melanopic excitation but the same mean photometric luminance. Our findings suggest that melanopsin light adaptation enhances cone signalling along the non-visual retina-brain axis. Parameters τ and I_p will allow estimation of the level of melanopsin bleaching in any light units; the data have implications for quantifying the relative contributions of putative melanopsin pathways to regulate the post-bleach photopigment regeneration and adaptation.

Prevalence of electronegative electroretinograms in a healthy adult cohort

Objective

An electronegative electroretinogram (ERG) can indicate important ocular or systemic disease. This study explored the prevalence of electronegative responses to dark-adapted stimuli in a largely healthy cohort.

Methods and Analysis

211 participants recruited from the TwinsUK cohort underwent ERG testing incorporating international standard (International Society for Clinical Electrophysiology of Vision (ISCEV)) protocols and additional stimuli. Responses were recorded using conductive fibre electrodes, following pupil dilation and 20 min dark adaptation. Responses analysed were to the ISCEV standard and strong flashes (3.0 and 10 cd/m² s), and to additional white flashes (0.67–67 cd/m² s). A-wave and b-wave amplitudes were extracted; b:a ratios were calculated and proportions of eyes with ratios<1 were noted.

Results

Mean (SD) age was 62.4 (11.4) years (median, 64.3; range 23–86 years). 93% were female. Mean (SD) b:a ratios for right and left eyes, respectively, were 1.86 (0.33) and 1.81 (0.29) for the standard flash, and 1.62 (0.25) and 1.58 (0.23) for the stronger flash; average b:a ratio was lower for the stronger flash (p<0.0001). No waveforms were electronegative. For additional flashes, b:a ratio decreased with increasing flash strength. No electronegative waveforms were seen except in three eyes (0.7%) for the strongest flash; in some cases, drift in the waveform may have artefactually reduced the b:a ratio.

Conclusion

For standard dark-adapted stimuli, no participants had electronegative waveforms. The findings support the notion that electronegative waveforms (in response to standard flash strengths) are unusual, and should prompt further investigation.

Functional Imaging of the Outer Retinal Complex using High Fidelity Imaging Retinal Densitometry

We describe a new technique, high fidelity Imaging Retinal Densitometry (IRD), which probes the functional integrity of the outer retinal complex. We demonstrate the ability of the technique to map visual pigment optical density and synthesis rates in eyes with and without macular disease. A multispectral retinal imaging device obtained precise measurements of retinal reflectance over space and time. Data obtained from healthy controls and 5 patients with intermediate AMD, before and after photopigment bleaching, were used to quantify visual pigment metrics. Heat maps were plotted to summarise the topography of rod and cone pigment kinetics and descriptive statistics conducted to highlight differences between those with and without AMD. Rod and cone visual pigment synthesis rates in those with AMD (v = 0.043 SD 0.019 min⁻¹ and v = 0.119 SD 0.046 min⁻¹, respectively) were approximately half those observed in healthy controls (v = 0.079 SD 0.024 min⁻¹ for rods and v = 0.206 SD 0.069 min⁻¹ for cones). By mapping visual pigment kinetics across the central retina, high fidelity IRD provides a unique insight into outer retinal complex function. This new technique will improve the phenotypic characterisation, diagnosis and treatment monitoring of various ocular pathologies, including AMD.

Impact of Bleaching on Photoreceptors in Different Intermediate AMD Phenotypes

Purpose

To investigate photoreceptors' structural changes after photobleaching exposure in intermediate age-related macular degeneration (iAMD) eyes with and without reticular pseudodrusen (RPD).

Methods

In this prospective, cross-sectional study, were enrolled iAMD patients and healthy controls. Patients and controls underwent repeated imaging with spectral-domain optical coherence tomography (SD-OCT), at baseline and at three intervals after bleaching, during the subsequent recovery in darkness. Structural changes in photoreceptors were investigated in the foveal region and in four perifoveal areas.

Results

Twenty eyes of 20 iAMD patients (12 with RPD and 8 without RPD) and 15 age-matched healthy controls were enrolled. At baseline, the photoreceptor outer segment (OS) volume was significantly reduced in iAMD eyes with RPD compared with controls, in the foveal and perifoveal regions. In healthy subjects, a precocious increase in OS volume was observed after bleaching in the foveal region, and a rapid recovery to baseline values was recorded. In the perifoveal regions, an increase in OS volume was observed 10 minutes after light onset. In contrast, in iAMD subjects with RPD an altered response to photobleaching, in the foveal and superior and inferior perifoveal regions, was recorded.

Conclusions

Our imaging evidences support the hypothesis that dark adaptation is more altered in eyes with RPD. The structural modifications may explain the functional increased damage of the retinal pigment epithelium and photoreceptors reported in eyes with RPD.

Translational Relevance

OCT imaging may be used to assess dark adaptation in AMD eyes.

Electrophysiological measures of dysfunction in early-stage diabetic retinopathy: No correlation between cone phototransduction and oscillatory potential abnormalities

PURPOSE

To define the relationship between abnormalities in the activation phase of cone phototransduction and the oscillatory potentials (OPs) of the light-adapted electroretinogram in diabetics who have mild or no retinopathy.

METHODS

Subjects included 20 non-diabetic controls and 40 type-2 diabetics (20 had no clinically apparent diabetic retinopathy [NDR] and 20 had mild nonproliferative DR). Single flash responses for a series of stimulus retinal illuminances were measured under light-adapted conditions using conventional techniques. The a-waves of the responses were fit with a delayed Gaussian model to derive R_mp3 (maximum amplitude of the massed photoreceptor response) and S (phototransduction sensitivity). OPs were extracted from the responses by conventional band-pass filtering.

RESULTS

Analysis of variance (ANVOA) indicated that both diabetic groups had significant OP amplitude and S reductions compared to the controls, whereas R_mp3 did not differ significantly among the groups. Although log OP amplitude and log R_mp3 were significantly correlated for the control subjects for each flash retinal illuminance (all r > 0.49, p < 0.03), log OP amplitude and log R_mp3 were not correlated for either diabetic group for any flash retinal illuminance (all r ≤ 0.36, p ≥ 0.13). Log OP amplitude and log S were generally not correlated significantly for the control or diabetic groups.

CONCLUSION

OP amplitude losses do not appear to be related to reduced cone sensitivity in early-stage diabetic retinopathy. This suggests that diabetes may separately affect cone function, as evidenced by cone phototransduction sensitivity losses, and inner-retina function, as evidenced by OP amplitude losses.

