Phototransduction in human cones measured using the alpha-wave of the ERG

Association between choroidopathy and photoreceptors during the early stage of diabetic retinopathy: a cross-sectional study

PURPOSE

To explore the role of choroidopathy in diabetic retinopathy (DR) by investigating the correlation between alterations of choroidal vessel and photoreceptors during the early stage of DR.

METHODS

We performed a cross-sectional comparison of diabetic patients without DR (NDR group; n=16) and those with mild nonproliferative diabetic retinopathy (NPDR group; n=39). Optical coherence tomography (OCT) images of choroidal vessel alterations and photoreceptor structures were evaluated using the choroidal vascularity index (CVI) and adjusted ellipsoid zone (EZ) reflectivity, respectively. To evaluate the function of cone photoreceptors, the fundamental, harmonic amplitudes, the parameters S and Rmp3 were calculated from the electroretinogram (ERG). These factors were compared between groups. The correlation between the CVI and parameters describing the function and structure of the photoreceptors was evaluated.

RESULTS

The significant decrease was observed in the CVI in the NPDR group compared to the NDR group (0.67 ± 0.04 vs. 0.70 ± 0.06; p = 0.028), but not in the adjusted EZ reflectivity or ERG parameters. In NPDR group and merging the 2 groups, CVI was moderately positively correlated with the fundamental amplitude obtained by the flicker ERG (NPDR only: r = 0.506; p = 0.001; merge the 2 groups: r = 0.423; p = 0.001), which was regulated by the response of the cone photoreceptors. The CVI was positively and moderately correlated with the logS (NPDR only: r = 0.462; p = 0.003; merge the 2 groups: r = 0.355; p = 0.008), indicating the sensitivity of cone cell light transduction.

CONCLUSION

Compared to eyes without DR, CVI decreased representing choroidal vascular changes in eyes with mild NPDR. These changes may be related to the functional impairment of cone photoreceptors, especially phototransduction sensitivity, as the DR develops.

Development of a translatable gene augmentation therapy for CNGB1-retinitis pigmentosa

In this study, we investigate a gene augmentation therapy candidate for the treatment of retinitis pigmentosa (RP) due to cyclic nucleotide-gated channel beta 1 (CNGB1) mutations. We use an adeno-associated virus serotype 5 with transgene under control of a novel short human rhodopsin promoter. The promoter/capsid combination drives efficient expression of a reporter gene (AAV5-RHO-eGFP) exclusively in rod photoreceptors in primate, dog, and mouse following subretinal delivery. The therapeutic vector (AAV5-RHO-CNGB1) delivered to the subretinal space of CNGB1 mutant dogs restores rod-mediated retinal function (electroretinographic responses and vision) for at least 12 months post treatment. Immunohistochemistry shows human CNGB1 is expressed in rod photoreceptors in the treated regions as well as restoration of expression and trafficking of the endogenous alpha subunit of the rod CNG channel required for normal channel formation. The treatment reverses abnormal accumulation of the second messenger, cyclic guanosine monophosphate, which occurs in rod photoreceptors of CNGB1 mutant dogs, confirming formation of a functional CNG channel. In vivo imaging shows long-term preservation of retinal structure. In conclusion, this study establishes the long-term efficacy of subretinal delivery of AAV5-RHO-CNGB1 to rescue the disease phenotype in a canine model of CNGB1-RP, confirming its suitability for future clinical development.

Rod photoreceptor activation and deactivation in early-stage diabetic eye disease

PURPOSE

To infer rod phototransduction activation and deactivation characteristics in diabetics who have mild or no clinically-apparent retinopathy.

METHODS

Fifteen non-diabetic controls, 15 diabetics with no clinically-apparent diabetic retinopathy (NDR), and 15 diabetics with mild non-proliferative diabetic retinopathy (MDR) participated. Dark-adapted flash electroretinograms (3.2 to 4.4 log scot td-s) were recorded to assess rod activation. The a-waves were fit with a Gaussian model to derive R_mp3 (maximum photoreceptor response amplitude) and S (phototransduction sensitivity). Rod deactivation was assessed with a paired flash paradigm, in which a-waves were measured for two flashes separated by inter-stimulus intervals (ISIs) of 0.125 to 16 s. The ISI needed for the a-wave amplitude of the second flash to recover to 50% of the first flash (t₅₀) was determined. The effect of stimulus retinal illuminance on activation and deactivation was evaluated in a subset of control subjects.

RESULTS

Analysis of variance indicated that both diabetic groups had significant log S reductions compared to controls (p < 0.001). Mean S was reduced by approximately 49% and 78% for the NDR and MDR groups, respectively. In contrast, log R_mp3 and log t₅₀ did not differ significantly among the groups (both p > 0.08). Reducing stimulus retinal illuminance significantly reduced S, but did not significantly affect R_max or t₅₀.

CONCLUSIONS

Only phototransduction sensitivity was abnormal in this sample of diabetic subjects. The normal deactivation kinetics suggests that circulating rod current is normal. These findings begin to constrain possible explanations for abnormal rod function in early diabetic retinal disease.

Characterization of scotopic and mesopic rod signaling pathways in dogs using the On-Off electroretinogram

BACKGROUND

The On-Off, or long flash, full field electroretinogram (ERG) separates retinal responses to flash onset and offset. Depending on degree of dark-adaptation and stimulus strength the On and Off ERG can be shaped by rod and cone photoreceptors and postreceptoral cells, including ON and OFF bipolar cells. Interspecies differences have been shown, with predominantly positive Off-response in humans and other primates and a negative Off-response in rodents and dogs. However, the rod signaling pathways that contribute to these differential responses have not been characterized. In this study, we designed a long flash protocol in the dog that varied in background luminance and stimulus strength allowing for some rod components to be present to better characterize how rod pathways vary from scotopic to mesopic conditions.

RESULTS

With low background light the rod a-wave remains while the b-wave is significantly reduced resulting in a predominantly negative waveform in mesopic conditions. Through modeling and subtraction of the rod-driven response, we show that rod bipolar cells saturate with dimmer backgrounds than rod photoreceptors, resulting in rod hyperpolarization contributing to a large underlying negativity with mesopic backgrounds.

CONCLUSIONS

Reduction in rod bipolar cell responses in mesopic conditions prior to suppression of rod photoreceptor responses may reflect the changes in signaling pathway of rod-driven responses needed to extend the range of lighting conditions over which the retina functions.