Bleaching effects and fluorescence lifetime imaging ophthalmoscopy

This study investigates the influence of photopigment bleaching on autofluorescence lifetimes in the fundus in 21 young healthy volunteers. Three measurements of 30° retinal fields in two spectral channels (SSC: 498-560 nm, LSC: 560-720 nm) were obtained for each volunteer using fluorescence lifetime imaging ophthalmoscopy (FLIO). After dark-adaptation by wearing a custom-made lightproof mask for 30 minutes, the first FLIO-measurement was recorded (dark-adapted state). Subsequently, the eye was bleached for 1 minute (luminance: 3200 cd/m²), followed by a second FLIO-measurement (bleached state). Following an additional 10 minute dark adaptation using the mask, a final FLIO-measurement was recorded (recovered state). Average values of the fluorescence lifetimes were calculated from within different areas of a standardized early treatment diabetic retinopathy study (ETDRS) grid (central area, inner and outer rings). The acquisition time in the bleached state was significantly shortened by approximately 20%. The SSC did not show any significant changes in fluorescence lifetimes with photopigment bleaching, only the LSC showed small but significant bleaching-related changes in the fluorescence lifetimes τ₁ and τ₂ from all regions, as well as the mean fluorescence lifetime in the central area. The fluorescence lifetime differences caused by bleaching were by far less significant than pathological changes caused by eye diseases. The magnitudes of fluorescence lifetime changes are <10% and do not interfere with healthy or disease related FLIO patterns. Thus, we conclude that bleaching is not a relevant confounder in current clinical applications of FLIO.

Photoreceptor Layer Thickness Changes During Dark Adaptation Observed With Ultrahigh-Resolution Optical Coherence Tomography

Purpose

To examine outer retinal band changes after flash stimulus and subsequent dark adaptation with ultrahigh-resolution optical coherence tomography (UHR-OCT).

Methods

Five dark-adapted left eyes of five normal subjects were imaged with 3-μm axial-resolution UHR-OCT during 30 minutes of dark adaptation following 96%, 54%, 23%, and 0% full-field and 54% half-field rhodopsin bleach. We identified the ellipsoid zone inner segment/outer segment (EZ[IS/OS]), cone interdigitation zone (CIZ), rod interdigitation zone (RIZ), retinal pigment epithelium (RPE), and Bruch's membrane (BM) axial positions and generated two-dimensional thickness maps of the EZ(IS/OS) to the four bands. The average thickness over an area of the thickness map was compared against that of the dark-adapted baselines. The time-dependent thickness changes (photoresponses) were statistically compared against 0% bleach. Dark adaptometry was performed with the same bleaching protocol.

Results

The EZ(IS/OS)-CIZ photoresponse was significantly different at 96% (P < 0.0001) and 54% (P = 0.006) bleach. At all three bleaching levels, the EZ(IS/OS)-RIZ, -RPE, and -BM responses were significantly different (P < 0.0001). The EZ(IS/OS)-CIZ and EZ(IS/OS)-RIZ time courses were similar to the recovery of rod- and cone-mediated sensitivity, respectively, measured with dark adaptometry. The maximal EZ(IS/OS)-CIZ and EZ(IS/OS)-RIZ response magnitudes doubled from 54% to 96% bleach. Both EZ(IS/OS)-RPE and EZ(IS/OS)-BM responses resembled dampened oscillations that were graded in amplitude and duration with bleaching intensity. Half-field photoresponses were localized to the stimulated retina.

Conclusions

With noninvasive, near-infrared UHR-OCT, we characterized three distinct, spatially localized photoresponses in the outer retinal bands. These photoresponses have potential value as physical correlates of photoreceptor function.

Relative Genetic and Environmental Contributions to Variations in Human Retinal Electrical Responses Quantified in a Twin Study

Purpose

To estimate heritability of parameters of human retinal electrophysiology and to explore which parameters change with age.

Design

Prospective, classic twin study.

Participants

Adult monozygotic and dizygotic twin pairs recruited from the TwinsUK cohort.

Methods

Electroretinogram responses were recorded using conductive fiber electrodes in response to stimuli incorporating standards set by the International Society for the Clinical Electrophysiology of Vision. These parameters were extracted; in addition, photopic negative-response (PhNR; originating from retinal ganglion cells) and i-wave components were extracted from responses to the photopic single flash. Parameter values were averaged from both eyes.

Main Outcome Measures

Mean values were calculated for the cohort. Correlation coefficients with age were calculated (averaging parameters from both twins from each pair). Coefficients of intrapair correlation were calculated for monozygotic and dizygotic twins. Age-adjusted heritability estimates were derived using standard maximum likelihood structural equation twin modeling.

Results

Responses were recorded from 210 participants in total (59 monozygotic and 46 dizygotic twin pairs). Ninety-three percent were women. Mean age for the cohort was 62.4 years (standard deviation, 11.4 years). In general, response amplitudes correlated negatively, and implicit times positively, with age. Correlations were statistically significant (P < 0.05) and moderate or strong (coefficient, >0.35) for the following parameters: scotopic standard and bright-flash a-wave implicit times, photopic 30-Hz flicker and single-flash b-wave implicit times, and PhNR and i-wave implicit times. Intrapair correlations were higher for monozygotic than dizygotic twins, suggesting important genetic influences. Age-adjusted estimates of heritability were significant for all parameters (except scotopic dim-flash b-wave implicit time), ranging from 0.34 to 0.85. Highest estimates were for photopic single-flash a-wave and b-wave amplitudes (0.84 and 0.85, respectively).

Conclusions

This study explored heritability of retinal electrophysiologic parameters and included measurements reflecting ganglion cell function. Most parameters showed significant heritability, indicating that genetic factors are important, determining up to 85% of the variance in some cone system response parameters. Scotopic responses tended to show lower heritability (possibly relating to greater rod system susceptibility to environmental factors). Future studies can explore the identity of these genetic factors, improving our understanding of how they shape retinal function.

Allosteric modulation of the substrate specificity of acyl-CoA wax alcohol acyltransferase 2

The esterification of alcohols with fatty acids is a universal mechanism to form inert storage forms of sterols, di- and triacylglycerols, and retinoids. In ocular tissues, formation of retinyl esters is an essential step in the enzymatic regeneration of the visual chromophore (11-cis-retinal). Acyl-CoA wax alcohol acyltransferase 2 (AWAT2), also known as multifunctional O-acyltransferase (MFAT), is an integral membrane enzyme with a broad substrate specificity that has been shown to preferentially esterify 11-cis-retinol and thus contribute to formation of a readily available pool of cis retinoids in the eye. However, the mechanism by which this promiscuous enzyme can gain substrate specificity is unknown. Here, we provide evidence for an allosteric modulation of the enzymatic activity by 11-cis retinoids. This regulation is independent from cellular retinaldehyde-binding protein (CRALBP), the major cis-retinoid binding protein. This positive-feedback regulation leads to decreased esterification rates for 9-cis, 13-cis, or all-trans retinols and thus enables preferential synthesis of 11-cis-retinyl esters. Finally, electron microscopy analyses of the purified enzyme indicate that this allosteric effect does not result from formation of functional oligomers. Altogether, these data provide the experimental basis for understanding regulation of AWAT2 substrate specificity.