Repeatability and reciprocity of the cone optoretinogram

Optoretinography has enabled noninvasive visualization of physiological changes in cone photoreceptors exposed to light. Understanding the cone optoretinogram in healthy subjects is essential for establishing it as a biomarker for cone function in disease. Here, we measure the population cone intensity optoretinogram in healthy adults, for multiple irradiance/duration combinations of visible stimuli with equal energy. We study the within and between session repeatability and reciprocity of the ORG in five healthy subjects. We find the cone optoretinogram exhibits equivalent amplitudes for equal-energy stimuli. We also find good within-subject repeatability, which allows us to show differences across the five subjects.

Use of extended protocols with nonstandard stimuli to characterize rod and cone contributions to the canine electroretinogram

PURPOSE

In this study, we assessed several extended electroretinographic protocols using nonstandard stimuli. Our aim was to separate and quantify the contributions of different populations of retinal cells to the overall response, both to assess normal function and characterize dogs with inherited retinal disease.

METHODS

We investigated three different protocols for measuring the full-field flash electroretinogram-(1) chromatic dark-adapted red and blue flashes, (2) increasing luminance blue-background, (3) flicker with fixed frequency and increasing luminance, and flicker with increasing frequency at a fixed luminance-to assess rod and cone contributions to electroretinograms recorded in phenotypically normal control dogs and dogs lacking rod function.

RESULTS

Temporal separation of the rod- and cone-driven responses is possible in the fully dark-adapted eye using dim red flashes. A- and b-wave amplitudes decrease at different rates with increasing background luminance in control dogs. Flicker responses elicited with extended flicker protocols are well fit with mathematical models in control dogs. Dogs lacking rod function demonstrated larger amplitude dark-adapted compared to light-adapted flicker responses.

CONCLUSIONS

Using extended protocols of the full-field electroretinogram provides additional characterization of the health and function of different populations of cells in the normal retina and enables quantifiable comparison between phenotypically normal dogs and those with retinal disease.

Clinical electroretinography in diabetic retinopathy: a review

The electroretinogram (ERG) is a noninvasive, objective technique to evaluate retinal function that has become increasingly important in the study of diabetic retinopathy. We summarize the principles and rationale of the ERG, present findings from recent clinical studies that have used the full-field ERG, multifocal ERG, and pattern ERG to evaluate neural dysfunction in patients with diabetes, and weigh the strengths and limitations of the technique as it applies to clinical studies and management of patients with diabetic retinopathy. Taken together, ERG studies have provided convincing evidence for dysfunction of the neural retina in patients with diabetes, including those who have no clinically-apparent retinal vascular abnormalities. Recent full-field ERG findings have pointed to the intriguing possibility that photoreceptor function is abnormal in early-stage disease. Pattern ERG data, in conjunction with recently developed photopic negative response analyses, indicate inner retina dysfunction. In addition, multifocal ERG studies have shown spatially localized neural abnormalities that can predict the location of future microaneurysms. Given the insights provided by the ERG, it is likely to play a growing role in understanding the natural history of neural dysfunction in diabetes, as well as providing an attractive outcome measure for future clinical trials that target neural preservation in diabetic retinopathy.

Sildenafil in ophthalmology: An update

Sildenafil citrate, a selective oral phosphodiesterase 5 inhibitor, is a widely used drug for erectile dysfunction that acts by elevating cGMP levels and causing smooth muscle relaxation. It also has 10% activity against PDE6, a key enzyme in phototransduction cascade in the retina. Recent ocular imaging developments have further revealed the influence of sildenafil on ocular hemodynamics, particularly choroidal perfusion. Choroidal thickness is increased, and choroidal perfusion is also enhanced by autoregulatory mechanisms that are further dependent on age and microvascular abnormalities. Studies demonstrating high intraocular pressure via a "parallel pathway" from increased choroidal volume and blood flow to the ciliary body have challenged previous concepts. Another new observation is the effect of sildenafil on bipolar cells and cyclic-nucleotide gated channels. We discuss potential deleterious effects (central serous chorioretinopathy, glaucoma, ischemic optic neuropathy, and risks to recessive carriers of retinitis pigmentosa), potential beneficial effects (ameliorate choroidal ischemia, prevent thickening of Bruch membrane, and promote recovery of the ellipsoid zone) in macular degeneration, as well as potential drug interactions of sildenafil.

ERG assessment of altered retinal function in canine models of retinitis pigmentosa and monitoring of response to translatable gene augmentation therapy

PURPOSE

To analyze ERG responses from two dog models of retinitis pigmentosa, one due to a PDE6A mutation and the other a CNGB1 mutation, both to assess the effect of these mutations on retinal function and the ability of gene augmentation therapy to restore normal function.

METHODS

Scotopic and photopic ERGs from young affected and normal control dogs and affected dogs following AAV-mediated gene augmentation therapy were analyzed. Parameters reflecting rod and cone function were collected by modeling the descending slope of the a-wave to measure receptor response and sensitivity. Rod-driven responses were further assessed by Naka-Rushton fitting of the first limb of the scotopic b-wave luminance-response plot.

RESULTS

PDE6A^-/- dogs showed a dramatic decrease in rod-driven responses with very reduced rod maximal responses and sensitivity. There was a minor reduction in the amplitude of maximal cone responses. In contrast, CNGB1^-/- dogs had some residual rod responses with reduced amplitude and sensitivity and normal cone responses. Following gene augmentation therapy, rod parameters were substantially improved in both models with restoration of sensitivity parameters log S and log K and a large increase in log R_max in keeping with rescue of normal rod phototransduction in the treated retinal regions.

CONCLUSIONS

Modeling of rod and cone a-waves and the luminance-response function of the scotopic b-wave characterized the loss of rod photoreceptor function in two dog models of retinitis pigmentosa and showed the effectiveness of gene augmentation therapy in restoring normal functional parameters.

A review of electroretinography waveforms and models and their application in the dog

Electroretinography (ERG) is a commonly used technique to study retinal function in both clinical and research ophthalmology. ERG responses can be divided into component waveforms, analysis of which can provide insight into the health and function of different types and populations of retinal cells. In dogs, ERG has been used in the characterization of normal retinal function, as well as the diagnosis of retinal diseases and measuring effects of treatment. While many components of the recorded waveform are similar across species, dogs have several notable features that should be differentiated from the responses in humans and other animals. Additionally, modifications of standard protocols, such as changing flash frequency and stimulus color, and mathematical models of ERG waveforms have been used in studies of human retinal function but have been infrequently applied to visual electrophysiology in dogs. This review provides an overview of the origins and applications of ERG in addition to potential avenues for further characterization of responses in the dog.

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.

Cone Photoreceptor Dysfunction in Early-Stage Diabetic Retinopathy: Association Between the Activation Phase of Cone Phototransduction and the Flicker Electroretinogram

Purpose

To define the nature and extent of cone photoreceptor abnormalities in diabetic individuals who have mild or no retinopathy by assessing the activation phase of cone phototransduction and the flicker ERG in these individuals.

Methods

Light-adapted single-flash and flicker ERGs were recorded from 20 diabetic individuals who have no clinically apparent retinopathy (NDR), 20 diabetic individuals who have mild nonproliferative diabetic retinopathy (NPDR), and 20 nondiabetic, age-equivalent controls. A-waves elicited by flashes of different retinal illuminance were fit with a delayed Gaussian model to derive R_mp3 (maximum amplitude of the massed photoreceptor response) and S (phototransduction sensitivity). Fundamental amplitude and phase of ERGs elicited by full-field sinusoidal flicker were obtained across a frequency range of 6 to 100 Hz.

Results

ANVOA indicated that both diabetic groups had significant S losses compared with the controls, whereas mean R_mp3 did not differ significantly among the groups. ANOVA also indicated significantly reduced flicker ERG amplitude for frequencies ≥56 Hz for both diabetic groups compared with the controls. Flicker ERG timing (phase) did not differ significantly among the groups. Log R_mp3 + log S was significantly correlated with the patients' high-frequency (62.5 Hz) flicker ERG amplitude loss (r = 0.69, P < 0.001).

Conclusions

The delayed Gaussian a-wave model is useful for characterizing abnormalities in the activation phase of cone phototransduction and can help explain flicker ERG abnormalities in early-stage diabetic retinopathy. Reduced cone sensitivity and attenuated high-frequency flicker ERGs provide evidence for impaired cone function in these individuals.

Haemodilution and head-down tilting induce functional injury in the rat optic nerve: A model for peri-operative ischemic optic neuropathy

BACKGROUND

Mechanisms of peri-operative ischaemic optic neuropathy remain poorly understood. Both specific pre-operative and intra-operative factors have been examined by retrospective studies, but no animal model currently exists.

OBJECTIVES

To develop a rodent model of peri-operative ischaemic optic neuropathy. In rats, we performed head-down tilt and/or haemodilution, theorising that the combination damages the optic nerve.

DESIGN

Animal study.

SETTING

Laboratory.

ANIMALS

A total of 36 rats, in four groups, completed the functional examination of retina and optic nerve after the interventions.

INTERVENTIONS

Anaesthetised groups (n>8) were supine (SUP) for 5 h, head-down tilted 70° for 5 h, head-down tilted/haemodiluted for 5 h or SUP/haemodiluted for 5 h. We measured blood pressure, heart rate, intra-ocular pressure and maintained constant temperature.

MAIN OUTCOME MEASUREMENTS

Retinal function (electroretinography), scotopic threshold response (STR) (for retinal ganglion cells) and visual evoked potentials (VEP) (for transmission through the optic nerve). We imaged the optic nerve in vivo and evaluated retinal histology, apoptotic cells and glial activation in the optic nerve. Retinal and optic nerve function were followed to 14 and 28 days after experiments.

RESULTS

At 28 days in head down tilted/haemodiluted rats, negative STR decreased (about 50% amplitude reduction, P = 0.006), VEP wave N2-P3 decreased (70% amplitude reduction, P = 0.01) and P2 latency increased (35%, P = 0.003), optic discs were swollen and glial activation was present in the optic nerve. SUP/haemodiluted rats had decreases in negative STR and increased VEP latency, but no glial activation.

CONCLUSION

An injury partly resembling human ischaemic optic neuropathy can be produced in rats by combining haemodilution and head-down tilt. Significant functional changes were also present with haemodilution alone. Future studies with this partial optic nerve injury may enable understanding of mechanisms of peri-operative ischaemic optic neuropathy and could help discover preventive or treatment strategies.

ISCEV extended protocol for the stimulus-response series for light-adapted full-field ERG

The International Society for Clinical Electrophysiology of Vision (ISCEV) standard for full-field electroretinography (ERG) describes a minimum protocol for clinical testing but encourages additional ERG testing when appropriate. This ISCEV extended protocol describes methods to record and evaluate a light-adapted (LA) ERG stimulus-response series with increasing flash strengths. The LA ERG stimulus-response series (also referred to as the luminance-response or intensity-response series in the published literature) can characterise generalised cone system function more comprehensively than the ISCEV standard LA ERGs alone. The amplitude of LA ERG a-waves, arising from cones and cone off-bipolar cells, typically shows a saturating function. The LA ERG b-wave amplitudes, which arise primarily from activity of retinal bipolar cells, show an amplitude peak followed by a nonzero plateau (the "photopic hill" phenomenon). This ISCEV extended protocol specifies a stimulus-response series suitable to evaluate generalised dysfunction affecting the LA retina, to aid in distinguishing between the on- and off-responses of the cone system and to monitor ERG changes in these characteristics. The LA ERG stimulus-response series for a- and b-waves is recorded to a sequence of nine flash stimuli ranging from 0.03 to 300 cd s m^-2, superimposed on a standard background of 30 cd m^-2. A shorter protocol is also presented to measure the mid-range of the function (the "photopic hill") using 5 flash stimuli.

Early light deprivation effects on human cone-driven retinal function

PURPOSE

To assess whether the early light deprivation induced by congenital cataract may influence the cone-driven retinal function in humans.

METHODS

Forty-one patients affected by congenital cataract (CC) who had undergone uncomplicated cataract extraction surgery and intraocular lens implant, and 14 healthy subjects (HS) were enrolled. All patients underwent complete ophthalmological and orthoptic evaluations and best-corrected visual acuity (BCVA) measurement; light-adapted full-field electroretinograms (ERG) and photopic negative responses (PhNR) were recorded to obtain a reliable measurement of the outer/inner retinal function and of the retinal ganglion cells' function respectively.

RESULTS

Mean values of light-adapted ERG a- and b-wave and PhNR amplitude of CC eyes were significantly reduced and photopic ERG b-wave implicit time mean values were significantly delayed when compared to HS ones. When studying photopic ERG mean amplitudes at 5 ms, significant differences were found when comparing CC and control eyes. In CC eyes, statistically significant correlations were found between a- and b- wave amplitudes and PhNR amplitudes. No significant correlations were found between ERG parameters and BCVA, as well as between the age of CC patients at surgery and the time elapsed from lens extraction. No significant differences were found when functional parameters of bilateral and unilateral congenital cataract (uCC) eyes were compared, however uCC eyes showed significant differences when compared with contralateral healthy eyes.