The neural retina in retinopathy of prematurity

Retinopathy of prematurity (ROP) is a neurovascular disease that affects prematurely born infants and is known to have significant long term effects on vision. We conducted the studies described herein not only to learn more about vision but also about the pathogenesis of ROP. The coincidence of ROP onset and rapid developmental elongation of the rod photoreceptor outer segments motivated us to consider the role of the rods in this disease. We used noninvasive electroretinographic (ERG), psychophysical, and retinal imaging procedures to study the function and structure of the neurosensory retina. Rod photoreceptor and post-receptor responses are significantly altered years after the preterm days during which ROP is an active disease. The alterations include persistent rod dysfunction, and evidence of compensatory remodeling of the post-receptor retina is found in ERG responses to full-field stimuli and in psychophysical thresholds that probe small retinal regions. In the central retina, both Mild and Severe ROP delay maturation of parafoveal scotopic thresholds and are associated with attenuation of cone mediated multifocal ERG responses, significant thickening of post-receptor retinal laminae, and dysmorphic cone photoreceptors. These results have implications for vision and control of eye growth and refractive development and suggest future research directions. These results also lead to a proposal for noninvasive management using light that may add to the currently invasive therapeutic armamentarium against ROP.

Intrinsic optical signal imaging of retinal physiology: a review

Intrinsic optical signal (IOS) imaging promises to be a noninvasive method for high-resolution examination of retinal physiology, which can advance the study and diagnosis of eye diseases. While specialized optical instruments are desirable for functional IOS imaging of retinal physiology, in depth understanding of multiple IOS sources in the complex retinal neural network is essential for optimizing instrument designs. We provide a brief overview of IOS studies and relationships in rod outer segment suspensions, isolated retinas, and intact eyes. Recent developments of line-scan confocal and functional optical coherence tomography (OCT) instruments have allowed in vivo IOS mapping of photoreceptor physiology. Further improvements of the line-scan confocal and functional OCT systems may provide a feasible solution to pursue functional IOS mapping of human photoreceptors. Some interesting IOSs have already been detected in inner retinal layers, but better development of the IOS instruments and software algorithms is required to achieve optimal physiological assessment of inner retinal neurons.

Detection of early age-related macular degeneration using novel functional parameters of the focal cone electroretinogram

The focal cone electroretinogram is a sensitive marker for macular disease, but have we unlocked its full potential? Typically assessment of waveform parameters is subjective and focuses on a small number of locations (e.g. the a-wave). This study evaluated the discriminatory and diagnostic potential of 4 conventional and 15 novel, objectively determined, parameters in patients with early Age-related Macular Degeneration. Focal cone electroretinograms were recorded in 54 participants with early Age-related Macular Degeneration (72.9±8.2 years) and 54 healthy controls (69±7.7 years). Conventional a and b wave amplitudes and implicit times were measured and compared to novel parameters derived from both the 1st and 2nd derivatives and the frequency-domain power spectrum of the electroretinogram.Statistically significant differences between groups were shown for all conventional parameters, the majority of 1st and 2nd derivative parameters and the power spectrum at 25 and 30 Hz. Receiver operating characteristics showed that both conventional and 1st and 2nd derivative implicit times had provided the best diagnostic potential. A regression model showed a small improvement over any individual parameter investigated. The non-conventional parameters enhanced the objective evaluation of the focal electroretinogram, especially when the amplitude was low. Furthermore, the novel parameters described here allow the implicit time of the electroretinogram to be probed at points other than the peaks of the a and b waves. Consequently these novel analysis techniques could prove valuable in future electrophysiological investigation, detection and monitoring of Age-related Macular Degeneration.

Measurement of cone dark adaptation: a comparison of four psychophysical methods

PURPOSE

Dark adaptometry is an important clinical tool for the diagnosis of a range of conditions, including age-related macular degeneration. In order to identify the most robust, clinically applicable technique for the measurement of cone dark adaptation, the repeatability and agreement of four psychophysical methods were assessed.

METHODS

Data were obtained from 31 healthy adults on two occasions, using four psychophysical methods. Participants' pupils were dilated, and 96 % of cone photopigment was bleached before threshold was monitored in the dark using one of the techniques, selected at random. This procedure was repeated for each of the remaining methods. An exponential recovery function was fitted to all threshold recovery data. The coefficient of repeatability (CoR) was calculated to assess the repeatability of the methods, and a repeated-measures analysis of variance was used to compare mean recovery parameters.

RESULTS

All four methods demonstrated a similar level of intersession repeatability for measurement of cone recovery, yielding CoRs between 1.18 and 1.56 min. There were no statistically significant differences in estimates of mean time constant of cone recovery (cone τ) between the four methods (p = 0.488); however, significant differences between initial and final cone thresholds were reported (p < 0.005).

CONCLUSIONS

All of the techniques were capable of monitoring the rapid changes in visual threshold that occur during cone dark adaptation, and the repeatability of the techniques was similar. This indicates that despite the respective advantages and disadvantages of these psychophysical techniques, all four methods would be suitable for measuring cone dark adaptation in clinical practice.

The effect of pre-adapting light intensity on dark adaptation in early age-related macular degeneration

BACKGROUND

This study aimed to identify the pre-adapting light intensity that generated the maximum separation in the parameters of dark adaptation between participants with early age-related macular degeneration (AMD) and healthy control participants in the minimum recording time.

METHODS

Cone dark adaptation was monitored in 10 participants with early AMD and 10 age-matched controls after exposure to three pre-adapting light intensities, using an achromatic annulus (12° radius) centred on the fovea. Threshold recovery data were modelled, and the time constant of cone recovery (τ), final cone threshold, and time to rod-cone-break (RCB) were determined. The diagnostic potential of these parameters at all pre-adapting intensities was evaluated by constructing receiver operating characteristic (ROC) curves.

RESULTS

There were significant differences between those with early AMD and healthy controls in cone τ and time to RCB (p < 0.05) at all pre-adapting 'bleaching' intensities. ROC curves showed that the diagnostic potential of dark adaptometry was high following exposure to all three pre-adapting intensities, generating an area under the curve in excess of 0.87 ± 0.08 for cone τ and time to RCB for all conditions.

CONCLUSIONS

Dark adaptation was shown to be highly diagnostic for early AMD across a range of pre-adapting light intensities, and therefore, the lower pre-adapting intensities evaluated in this study may be used to expedite dark adaptation measurement in the clinic without compromising the integrity of the data obtained. This study reinforces the suggestion that cone and rod dark adaptation are good candidate biomarkers for early AMD.

Photoreceptor and postreceptor responses in congenital stationary night blindness

PURPOSE

To investigate photoreceptor and postreceptor retinal function in patients with congenital stationary night blindness (CSNB).

METHODS

Forty-one patients with CSNB (ages 0.19-32 years) were studied. ERG responses to a series of full-field stimuli were obtained under scotopic and photopic conditions and were used to categorize the CSNB patients as complete (cCSNB) or incomplete (iCSNB). Rod and cone photoreceptor (R(ROD), S(ROD), R(CONE), S(CONE)) and rod-driven postreceptor (V(MAX), log σ) response parameters were calculated from the a- and b-waves. Cone-driven responses to 30 Hz flicker and ON and OFF responses to a long duration (150 ms) flash were also obtained. Dark-adapted thresholds were measured. Analysis of variance was used to compare data from patients with cCSNB, patients with iCSNB, and controls.