CONCLUSION

We found a significant impairment of cone-driven retinal responses in patients with a history of congenital cataract. These changes might result from the long-lasting effects of early light deprivation on the cone retinal pathways. Our findings support the relevance of retinal involvement in deficits induced by early light deprivation.

Impact of motion-associated noise on intrinsic optical signal imaging in humans with optical coherence tomography

A growing body of evidence suggests that phototransduction can be studied in the human eye in vivo by imaging of fast intrinsic optical signals (IOS). There is consensus concerning the limiting influence of motion-associated imaging noise on the reproducibility of IOS-measurements, especially in those employing spectral-domain optical coherence tomography (SD-OCT). However, no study to date has conducted a comprehensive analysis of this noise in the context of IOS-imaging. In this study, we discuss biophysical correlates of IOS, and we address motion-associated imaging noise by providing correctional post-processing methods. In order to avoid cross-talk of adjacent IOS of opposite signal polarity, cellular resolution and stability of imaging to the level of individual cones is likely needed. The optical Stiles-Crawford effect can be a source of significant IOS-imaging noise if alignment with the peak of the Stiles-Crawford function cannot be maintained. Therefore, complete head stabilization by implementation of a bite-bar may be critical to maintain a constant pupil entry position of the OCT beam. Due to depth-dependent sensitivity fall-off, heartbeat and breathing associated axial movements can cause tissue reflectivity to vary by 29% over time, although known methods can be implemented to null these effects. Substantial variations in reflectivity can be caused by variable illumination due to changes in the beam pupil entry position and angle, which can be reduced by an adaptive algorithm based on slope-fitting of optical attenuation in the choriocapillary lamina.

The rod-driven a-wave of the dark-adapted mammalian electroretinogram

The a-wave of the electroretinogram (ERG) reflects the response of photoreceptors to light, but what determines the exact waveform of the recorded voltage is not entirely understood. We have now simulated the trans-retinal voltage generated by the photocurrent of dark-adapted mammalian rods, using an electrical model based on the in vitro measurements of Hagins et al. (1970) and Arden (1976) in rat retinas. Our simulations indicate that in addition to the voltage produced by extracellular flow of photocurrent from rod outer to inner segments, a substantial fraction of the recorded a-wave is generated by current that flows in the outer nuclear layer (ONL) to hyperpolarize the rod axon and synaptic terminal. This current includes a transient capacitive component that contributes an initial negative "nose" to the trans-retinal voltage when the stimulus is strong. Recordings in various species of the a-wave, including the peak and initial recovery towards the baseline, are consistent with simulations showing an initial transient primarily related to capacitive currents in the ONL. Existence of these capacitive currents can explain why there is always a substantial residual transient a-wave when post-receptoral responses are pharmacologically inactivated in rodents and nonhuman primates, or severely genetically compromised in humans (e.g. complete congenital stationary night blindness) and nob mice. Our simulations and analysis of ERGs indicate that the timing of the leading edge and peak of dark-adapted a-waves evoked by strong stimuli could be used in a simple way to estimate rod sensitivity.

Optical imaging of human cone photoreceptors directly following the capture of light

Capture of light in the photoreceptor outer segment initiates a cascade of chemical events that inhibit neurotransmitter release, ultimately resulting in vision. The massed response of the photoreceptor population can be measured non-invasively by electrical recordings, but responses from individual cells cannot be measured without dissecting the retina. Here we used optical imaging to observe individual human cones in the living eye as they underwent bleaching of photopigment and associated phototransduction. The retina was simultaneously stimulated and observed with high intensity visible light at 1 kHz, using adaptive optics. There was marked variability between individual cones in both photosensitivity and pigment optical density, challenging the conventional assumption that photoreceptors act as identical subunits (coefficient of variation in rate of photoisomerization = 23%). There was also a pronounced inverse correlation between these two parameters (p<10(-7)); the temporal evolution of image statistics revealed this to be a dynamic relationship, with cone waveguiding efficiency beginning a dramatic increase within 3 ms of light onset. Beginning as early as 2 ms after light onset and including half of cells by ∼7 ms, cone intensity showed reversals characteristic of interference phenomena, with greater delays in reversal corresponding to cones with more photopigment (p<10(-3)). The timing of these changes is argued to best correspond with either the cessation of dark current, or to related events such as changes in intracellular cGMP. Cone intensity also showed fluctuations of high frequency (332±25 Hz) and low amplitude (3.0±0.85%). Other groups have shown similar fluctuations that were directly evoked by light; if this corresponds to the same phenomenon, we propose that the amplitude of fluctuation may be increased by the use of a bright flash followed by a brief pause, to allow recovery of cone circulating current.

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.

Electroretinographic (ERG) responses in pediatric patients using vigabatrin

The antiepileptic drug vigabatrin is known to cause retinal and visual dysfunction, particularly visual field defects, in some patients. Electroretinography (ERG) is used in an attempt to identify adverse effects of vigabatrin (VGB) in patients who are not candidates for conventional perimetry. We report data from 114 pediatric patients taking VGB referred for clinical evaluation; median age at test was 22.9 (2.4 to 266.1) months, and median duration of VGB use was 9.7 (0.3 to 140.7) months. Twenty-seven of them were tested longitudinally (3 to 12 ERG tests). ERG responses to full-field stimuli were recorded in scotopic and photopic conditions, and results were compared to responses from healthy control subjects. We found that abnormalities of photoreceptor and post-receptor ERG responses are frequent in these young patients. The most frequently abnormal scotopic parameter was post-receptor sensitivity, log σ, derived from the b-wave stimulus-response function; the most frequently abnormal photopic parameter was the implicit time of the OFF response (d-wave) to a long (150 ms) flash. Abnormal 30-Hz flicker response amplitude, previously reported to be a predictor of visual field loss, occurred infrequently. For the group as a whole, none of the ERG parameters changed significantly with increasing duration of VGB use. Four of the 27 patients tested longitudinally showed systematic worsening of log σ with duration of VGB use. In a subset of patients who underwent perimetry (N = 39), there was no significant association of any ERG parameter with visual field defects. We cannot determine whether the ERG abnormalities we found were due solely to the effects of VGB. We caution against over-reliance on the ERG to monitor pediatric patients for VGB toxicity and recommend further development of a reliable test of peripheral vision to supplant ERG testing.