RESULTS

We found significant reduction in saturated photoreceptor amplitude (R(ROD), R(CONE)) but normal photoreceptor sensitivity (S(ROD), S(CONE)) in both CSNB groups. Rod-driven postreceptor response amplitude (V(MAX)) and sensitivity (log σ) were significantly reduced in CSNB. Log σ was significantly worse in cCSNB than in iCSNB; this was the only scotopic parameter that differed between the two CSNB groups. Photopic b-wave amplitude increased monotonically with stimulus strength in CSNB patients rather than showing a normal photopic hill. The amplitude of the 30-Hz flicker response was reduced compared with controls, more so in iCSNB than in cCSNB. The mean dark-adapted threshold was significantly elevated in CSNB, more so in cCSNB than in iCSNB.

CONCLUSIONS

These results are evidence of normal photoreceptor function (despite the low saturated photoresponse amplitude) and anomalous postreceptor retinal circuitry.

In vivo confocal intrinsic optical signal identification of localized retinal dysfunction

PURPOSE

The purposes of this study were to investigate the physiological mechanism of stimulus-evoked fast intrinsic optical signals (IOSs) recorded in dynamic confocal imaging of the retina, and to demonstrate the feasibility of in vivo confocal IOS mapping of localized retinal dysfunctions.

METHODS

A rapid line-scan confocal ophthalmoscope was constructed to achieve in vivo confocal IOS imaging of frog (Rana pipiens) retinas at cellular resolution. In order to investigate the physiological mechanism of confocal IOS, comparative IOS and electroretinography (ERG) measurements were made using normal frog eyes activated by variable-intensity stimuli. A dynamic spatiotemporal filtering algorithm was developed to reject the contamination of hemodynamic changes on fast IOS recording. Laser-injured frog eyes were employed to test the potential of confocal IOS mapping of localized retinal dysfunctions.

RESULTS

Comparative IOS and ERG experiments revealed a close correlation between the confocal IOS and retinal ERG, particularly the ERG a-wave, which has been widely used to evaluate photoreceptor function. IOS imaging of laser-injured frog eyes indicated that the confocal IOS could unambiguously detect localized (30 μm) functional lesions in the retina before a morphological abnormality is detectable.

CONCLUSIONS

The confocal IOS predominantly results from retinal photoreceptors, and can be used to map localized photoreceptor lesion in laser-injured frog eyes. We anticipate that confocal IOS imaging can provide applications in early detection of age-related macular degeneration, retinitis pigmentosa, and other retinal diseases that can cause pathological changes in the photoreceptors.

Origin and control of the dominant time constant of salamander cone photoreceptors

Recovery of the light response in vertebrate photoreceptors requires the shutoff of both active intermediates in the phototransduction cascade: the visual pigment and the transducin–phosphodiesterase complex. Whichever intermediate quenches more slowly will dominate photoresponse recovery. In suction pipette recordings from isolated salamander ultraviolet- and blue-sensitive cones, response recovery was delayed, and the dominant time constant slowed when internal [Ca²⁺] was prevented from changing after a bright flash by exposure to 0Ca²⁺/0Na⁺ solution. Taken together with a similar prior observation in salamander red-sensitive cones, these observations indicate that the dominance of response recovery by a Ca²⁺-sensitive process is a general feature of amphibian cone phototransduction. Moreover, changes in the external pH also influenced the dominant time constant of red-sensitive cones even when changes in internal [Ca²⁺] were prevented. Because the cone photopigment is, uniquely, exposed to the external solution, this may represent a direct effect of protons on the equilibrium between its inactive Meta I and active Meta II forms, consistent with the notion that the process dominating recovery of the bright flash response represents quenching of the active Meta II form of the cone photopigment.

Speed, adaptation, and stability of the response to light in cone photoreceptors: the functional role of Ca-dependent modulation of ligand sensitivity in cGMP-gated ion channels

The response of cone photoreceptors to light is stable and reproducible because of the exceptional regulation of the cascade of enzymatic reactions that link visual pigment (VP) excitation to the gating of cyclic GMP (cGMP)-gated ion channels (cyclic nucleotide–gated [CNG]) in the outer segment plasma membrane. Regulation is achieved in part through negative feedback control of some of these reactions by cytoplasmic free Ca²⁺. As part of the control process, Ca²⁺ regulates the phosphorylation of excited VP, the activity of guanylate cyclase, and the ligand sensitivity of the CNG ion channels. We measured photocurrents elicited by stimuli in the form of flashes, steps, and flashes superimposed on steps in voltage-clamped single bass cones isolated from striped bass retina. We also developed a computational model that comprises all the known molecular events of cone phototransduction, including all Ca-dependent controls. Constrained by available experimental data in bass cones and cone transduction biochemistry, we achieved an excellent match between experimental photocurrents and those simulated by the model. We used the model to explore the physiological role of CNG ion channel modulation. Control of CNG channel activity by both cGMP and Ca²⁺ causes the time course of the light-dependent currents to be faster than if only cGMP controlled their activity. Channel modulation also plays a critical role in the regulation of the light sensitivity and light adaptation of the cone photoresponse. In the absence of ion channel modulation, cone photocurrents would be unstable, oscillating during and at the offset of light stimuli.

Visible light induced ocular delayed bioluminescence as a possible origin of negative afterimage

The delayed luminescence of biological tissues is an ultraweak reemission of absorbed photons after exposure to external monochromatic or white light illumination. Recently, Wang, Bókkon, Dai and Antal (2011) [10] presented the first experimental proof of the existence of spontaneous ultraweak biophoton emission and visible light induced delayed ultraweak photon emission from in vitro freshly isolated rat's whole eye, lens, vitreous humor and retina. Here, we suggest that the photobiophysical source of negative afterimage can also occur within the eye by delayed bioluminescent photons. In other words, when we stare at a colored (or white) image for few seconds, external photons can induce excited electronic states within different parts of the eye that is followed by a delayed reemission of absorbed photons for several seconds. Finally, these reemitted photons can be absorbed by non-bleached photoreceptors that produce a negative afterimage. Although this suggests the photobiophysical source of negative afterimages is related retinal mechanisms, cortical neurons have also essential contribution in the interpretation and modulation of negative afterimages.

Contribution of post-receporal cells to the cone a-wave of the human electroretinogram in congenital stationary night blindness and autoimmune-like retinopathy

In normal subjects the later part of the cone a-wave to a brief flash increases in amplitude after 50-100 ms darkness due to a contribution from secondary hyperpolarising cells. We recorded these responses along with clinical ON and OFF ERGs in patients with inner retinal dysfunction to see if this part of the a-wave is affected. Patients with autoimmune-like retinopathy and CSNB2 had abnormal ON and OFF responses but the a-wave increased in amplitude in the dark as in normals. Conversely, the OFF-response was normal in CSNB1 but the a-wave did not increase in the dark. Contrary to expectation these results show some hyperpolarising cell function in autoimmune-like disease and CSNB2 and some OFF-pathway abnormality in CSNB1. The a- and d-wave are needed to assess OFF-pathway function.