Modelling the initial phase of the human rod photoreceptor response to the onset of steady illumination

The initial time course of the change in photoreceptor outer segment membrane conductance in response to light flashes has been modelled using biochemical analysis of phototransduction, and the model has been successfully applied to a range of in vitro recordings and has also been shown to provide a good fit to the leading edge of the electroretinogram a-wave recorded in vivo. We investigated whether a simple modification of the model's equation would predict responses to the onset of steady illumination and tested this against electroretinogram recordings. Scotopic electroretinograms were recorded from three normal human subjects, using conductive fibre electrodes, in response to light flashes (0.30-740 scotopic cd m(-2) s) and to the onset of steady light (11-1,900 scotopic cd m(-2)). Subjects' pupils were dilated pharmacologically. The standard form of the model was applied to flash responses, as in previous studies, to obtain values for the three parameters: maximal response amplitude r (max), sensitivity S and effective delay time t (eff). A new "step response" equation was derived, and this equation provided a good fit to rod responses to steps of light using the same parameter values as for the flash responses. The results support the applicability of the model to the leading edge of electroretinogram responses: in each subject, the model could be used to fit responses both to flashes of light and to the onset of backgrounds with a single set of parameter values.

Phenotypic characterization of 3 families with autosomal dominant retinitis pigmentosa due to mutations in KLHL7

OBJECTIVE

To characterize the visual phenotype caused by mutations in the BTB-Kelch protein, KLHL7, responsible for the RP42 form of autosomal dominant retinitis pigmentosa (RP).

METHODS

Comprehensive ophthalmic testing included visual acuity, static visual field, kinetic visual field, dark adaptometry, full-field electroretinography, spectral-domain optical coherence tomography, and fundus photography. Longitudinal visual function data (range, 15-27 years) were available for some of the affected individuals.

RESULTS

We report a phenotypic assessment of 3 unrelated families, each harboring different KLHL7 mutations (c.458C>T, c.449G>A, and c.457G>A). The fundi showed classic signs of RP. Best-corrected visual acuity was 20/50 or better in at least one eye up to age 65 years. Static and kinetic visual fields showed concentric constriction to central 10° to 20° by age 65 years; 2 patients with Goldmann perimetry exhibited bilateral visual field retention in the far periphery. Both rod and cone full-field electroretinographic amplitudes were substantially lower than normal, with a decline rate of 3% per year in cone 31-Hz flicker response. Rod and cone activation and inactivation variables were abnormal. Spectral-domain optical coherence tomography indicated retention of foveal inner segment-outer segment junction through age 65 years.

CONCLUSIONS

Mutations in KLHL7 are associated with a late-onset form of autosomal dominant retinal degeneration that preferentially affects the rod photoreceptors. Full-field electroretinographic findings, including recovery kinetics, are consistent with those observed in other forms of autosomal dominant RP.

CLINICAL RELEVANCE

The phenotypes are similar among patients with 3 types of KLHL7 mutations (c.458C>T, c.449G>A, and c.457G>A). Strong retention of foveal function and bilateral concentric constriction of visual fields with far periphery sparing may guide mutation screening in autosomal dominant RP.

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.

Cone ERG responses in patients with Smith-Lemli-Opitz Syndrome (SLOS)

PURPOSE

To evaluate cone and cone-driven retinal function in patients with Smith-Lemli-Opitz syndrome (SLOS), a condition characterized by low cholesterol. Rod and rod-driven function in patients with SLOS are known to be abnormal.

METHODS

Electroretinographic (ERG) responses to full-field stimuli presented on a steady, rod suppressing background were recorded in 13 patients who had received long-term cholesterol supplementation. Cone photoresponse sensitivity (S(CONE)) and saturated amplitude (R(CONE)) parameters were estimated using a model of the activation of phototransduction, and post-receptor b-wave and 30 Hz flicker responses were analyzed. The responses of the patients were compared to those of control subjects (N = 13).

RESULTS

Although average values of both S(CONE) and R(CONE) were lower than in controls, the differences were not statistically significant. Post-receptor b-wave amplitude and implicit time and flicker responses were normal.

CONCLUSIONS

The normal cone function contrasts with the significant abnormalities in rod function that were found previously in these same patients. Possibly, cholesterol supplementation has a greater protective effect on cones than on rods as has been demonstrated in the rat model of SLOS.

Variable optical activation of human cone photoreceptors visualized using a short coherence light source

It has been shown that after a visible stimulus, optical oscillations of nearly all cone photoreceptors can be observed using long coherence length light and in a few cones using short coherence length light. Here, we show that after exposure to a visible stimulus, a short coherence length imaging source reveals light-evoked oscillation signals in a large number of cones. More than 80% of cones in a given retinal area are activated (modulation in the reflectance signal) after stimulation, and the pattern of their activation can be subjectively classified into one of four categories. The application of light-evoked signal detection techniques for in vivo retinal imaging may prove useful for assessing the functional status of cones in normal and diseased retinae.

The neurovascular retina in retinopathy of prematurity

The continuing worldwide epidemic of retinopathy of prematurity (ROP), a leading cause of childhood visual impairment, strongly motivates further research into mechanisms of the disease. Although the hallmark of ROP is abnormal retinal vasculature, a growing body of evidence supports a critical role for the neural retina in the ROP disease process. The age of onset of ROP coincides with the rapid developmental increase in rod photoreceptor outer segment length and rhodopsin content of the retina with escalation of energy demands. Using a combination of non-invasive electroretinographic (ERG), psychophysical, and image analysis procedures, the neural retina and its vasculature have been studied in prematurely born human subjects, both with and without ROP, and in rats that model the key vascular and neural parameters found in human ROP subjects. These data are compared to comprehensive numeric summaries of the neural and vascular features in normally developing human and rat retina. In rats, biochemical, anatomical, and molecular biological investigations are paired with the non-invasive assessments. ROP, even if mild, primarily and persistently alters the structure and function of photoreceptors. Post-receptor neurons and retinal vasculature, which are intimately related, are also affected by ROP; conspicuous neurovascular abnormalities disappear, but subtle structural anomalies and functional deficits may persist years after clinical ROP resolves. The data from human subjects and rat models identify photoreceptor and post-receptor targets for interventions that promise improved outcomes for children at risk for ROP.

Structure of cone photoreceptors

Although outnumbered more than 20:1 by rod photoreceptors, cone cells in the human retina mediate daylight vision and are critical for visual acuity and color discrimination. A variety of human diseases are characterized by a progressive loss of cone photoreceptors but the low abundance of cones and the absence of a macula in non-primate mammalian retinas have made it difficult to investigate cones directly. Conventional rodents (laboratory mice and rats) are nocturnal rod-dominated species with few cones in the retina, and studying other animals with cone-rich retinas presents various logistic and technical difficulties. Originating in the early 1900s, past research has begun to provide insights into cone ultrastructure but has yet to afford an overall perspective of cone cell organization. This review summarizes our past progress and focuses on the recent introduction of special mammalian models (transgenic mice and diurnal rats rich in cones) that together with new investigative techniques such as atomic force microscopy and cryo-electron tomography promise to reveal a more unified concept of cone photoreceptor organization and its role in retinal diseases.

Diffusion of the second messengers in the cytoplasm acts as a variability suppressor of the single photon response in vertebrate phototransduction

The single photon response in vertebrate phototransduction is highly reproducible despite a number of random components of the activation cascade, including the random activation site, the random walk of an activated receptor, and its quenching in a random number of steps. Here we use a previously generated and tested spatiotemporal mathematical and computational model to identify possible mechanisms of variability reduction. The model permits one to separate the process into modules, and to analyze their impact separately. We show that the activation cascade is responsible for generation of variability, whereas diffusion of the second messengers is responsible for its suppression. Randomness of the activation site contributes at early times to the coefficient of variation of the photoresponse, whereas the Brownian path of a photoisomerized rhodopsin (Rh*) has a negligible effect. The major driver of variability is the turnoff mechanism of Rh*, which occurs essentially within the first 2-4 phosphorylated states of Rh*. Theoretically increasing the number of steps to quenching does not significantly decrease the corresponding coefficient of variation of the effector, in agreement with the biochemical limitations on the phosphorylated states of the receptor. Diffusion of the second messengers in the cytosol acts as a suppressor of the variability generated by the activation cascade. Calcium feedback has a negligible regulatory effect on the photocurrent variability. A comparative variability analysis has been conducted for the phototransduction in mouse and salamander, including a study of the effects of their anatomical differences such as incisures and photoreceptors geometry on variability generation and suppression.

The cone electroretinogram in retinopathy of prematurity

PURPOSE

To test the hypothesis that retinopathy of prematurity (ROP) affects the cone photoreceptors less than the rod photoreceptors.

METHODS

Electroretinogram (ERG) responses to a 1.8-log-unit range of red flashes on a white, rod-saturating background were recorded in 42 subjects with a history of preterm birth and ROP (28 untreated; 6 treated) or no ROP (n = 8). The sensitivity (S(CONE)) and saturated amplitude (R(CONE)) of the cone photoresponse were calculated by fit of a model of the activation of cone phototransduction to the a-waves. The cone-driven b-wave amplitude was evaluated as a function of stimulus intensity. S(CONE) and R(CONE) were compared to the rod response parameters (S(ROD), R(ROD)) recorded from the same preterm subjects. Responses in the former preterm subjects were compared to those in control subjects.

RESULTS

The values of S(CONE) and R(CONE) in the preterm subjects overlapped broadly with those in the control subjects. The shapes of the b-wave stimulus-response functions did not differ between preterm and control subjects. The relative value of S(CONE) was significantly greater than that of S(ROD).

CONCLUSIONS

ROP has less effect on the cone than on the rod photoresponses, suggesting that cones are more resistant to the ROP disease process. The similar shape of the b-wave stimulus-response function in preterms and control subjects is evidence that ROP does not alter the balance of ON and OFF signals in the cone pathway.

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.

In vivo functional imaging of human cone photoreceptors

We evaluate a novel non-invasive optical technique for observing fast physiological processes, in particular phototransduction, in single photoreceptor cells in the living human eye. The method takes advantage of the interference of multiple reflections within the outer segments (OS) of cones. This self-interference phenomenon is highly sensitive to phase changes such as those caused by variations in refractive index and scatter within the photoreceptor cell. A high-speed (192 Hz) flood-illumination retina camera equipped with adaptive optics (AO) is used to observe individual photoreceptors, and to monitor changes in their reflectance in response to visible stimuli ("scintillation"). AO and high frame rates are necessary for resolving individual cones and their fast temporal dynamics, respectively. Scintillation initiates within 5 to 10 ms after the onset of the stimulus flash, lasts 300 to 400 ms, is observed at visible and near-infrared (NIR) wavelengths, and is highly sensitive to the coherence length of the imaging light source. To our knowledge this is the first demonstration of in vivo optical imaging of the fast physiological processes that accompany phototransduction in individual photoreceptors.

The photopic negative response of the flash electroretinogram in retinal vein occlusion

The photopic negative response (PhNR) has recently been shown to be severely affected in central retinal artery occlusion (CRAO), despite relative preservation of the cone b-wave compared to that in the healthy unaffected fellow eye. The aim of this study was to test how the PhNR of the flash electroretinogram (ERG) is affected in human retinal vein occlusion. PhNR was elicited with red stimuli (1 cd s/m2, 5 cd s/m2, and 7 cd s/m2 with 4 ms duration) and blue background (10 cd/m2). Standard Ganzfeld flash ERG was produced according to the ISCEV standard for the clinical electroretinogram (2004). Sixteen patients with central retinal vein occlusion (CRVO), 14 patients with branch retinal vein occlusion (BRVO), and 16 controls were analyzed. The amplitude of the PhNRs was significantly smaller in the CRVO and BRVO eyes than those in the unaffected fellow or control eyes (p = 0.000). There was a significantly greater reduction of PhNR amplitudes than that of other waves including the OPs, rod b-wave, combined a-wave and b-wave, cone a-wave and b-wave, and 30 Hz flicker ERG. Thus, PhNR amplitude in retinal vein occlusion is severely affected. There is a potential role for PhNR in assessing inner retinal damage and evaluating the effect of treatment.

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.

ERG oscillatory potentials in infants

PURPOSE

Study the scotopic and photopic oscillatory potentials (OPs) of the electroretinogram (ERG) in 10-week old infants.

METHODS

Term-born 10-week old infants (n = 15) and adults (n = 12) were tested. Full-field ERGs were recorded under scotopic and photopic conditions. The records were filtered (75-300 Hz) to demonstrate the oscillatory wavelets. The amplitudes and implicit times of the infants' OPs were compared to those in adults and also to amplitudes of the saturated photoreceptor responses.