Dark adaptation recovery of human rod bipolar cell response kinetics estimated from scotopic b-wave measurements

We recorded ganzfeld scotopic ERGs to examine the responses of human rod bipolar cells in vivo, during dark adaptation recovery following bleaching exposures, as well as during adaptation to steady background lights. In order to be able to record responses at relatively early times in recovery, we utilized a 'criterion response amplitude' protocol in which the test flash strength was adjusted to elicit responses of nearly constant amplitude. In order to provide accurate and unbiased measures of response kinetics, we utilized a curve-fitting procedure to fit a smooth function to the measured responses in the vicinity of the peak, thereby extracting both the time-to-peak and the amplitude of the responses. Following bleaching exposures, the responses exhibited both desensitization and accelerated kinetics. During early post-bleach recovery, the flash sensitivity and time-to-peak varied according to a power-law expression (with an exponent of 6), as found in the presence of steady background light. This light-like phenomenon, however, appeared to be set against the backdrop of a second, more slowly recovering 'pure' desensitization, most clearly evident at late post-bleach times. The post-bleach 'equivalent background intensity' derived from measurements of flash sensitivity faded initially with an S2 slope of approximately 0.24 decades min(-1), and later as a gentle S3 tail. When calculated from kinetics, the results displayed only the S2 slope. While the recovery of rod bipolar cell response kinetics can be described accurately by a declining level of opsin in the rods, the sensitivity of these cells is reduced further than expected by this mechanism alone.

Relative course of retinal nerve fiber layer birefringence and thickness and retinal function changes after optic nerve transection

PURPOSE

To test the hypothesis that alterations of RNFL birefringence precede changes in RNFL thickness in an experimental model of RGC injury and, secondarily, to determine the time course of RGC functional abnormalities relative to RNFL birefringence and thickness changes.

METHODS

RNFL birefringence was measured by scanning laser polarimetry (GDx VCC; Carl Zeiss Meditec, Inc., Dublin, CA). RNFL thickness was measured by spectral domain optical coherence tomography (SD-OCT, Spectralis HRA+OCT; Heidelberg Engineering, GmbH, Heidelberg, Germany). Retinal function was assessed by three forms of electroretinography (ERG): slow-sequence multifocal (mf)ERG (VERIS; EDI, San Mateo, CA); pattern-reversal (P)ERG (Utas-E3000; LKC Technologies, Inc. Gaithersburg, MD); and photopic full-field flash (ff)ERG (Utas-E3000; LKC Technologies). All measurements were obtained in both eyes of four adult rhesus macaque monkeys (Macaca mulatta) during two baseline sessions, and again 1 week and 2 weeks after unilateral optic nerve transection (ONT).

RESULTS

ONT was successfully completed in three subjects. RNFL birefringence declined by 15% 1 week after ONT (P = 0.043), whereas there was no significant change in RNFL thickness (+1%, P = 0.42). Two weeks after ONT, RNFL retardance had declined by 39% (P = 0.018), whereas RNFL thickness had declined by only 15% (P = 0.025). RGC functional abnormalities were present 1 week after ONT, including decreased amplitudes relative to baseline of the mfERG high-frequency components (-65%, P = 0.018), the PERG N95 component (-70%, P = 0.007), and the photopic negative response of the ffERG (-44%, P = 0.005).

CONCLUSIONS

RNFL birefringence declined before and faster than RNFL thickness after ONT. RGC functional abnormalities were present 1 week after ONT, when RNFL thickness had not yet begun to change. RNFL birefringence changes after acute RGC injury are associated with RGC dysfunction. Together, they reflect RGC abnormalities that precede axonal caliber changes and loss.

Contribution of voltage-gated sodium channels to the b-wave of the mammalian flash electroretinogram

Voltage-gated sodium channels (Na(v) channels) in retinal neurons are known to contribute to the mammalian flash electroretinogram (ERG) via activity of third-order retinal neurons, i.e. amacrine and ganglion cells. This study investigated the effects of tetrodotoxin (TTX) blockade of Na(v) channels on the b-wave, an ERG wave that originates mainly from activity of second-order retinal neurons. ERGs were recorded from anaesthetized Brown Norway rats in response to brief full-field flashes presented over a range of stimulus energies, under dark-adapted conditions and in the presence of steady mesopic and photopic backgrounds. Recordings were made before and after intravitreal injection of TTX (approximately 3 microm) alone, 3-6 weeks after optic nerve transection (ONTx) to induce ganglion cell degeneration, or in combination with an ionotropic glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 200 microm) to block light-evoked activity of inner retinal, horizontal and OFF bipolar cells, or with the glutamate agonist N-methyl-D-aspartate (NMDA, 100-200 microm) to reduce light-evoked inner retinal activity. TTX reduced ERG amplitudes measured at fixed times corresponding to b-wave time to peak. Effects of TTX were seen under all background conditions, but were greatest for mesopic backgrounds. In dark-adapted retina, b-wave amplitudes were reduced only when very low stimulus energies affecting the inner retina, or very high stimulus energies were used. Loss of ganglion cells following ONTx did not affect b-wave amplitudes, and injection of TTX in eyes with ONTx reduced b-wave amplitudes by the same amount for each background condition as occurred when ganglion cells were intact, thereby eliminating a ganglion cell role in the TTX effects. Isolation of cone-driven responses by presenting test flashes after cessation of a rod-saturating conditioning flash indicated that the TTX effects were primarily on cone circuits contributing to the mixed rod-cone ERG. NMDA significantly reduced only the additional effects of TTX on the mixed rod-cone ERG observed under mesopic conditions, implicating inner retinal involvement in those effects. After pharmacological blockade with CNQX, TTX still reduced b-wave amplitudes in cone-isolated ERGs indicating Na(v) channels in ON cone bipolar cells themselves augment b-wave amplitude and sensitivity. This augmentation was largest under dark-adapted conditions, and decreased with increasing background illumination, indicating effects of background illumination on Na(v) channel function. These findings indicate that activation of Na(v) channels in ON cone bipolar cells affects the b-wave of the rat ERG and must be considered when analysing results of ERG studies of retinal function.

Contribution of post-receptoral cells to the a-wave of the human photopic electroretinogram

ERGs were recorded to red flashes (0.01-50 phcdsm(2)) presented against a steady background (2000 sctd) or 0-300 ms after its suppression. The cone a-wave was altered in form and increased in amplitude in the dark. Peak amplitudes were doubled when the dark period was 50-100 ms and also when it was 150-200 ms. Measurement of the a-wave at fixed times showed that amplitude increase occurred at times later than 6-8 ms. The a-wave receives a significant negative-signal contribution from two post-receptoral mechanisms. These are adapted by weak backgrounds and recover their sensitivity extremely rapidly in the dark. The cone photocurrent alone contributes 40-70% of peak amplitude depending on stimulus intensity.