RESULTS

In infants, the mean OP amplitudes are similar in scotopic and photopic conditions and do not vary significantly with OP number. Infants' OPs are significantly smaller than in adults, with scotopic OPs averaging 19% of that in adults and photopic OPs averaging 47%, whereas the amplitudes of the saturated photoreceptor responses are 43% and 66% of those in adults. Mean interpeak intervals are similar in infants and adults, indicating oscillatory behavior at a frequency of 155 Hz in scotopic conditions and 135 Hz in photopic conditions.

CONCLUSIONS

In young infants, the OPs are relatively immature compared to the photoreceptor responses, with the immaturity of the scotopic responses being more marked than that of the photopic responses.

Cone phototransduction and growth of the ERG b-wave during light adaptation

The purpose of this study was to determine whether cone redepolarization accounts for the amplitude increase of the b-wave of the human electroretinogram (ERG) during light adaptation. The time course of the b-wave amplitude increase was compared to the time course of the change in the activation phase of cone phototransduction, as derived from a delayed Gaussian model applied to the leading edge of the ERG a-wave. ERG recordings were obtained from five visually normal subjects, alternately in the presence of the adapting field (adapt-on condition) and 300ms after its temporary extinction (adapt-off condition). The proportional increase in amplitude was less for R(mp3) (maximum amplitude of P3, the massed cone photoreceptor response) than for the b-wave for both adaptation conditions, and the time course of the amplitude increase for R(mp3) was faster than that for the b-wave in the adapt-off condition. The results demonstrate that time-dependent changes in the activation phase of cone phototransduction have only a minimal role in governing the increase in the amplitude of the human cone-derived ERG b-wave during light adaptation. In addition, the systematic increase in b-wave amplitude and the decrease in b-wave implicit time in the adapt-off condition indicates that the ERG response measured shortly after adapting field offset does not necessarily represent the waveform of the dark-adapted cone ERG.

Night blindness with depolarizing pattern of ON/OFF response in electroretinogram: a case report

PURPOSE

To present a patient who has night blindness with a depolarizing pattern of ON/OFF response by electroretinography (ERG).

CASE

A 43-year-old woman had had night blindness and poorly corrected visual acuity since childhood. Parental consanguinity was noted. The patient had suffered from mental retardation, epilepsy, and mild cerebellar ataxia. Corrected visual acuity was 20/30 in the right eye and 20/25 in the left. Goldmann perimetry showed no scotoma but slight depression with internal isoptors. No evidence for rod activity was observed by Goldmann-Weekers adaptometry. The ocular fundi appeared normal.

METHODS

Conventional full-field ERGs to scotopic (dim and bright flash) and photopic (bright flash and flicker) stimuli were recorded. Photopic ERG responses to long flash stimulation (200 ms) were also examined.

RESULTS

The scotopic responses to dim flash were non-recordable, while those to bright flash were severely reduced. The photopic responses to bright flash were decreased. The amplitudes of flicker responses also were significantly decreased, and the implicit times of those responses were prolonged. Although the amplitudes of a- and d-waves to long flash stimulation were reduced, those of the b-wave were within normal range. The implicit times of a-, b- and d-waves were significantly prolonged. The patient showed a normal rise for the photopic b-wave but lacked a normal falling slope after the b-wave peak. An OFF-response late-negativity was also noted.

CONCLUSIONS

The abnormal ON/OFF response found in the patient could be diagnosed as depolarizing pattern, characterized by elevation of the plateau to a positive potential above the prestimulus baseline. Since the ERG waveforms and clinical features found in our patient were different from those in previous reports, her ERG findings might reflect another retinal physiological mechanism.

Activation phase of cone phototransduction and the flicker electroretinogram in retinitis pigmentosa

This study examined the relationship between the activation phase of cone phototransduction and the flicker electroretinogram (ERG) in 15 patients with retinitis pigmentosa (RP) and 12 age-equivalent, visually normal control subjects. Values of Rmp3 (maximum amplitude of P3, the massed cone photoreceptor response) and S (sensitivity of cone phototransduction) were derived from a delayed Gaussian model applied to the leading edge of the ERG a-wave. Fundamental amplitude and phase of the flicker ERG were derived from responses to sinusoidal flicker presented at temporal frequencies ranging from 7.8 to 100 Hz. Patients with RP who had a reduced value of Rmp3 alone had an overall reduction in flicker ERG amplitude with a normal response phase across temporal frequency. Patients with RP who had a reduced value of S, whether or not Rmp3 was reduced, had the greatest amplitude reduction at temporal frequencies above 40 Hz and phase lags across a range of temporal frequencies. At high temporal frequencies, the amplitude reduction of the flicker ERG was predicted by the product of Rmp3 and S for all of the subjects except the three patients with RP who had the lowest fundamental amplitudes. The results indicate that there is a systematic relationship between the derived parameters of the activation phase of cone phototransduction and the characteristics of the flicker ERG in patients with RP, although the phase changes in the flicker ERG were generally greater than predicted by the derived parameters alone.

Development of the cone ERG in infants

PURPOSE

To assess cone photoreceptor and cone-mediated postreceptoral retinal function in infants.

METHODS

ERG responses to a 1.8-log unit range of long-wavelength flashes on a white, rod-saturating background were recorded in 4-week-old (n = 22) and 10-week-old (n = 28) infants and control adults and children, 8 to 40 years of age (n = 13). A model of the activation of cone phototransduction was fit to the a-waves. Sensitivity (S(CONE)) and saturated-response amplitude (R(CONE)) were calculated. The amplitude and implicit time of the b-wave were examined as a function of stimulus intensity. The cone photoresponse parameters were compared to the rod photoresponse parameters (S(ROD) and R(ROD)) in the same subjects.

RESULTS

S(CONE) and R(CONE) in infants were significantly smaller than in the mature control subjects. The mean S(CONE) was 64% and 68%, and the mean R(CONE) was 63% and 72% in 4- and 10-week-olds, respectively. The mean rod photoresponse parameters were considerably less mature, as the mean S(ROD) was 35% and 46%, and the mean R(ROD) was 39% and 43% of mature values at 4 and 10 weeks. The b-wave stimulus-response functions in the 4- and 10-week-old infants did not show the photopic hill that was characteristic of the children's and adults' photopic b-waves.

CONCLUSIONS

Peripheral cone function is relatively more mature than rod function in young infants. The lack of a photopic hill is hypothesized to result from immaturity in the relative contributions of ON and OFF bipolar cell responses.

Rod and Cone a-Waves in Central Retinal Vein Occlusion

PURPOSE

To evaluate rod and cone a-waves in cases with unilateral central retinal vein occlusion (CRVO).