Physiological features of the S- and M-cone photoreceptors of wild-type mice from single-cell recordings

Cone cells constitute only 3% of the photoreceptors of the wild-type (WT) mouse. While mouse rods have been thoroughly investigated with suction pipette recordings of their outer segment membrane currents, to date no recordings from WT cones have been published, likely because of the rarity of cones and the fragility of their outer segments. Recently, we characterized the photoreceptors of Nrl^−/− mice, using suction pipette recordings from their “inner segments” (perinuclear region), and found them to be cones. Here we report the use of this same method to record for the first time the responses of single cones of WT mice, and of mice lacking the α-subunit of the G-protein transducin (G_tα^−/−), a loss that renders them functionally rodless. Most cones were found to functionally co-express both S- (λ_max = 360 nm) and M- (λ_max = 508 nm) cone opsins and to be maximally sensitive at 360 nm (“S-cones”); nonetheless, all cones from the dorsal retina were found to be maximally sensitive at 508 nm (“M-cones”). The dim-flash response kinetics and absolute sensitivity of S- and M-cones were very similar and not dependent on which of the coexpressed cone opsins drove transduction; the time to peak of the dim-flash response was ∼70 ms, and ∼0.2% of the circulating current was suppressed per photoisomerization. Amplification in WT cones (A ∼4 s⁻²) was found to be about twofold lower than in rods (A ∼8 s⁻²). Mouse M-cones maintained their circulating current at very nearly the dark adapted level even when >90% of their M-opsin was bleached. S-cones were less tolerant to bleached S-opsin than M-cones to bleached M-opsin, but still far more tolerant than mouse rods to bleached rhodopsin, which exhibit persistent suppression of nearly 50% of their circulating current following a 20% bleach. Thus, the three types of mouse opsin appear distinctive in the degree to which their bleached, unregenerated opsins generate “dark light.”

Origin of electroretinogram amplitude growth during light adaptation in pigmented rats

We assessed the growth of the rat photopic electroretinogram (ERG) during light adaptation and the mechanisms underlying this process. Full field ERG responses were recorded from anesthetized adult Brown-Norway rats at each minute for 20 min of light adaptation (backgrounds: 1.8, 2.1, 2.4 log scotopic cd m(-2)). The rat photopic b-wave amplitude increased with duration of light adaptation and its width at 33% maximal amplitude narrowed (by approximately 40 ms). These effects peaked 12-15 min after background onset. The narrowing of the b-wave reflected steepening of the b-wave recovery phase, with little change in the rising phase. OP amplitudes grew in proportion to the b-wave. Inhibition of inner retinal responses using TTX resulted in a greater relative growth of b-wave and OP amplitude compared with fellow control eyes, and delayed the change in recovery phase by approximately 5 min. Inhibition of all ionotropic glutamate receptors with CNQX/D-AP7 delayed both rising and recovery phases equally (approximately 12 ms) without altering b-wave width or the time course of adaptation changes. These outcomes suggest that inner retinal light responses are not directly responsible for b-wave amplitude growth, but may contribute to the change in its recovery phase during adaptation. A TTX-sensitive mechanism may help to hasten this process. The cone a-wave was isolated using PDA/L-AP4 or CNQX/L-AP4. A-wave amplitude (35 ms after stimulus onset) also increased with time during light adaptation and reached a maximum (130 +/- 29% above baseline) 12-15 min after background onset. B-wave amplitude growth in fellow control eyes closely followed the course and relative magnitude of cone a-wave amplitude growth. Hence, the increase of the cone response during light adaptation is sufficient to explain b-wave amplitude growth.

Dark adaptation of human rod bipolar cells measured from the b-wave of the scotopic electroretinogram

To examine the dark adaptation of human rod bipolar cells in vivo, we recorded ganzfeld ERGs to (a) a family of flashes of increasing intensity, (b) dim test flashes presented on a range of background intensities, and (c) dim test flashes presented before, and up to 40 min after, exposure to intense illumination eliciting bleaches from a few per cent to near total. The dim flash ERG was characterized by a prominent b-wave response generated principally by rod bipolar cells. In the presence of background illumination the response reached peak earlier and desensitized according to Weber's Law. Following bleaching exposures, the response was initially greatly desensitized, but thereafter recovered slowly with time. For small bleaches, the desensitization was accompanied by acceleration, in much the same way as for real light. Following a near-total bleach, the response was unrecordable for >10 min, but after approximately 23 min half-maximal sensitivity was reached, and full sensitivity was restored between approximately 35 and 40 min. With smaller bleaches, recovery commenced earlier. We converted the post-bleach measurements of desensitization into 'equivalent background intensities' using a Crawford transformation. Across the range of bleaching levels, the results were described by a prominent 'S2' component (0.24 decades min(-1)) together with a smaller and slower 'S3' component (0.06 decades min(-1)), as is found for dark adaptation of the scotopic visual system. We attribute the S2 component to the presence of unregenerated opsin, and we speculate that the S3 component results from ion channel closure by all-trans retinal.

Evaluation of retinal function using the Dynamic Focal Cone ERG

The development of effective means of assessing visual function in retinal disease holds the key to improved understanding of pathogenesis, and better monitoring of treatment outcomes. In diseases such as age-related macular degeneration, in which the primary locus of dysfunction is the outer retina, tests which provide a direct measure of the functional integrity of the photoreceptor/retinal pigment epithelium (RPE) complex are of great importance. Recovery of retinal function following adaptation to a bright light requires the healthy function of photoreceptors, RPE, Bruch's membrane and choroidal circulation, making an assessment of this recovery a potentially useful clinical tool. However, current techniques are either subjective in nature, or are influenced by post-retinal processing of visual information. This report describes a novel technique, the 'Dynamic Focal Cone Electro-retinogram (ERG)', which allows direct, objective assessment of the recovery of macular function following photopigment bleach. A series of 41 Hz ERGs was recorded, and ERG amplitude was plotted as a function of time following cessation of the bleach. Normative data was collected from 10 healthy subjects. For all subjects, there was no measurable ERG immediately after the bleach, but the amplitude had returned to a pre-bleach level within 4 min. The amplitude recovery data were adequately described both by an exponential recovery function and by a model based on a rate-limited recovery process. We conclude that this technique provides a clinically applicable, objective measure of outer retinal recovery.

Photoreceptors of Nrl -/- mice coexpress functional S- and M-cone opsins having distinct inactivation mechanisms

The retinas of mice null for the neural retina leucine zipper transcription factor (Nrl-/-) contain no rods but are populated instead with photoreceptors that on ultrastructural, histochemical, and molecular criteria appear cone like. To characterize these photoreceptors functionally, responses of single photoreceptors of Nrl-/- mice were recorded with suction pipettes at 35-37 degrees C and compared with the responses of rods of WT mice. Recordings were made either in the conventional manner, with the outer segment (OS) drawn into the pipette ("OS in"), or in a novel configuration with a portion of the inner segment drawn in ("OS out"). Nrl-/- photoreceptor responses recorded in the OS-out configuration were much faster than those of WT rods: for dim-flash responses tpeak = 91 ms vs. 215 ms; for saturating flashes, dominant recovery time constants, tau(D) = 110 ms vs. 240 ms, respectively. Nrl-/- photoreceptors in the OS-in configuration had reduced amplification, sensitivity, and slowed recovery kinetics, but the recording configuration had no effect on rod response properties, suggesting Nrl-/- outer segments to be more susceptible to damage. Functional coexpression of two cone pigments in a single mammalian photoreceptor was established for the first time; the responses of every Nrl-/- cell were driven by both the short-wave (S, lambda(max) approximately 360 nm) and the mid-wave (M, lambda(max) approximately 510 nm) mouse cone pigment; the apparent ratio of coexpressed M-pigment varied from 1:1 to 1:3,000 in a manner reflecting a dorso-ventral retinal position gradient. The role of the G-protein receptor kinase Grk1 in cone pigment inactivation was investigated in recordings from Nrl-/-/Grk1-/- photoreceptors. Dim-flash responses of cells driven by either the S- or the M-cone pigment were slowed 2.8-fold and 7.5-fold, respectively, in the absence of Grk1; the inactivation of the M-pigment response was much more seriously retarded. Thus, Grk1 is essential to normal inactivation of both S- and M-mouse cone opsins, but S-opsin has access to a relatively effective, Grk1-independent inactivation pathway.

Extremely rapid recovery of human cone circulating current at the extinction of bleaching exposures

We used a conductive fibre electrode placed in the lower conjunctival sac to record the a-wave of the human photopic electroretinogram elicited by bright white flashes, delivered during, or at different times after, exposure of the eye to bright white illumination that bleached a large fraction (approximately 90%) of the cone photopigment. During steady-state exposures of this intensity, the amplitude of the bright-flash response declined to approximately 50% of its dark-adapted level. After the intense background was turned off, the amplitude of the bright-flash response recovered substantially, for flashes presented within 20 ms of background extinction, and fully, for flashes presented 100 ms after extinction. In addition, a prominent 'background-off a-wave' was observed, beginning within 5-10 ms of background extinction. We interpret these results to show, firstly, that human cones are able to preserve around half of their circulating current during steady-state illumination that bleaches 90% of their pigment and, secondly, that following extinction of such illumination, the cone circulating current is restored within a few tens of milliseconds. This behaviour is in stark contrast to that in human rods, where the circulating current is obliterated by a background that bleaches only a few percent of the pigment, and where full recovery following a large bleach takes at least 20 min, some 50,000 times more slowly than shown here for human cones.

Photoreceptors and photopigments in a subterranean rodent, the pocket gopher (Thomomys bottae)

Pocket gophers (Thomomys bottae) are rodents that spend much of their lives in near-lightless subterranean burrows. The visual adaptations associated with this extreme environment were investigated by making anatomical observations of retinal organization and by recording retinal responses to photic stimulation. The size of the eye is within the normal range for rodents, the lens transmits light well down into the ultraviolet, and the retina conforms to the normal mammalian plan. Electroretinogram recording revealed the presence of three types of photopigments, a rod pigment with a spectral peak of about 495 nm and two types of cone pigment with respective peak values of about 367 nm (UV) and 505 nm (medium-wavelength sensitive). Both in terms of responsivity to lights varying in temporal frequency and in response recovery following intense light adaptation, the cone responses of the pocket gopher are similar to those of other rodents. Labeling experiments indicate an abundance of cones that reach densities in excess of 30,000 mm-2. Cones containing UV opsin are found throughout the retina, but those containing medium-wavelength sensitive opsin are mostly restricted to the dorsal retina where coexpression of the two photopigments is apparently the rule. Rod densities are lower than those typical for nocturnal mammals.

In intact mammalian photoreceptors, Ca2+-dependent modulation of cGMP-gated ion channels is detectable in cones but not in rods

In the mammalian retina, cone photoreceptors efficiently adapt to changing background light intensity and, therefore, are able to signal small differences in luminance between objects and backgrounds, even when the absolute intensity of the background changes over five to six orders of magnitude. Mammalian rod photoreceptors, in contrast, adapt very little and only at intensities that nearly saturate the amplitude of their photoresponse. In search of a molecular explanation for this observation we assessed Ca²⁺-dependent modulation of ligand sensitivity in cyclic GMP–gated (CNG) ion channels of intact mammalian rods and cones. Solitary photoreceptors were isolated by gentle proteolysis of ground squirrel retina. Rods and cones were distinguished by whether or not their outer segments bind PNA lectin. We measured membrane currents under voltage-clamp in photoreceptors loaded with Diazo-2, a caged Ca²⁺ chelator, and fixed concentrations of 8Br-cGMP. At 600 nM free cytoplasmic Ca²⁺ the midpoint of the cone CNG channels sensitivity to 8BrcGMP, ^8BrcGMPK_1/2, is ∼2.3 μM. The ligand sensitivity is less in rod than in cone channels. Instantly decreasing cytoplasmic Ca²⁺ to <30 nM activates a large inward membrane current in cones, but not in rods. Current activation arises from a Ca²⁺ -dependent modulation of cone CNG channels, presumably because of an increase in their affinity to the cyclic nucleotide. The time course of current activation is temperature dependent; it is well described by a single exponential process of ∼480 ms time constant at 20–21°C and 138 ms at 32°C. The absence of detectable Ca²⁺-dependent CNG current modulation in intact rods, in view of the known channel modulation by calmodulin in-vitro, affirms the modulation in intact rods may only occur at low Ca²⁺ concentrations, those expected at intensities that nearly saturate the rod photoresponse. The correspondence between Ca²⁺ dependence of CNG modulation and the ability to light adapt suggest these events are correlated in photoreceptors.

Contribution of cone photoreceptors and post-receptoral mechanisms to the human photopic electroretinogram

We recorded the electroretinogram (ERG) from human subjects with normal vision, using ganzfeld stimulation in the presence of rod-suppressing blue background light. In families of responses to flashes of increasing intensity, we investigated features of both receptoral and post-receptoral origin. Firstly, we found that the oscillatory potentials (OPs, that have long been known to be post-receptoral) exhibited a time course that was invariant over a range of bright flash intensities. Secondly, we found that the photopic b-wave (which probably originates in cone ON bipolar cells) was most pronounced after test flashes of around 20 Td s, and could be suppressed either by increasing the test flash intensity or by applying a second flash after the test flash. We obtained estimates of the time course of the cone photoreceptor response using the paired-flash technique, in which an intense 'probe' flash was delivered at different times after a test flash. The response to the probe flash was recorded and, its amplitude was measured at early times after the probe flash. Estimates obtained in this way were of normalized amplitude, but could be scaled to an absolute amplitude by making an assumption about the level of probe-flash response that corresponded to complete suppression of photoreceptor current. For moderately bright test flashes the estimated cone photoreceptor response at early times coincided closely with the a-wave of the test flash ERG. However, the maximal size of this estimated response accounted for only about 70% of the peak a-wave amplitude in the case of bright flashes, and for an even smaller proportion after flashes of lower intensity, and we take this to indicate the existence of a third substantial post-receptoral contribution to the a-wave. For dim flashes, the time-to-peak of the cone response was around 15-20 ms, and for saturating flashes the dominant time constant of recovery was about 18 ms. The intensity dependence of the estimated cone response amplitude at fixed times followed an exponential saturation relation. We provide a comparison between our estimates of photoreceptor responses from human cones, and recent estimates from monkey cones obtained using related ERG approaches, and earlier single-cell measurements from isolated primate cones.