METHODS

Scotopic and photopic flash electroretinograms (ERGs) were recorded in seven patients aged 54-84 with unilateral hemorrhagic CRVO. Rod and cone a-waves were analyzed using photoreceptor models, and Rm(p3) (maximum a-wave amplitude) and S (sensitivity) were calculated.

RESULTS

Decreased rod log S was found in all seven cases, and decreased cone log S was found in five cases. In only one case, rod log S in the fellow eye was decreased. The alterations in rod and cone log Rm(p3) were smaller than those in rod and cone log S. Of three cases in which ERGs could be recorded again after a certain follow-up period, rod log S and cone logS became larger in two cases and smaller in one case.

CONCLUSIONS

The change in the phototransduction cascade was confirmed not only in rods but also in cones in five of our seven cases of CRVO. The ERG findings might reflect the functional change in the photoreceptor layer after the onset of CRVO.

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.

PIII and Derived PII Analysis in a Patient with Retinal Dysfunction with Supernormal Scotopic ERG

PURPOSE

To present electroretinographic (ERG) findings in a patient with retinal dysfunction with supernormal scotopic ERG, and to analyze rod and cone PIII components and rod inner nuclear layer (derived PII) responses.

PATIENT

A Japanese 11-year-old girl complained of poor visual acuity. There was no parental consanguinity in her family. The corrected visual acuity was 0.7 in both eyes. No abnormal finding was observed in both fundi.

METHODS

The patient underwent full-field ERGs. Rod and cone a-waves were analyzed using photoreceptor models. The derived PII responses were analyzed using a technique described by Hood and Birch.

RESULTS

In the photopic ERG, responses to single flash and 30-Hz flicker were attenuated. In the scotopic ERG, b-wave was supernormal in amplitude in response to intense flashes, but smaller than normal and markedly delayed over a lower range of flash intensities. By the PIII analysis, phototransductions (values of S) of both rod and cone were remarkably decreased. The derived PII responses for this patient were larger than the responses for normal subjects, and the onset of the PII responses in this patient are significantly delayed compared to those in normal subjects.

CONCLUSIONS

The ophthalmological findings in this patient are consistent with previous publications of this disease. Although it has been reported that the sites of disease action were beyond the outer segment (values of S were within the normal range), our results suggest that photoreceptors could be involved in sites of disease action in at least some patients with this disease.

ERG rod a-wave in Oguchi disease

We analyzed the change in the ERG rod a-wave waveform during the course of dark adaptation in two patients with Oguchi disease. Two Japanese patients showed a homozygous arrestin 1147delA mutation. Scotopic flash ERGs were recorded after different periods of dark adaptation. ERG rod a-waves were obtained after subtraction of the cone ERG contribution. The rod a-waves were fitted with a model of the rod receptor signal. The parameters, Rm(p3) (maximum a-wave amplitude) and S (sensitivity) were calculated. Longer periods in the dark produced larger rod a-wave but only to the first flash presented. The amplitude of the response to subsequent flashes was essentially independent of the period of dark adaptation. Rm(p3) increased with advance of dark adaptation. However, S was nearly constant. Our results suggest that the cause of delayed dark adaptation is not to be sought in the activation of phototransduction process or the regeneration of rhodopsin per se but rather in the deactivation process of the phototransduction cascade.

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.

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.

Morphological and functional abnormalities in the inner retina of the rd/rd mouse

We investigated the effects of photoreceptor degeneration on the anatomy and physiology of inner retinal neurons in a mouse model of retinitis pigmentosa, the retinal degeneration (rd) mutant mouse. Although there is a general assumption that the inner retinal cells do not suffer from photoreceptor death, we confirmed major changes both accompanying and after this process. Changes include sprouting of horizontal cells, lack of development of dendrites of rod bipolar cells, and progressive atrophy of dendrites in cone bipolar cells. Electrophysiological recordings demonstrate a selective impairment of second-order neurons that is not predictable on the basis of a pure photoreceptor dysfunction. Our data point out the necessity to prove integrity of the inner retina before attempting restoring visual function through photoreceptor intervention. This is even more important when considering that although intervention can be performed before the onset of any symptoms in animals carrying inherited retinopathies, this is obviously not true for human subjects.

Visual function in patients with cone-rod dystrophy (CRD) associated with mutations in the ABCA4(ABCR) gene

Mutations in the ABCA4(ABCR) gene cause autosomal recessive Stargardt disease (STGD). ABCR mutations were identified in patients with cone-rod dystrophy (CRD) and retinitis pigmentosa (RP) by direct sequencing of all 50 exons in 40 patients. Of 10 patients with RP, one contained two ABCR mutations suggesting a compound heterozygote. This patient had a characteristic fundus appearance with attenuated vessels, pale disks and bone-spicule pigmentation. Rod electroretinograms (ERGs) were non-detectable, cone ERGs were greatly reduced in amplitude and delayed in implicit time, and visual fields were constricted to 10 degrees diameter. Eleven of 30 (37%) patients with CRD had mutations in ABCR. In general, these patients showed reduced but detectable rod ERG responses, reduced and delayed cone responses, and poor visual acuity. Rod photoresponses to high intensity flashes were of reduced maximum amplitude but showed normal values for the gain of phototransduction. Most CRD patients with mutations in ABCR showed delayed recovery of sensitivity (dark adaptation) following exposure to bright light. Pupils were also significantly smaller in these patients compared to controls at 30 min following light exposure, consistent with a persistent 'equivalent light' background due to the accumulation of a tentatively identified 'noisy' photoproduct.

Analysis of photoreceptor function and inner retinal activity in juvenile X-linked retinoschisis

Thirteen retinoschisis males with genotyped XLRS1 gene mutations were examined by electroretinogram (ERG) techniques to determine photoreceptor involvement and ON-pathway and OFF-pathway sites of dysfunction. Parameters R(max) and logS determined by fitting the mathematical model of the activation phase of phototransduction to the scotopic and photopic a-wave responses, were not significantly different from normal. However, the XLRS photopic a-wave amplitudes were significantly lower than normal across all intensities, consistent with defective signaling in the OFF pathway. Long flash (150 ms) ON-OFF photopic responses showed reduced b-wave amplitude but normal d-wave amplitude, giving a reduced b/d ratio of <1.32 Hz photopic flicker ERG fundamental frequency responses showed reduced amplitude and delayed phase, consistent with abnormal signaling by both the ON- and OFF-pathway components. These results indicate that the XLRS1 protein appears not to affect photoreceptor function directly for most XLRS males, and that ERG signaling abnormalities occur in both the ON- and OFF-pathway components that originate in the proximal retina.