Recovery of the human photopic electroretinogram after bleaching exposures: estimation of pigment regeneration kinetics

We used a fibre electrode in the lower conjunctival sac of the human eye to record the a-wave of the photopic electroretinogram elicited in response to dim red flashes, delivered in the presence of a rod-saturating blue background, before and after exposure of the eye to bright white illumination that bleached a significant fraction of cone photopigment. Responses were recorded from two normal subjects whose pupils were maximally dilated. A range of intensities of bleaching light were used, from 500 to 3000 photopic cd m(-2), and exposures were made sufficiently long in duration to achieve a steady-state bleach. In addition, responses were also recorded following shorter durations of exposures to the highest intensity (3000 cd m(-2)); these durations ranged from 5 to 60 s. The amplitude of the a-wave response to dim flashes was reduced following the exposures, with brighter or longer exposures causing greater reduction. The amplitude then recovered within about 4 min to the prebleach level. The amplitudes measured at ca 15 ms after the flash were used to derive the effective intensity of the flashes, thereby quantifying the fraction of photopigment available at the time of delivery of each flash. Recovery from all exposures in both subjects followed a common time course, which could be described well by a model of pigment kinetics based on rate-limited regeneration, where the initial rate of recovery following a total bleach was ca 50% of the total pigment per minute, and the residual pigment level for half the maximal rate was ca 20% of the total pigment. The same parameters, together with a fixed photosensitivity, could account for the steady-state pigment levels seen at each bleaching intensity, and also for the fraction of pigment bleached following exposures of different duration at the highest intensity. The dim-flash ERG thus provides a novel method for assessing pigment regeneration in vivo. Our finding that pigment regeneration follows rate-limited kinetics may explain previous reports of pigment regeneration deviating from first order kinetics. We present a model of regeneration in which the rate limit arises from a limitation in the delivery of 11-cis-retinoid to the photoreceptor outer segments.

Visual adaptations in a diurnal rodent, Octodon degus

The degu (Octodon degus) is a diurnal rodent, native to Chile. Basic features of vision and visual organization in this species were examined in a series of anatomical, electrophysiological and behavioral experiments. The lens of the degu eye selectively absorbs short-wavelength light and shows a progressive increase in optical density as a function of age. Electroretinograms recorded using a flicker-photometric procedure reveal three spectral mechanisms: a rod with peak sensitivity of about 500 nm and two types of cone having respective spectral peaks of about 362 nm and 507 nm. Opsin antibody labeling was used to determine the retinal distributions of the three receptor types. A total of about one-third of the approximately 9 million photoreceptors of the degu retina are cones with the two types (507 nm/362 nm) represented in a ratio of about 13:1. The contributions to vision of all three receptor types were examined in a series of behavioral experiments. A consistent feature of both the electrophysiological and behavioral results is that relatively high levels of light adaptation are required to effect the full transition from rod-based to cone-based vision. In behavioral tests degus were shown to be able to make color discriminations between ultraviolet and visible lights.

Rod and cone contributions to the a-wave of the electroretinogram of the macaque

The electroretinogram (ERG) of anaesthetised dark-adapted macaque monkeys was recorded in response to ganzfeld stimulation and rod- and cone-driven receptoral and postreceptoral components were separated and modelled. The test stimuli were brief (< 4.1 ms) flashes. The cone-driven component was isolated by delivering the stimulus shortly after a rod-saturating background had been extinguished. The rod-driven component was derived by subtracting the cone-driven component from the mixed rod-cone ERG. The initial part of the leading edge of the rod-driven a-wave scaled linearly with stimulus energy when energy was sufficiently low and, for times less than about 12 ms after the stimulus, it was well described by a linear model incorporating a distributed delay and three cascaded low-pass filter elements. Addition of a simple static saturating non-linearity with a characteristic intermediate between a hyperbolic and an exponential function was sufficient to extend application of the model to most of the leading edge of the saturated responses to high energy stimuli. It was not necessary to assume involvement of any other non-linearity or that any significant low-pass filter followed the non-linear stage of the model. A negative inner-retinal component contributed to the later part of the rod-driven a-wave. After suppressing this component by blocking ionotropic glutamate receptors, the entire a-wave up to the time of the first zero-crossing scaled with stimulus energy and was well described by summing the response of the rod model with that of a model describing the leading edge of the rod-bipolar cell response. The negative inner-retinal component essentially cancelled the early part of the rod-bipolar cell component and, for stimuli of moderate energy, made it appear that the photoreceptor current was the only significant component of the leading edge of the a-wave. The leading edge of the cone-driven a-wave included a slow phase that continued up to the peak, and was reduced in amplitude either by a rod-suppressing background or by the glutamate analogue, cis-piperidine-2,3-dicarboxylic acid (PDA). Thus the slow phase represents a postreceptoral component present in addition to a fast component of the a-wave generated by the cones themselves. At high stimulus energies, it appeared less than 5 ms after the stimulus. The leading edge of the cone-driven a-wave was adequately modelled as the sum of the output of a cone photoreceptor model similar to that for rods and a postreceptoral signal obtained by a single integration of the cone output. In addition, the output of the static non-linear stage in the cone model was subject to a low-pass filter with a time constant of no more than 1 ms. In conclusion, postreceptoral components must be taken into account when interpreting the leading edge of the rod- and cone-driven a-waves of the dark-adapted ERG.

Chromatic light adaptation measured using functional magnetic resonance imaging

Sensitivity changes, beginning at the first stages of visual transduction, permit neurons with modest dynamic range to respond to contrast variations across an enormous range of mean illumination. We have used functional magnetic resonance imaging (fMRI) to investigate how these sensitivity changes are controlled within the visual pathways. We measured responses in human visual area V1 to a constant-amplitude, contrast-reversing probe presented on a range of mean backgrounds. We found that signals from probes initiated in the L and M cones were affected by backgrounds that changed the mean absorption rates in the L and M cones, but not by background changes seen only by the S cones. Similarly, signals from S cone-initiated probes were altered by background changes in the S cones, but not by background changes in the L and M cones. Performance in psychophysical tests under similar conditions closely mirrored the changes in V1 fMRI signals. We compare our data with simulations of the visual pathway from photon catch rates to cortical blood-oxygen level-dependent signals and show that the quantitative fMRI signals are consistent with a simple model of mean-field adaptation based on Naka-Rushton (Naka and Rushton, 1966) adaptation mechanisms within cone photoreceptor classes.

Chromatic light adaptation measured using functional magnetic resonance imaging

Sensitivity changes, beginning at the first stages of visual transduction, permit neurons with modest dynamic range to respond to contrast variations across an enormous range of mean illumination. We have used functional magnetic resonance imaging (fMRI) to investigate how these sensitivity changes are controlled within the visual pathways. We measured responses in human visual area V1 to a constant-amplitude, contrast-reversing probe presented on a range of mean backgrounds. We found that signals from probes initiated in the L and M cones were affected by backgrounds that changed the mean absorption rates in the L and M cones, but not by background changes seen only by the S cones. Similarly, signals from S cone-initiated probes were altered by background changes in the S cones, but not by background changes in the L and M cones. Performance in psychophysical tests under similar conditions closely mirrored the changes in V1 fMRI signals. We compare our data with simulations of the visual pathway from photon catch rates to cortical blood-oxygen level-dependent signals and show that the quantitative fMRI signals are consistent with a simple model of mean-field adaptation based on Naka-Rushton (Naka and Rushton, 1966) adaptation mechanisms within cone photoreceptor classes.