The directly recorded standing potential of the human eye

Natural protection of ocular surface from viral infections - A hypothesis

A pandemic outbreak of a viral respiratory infection (COVID-19) caused by a coronavirus (SARS-CoV-2) prompted a multitude of research focused on various aspects of this disease. One of the interesting aspects of the clinical manifestation of the infection is an accompanying ocular surface viral infection, viral conjunctivitis. Although occasional reports of viral conjunctivitis caused by this and the related SARS-CoV virus (causing the SARS outbreak in the early 2000s) are available, the prevalence of this complication among infected people appears low (~1%). This is surprising, considering the recent discovery of the presence of viral receptors (ACE2 and TMPRSS2) in ocular surface tissue. The discrepancy between the theoretically expected high rate of concurrence of viral ocular surface inflammation and the observed relatively low occurrence can be explained by several factors. In this work, we discuss the significance of natural protective factors related to anatomical and physiological properties of the eyes and preventing the deposition of large number of virus-loaded particles on the ocular surface. Specifically, we advance the hypothesis that the standing potential of the eye plays an important role in repelling aerosol particles (microdroplets) from the surface of the eye and discuss factors associated with this hypothesis, possible ways to test it and its implications in terms of prevention of ocular infections.

Electrophysiology in pigment epithelial changes

The c-wave of the ERG and the slow SP variations reflect mainly the activity of the pigment epithelium. However, both potentials are dependent upon the photoreceptors and/or the inner retina as well. In pigment epithelial abnormalities the c-wave is reduced or abolished, and the slow SP variations, d.c. recorded directly or investigated with the EOG, reduced or abolished as well.

Clinical electro-oculography

Electro-oculography (EOG), a retinal function test, indirectly records slow, light-induced changes in the potential difference between the anterior and posterior poles of the eye. EOG is a valuable clinical tool in diagnosis of tapetoretinal degenerations, vitelliform foveal dystrophy including the carriers of this disease, retinal intoxications and in differential diagnosis of choroidal malignant melanomas. EOG is not an alternative to electroretinography but yields additional information on the function of the retinal pigment epithelium. The main drawback of the present technique is the high inter- and intraindividual variability of the results.

Studies on acute and late stages of experimental central retinal artery occlusion in the Cynomolgus monkey. I. Intensity-amplitude relations of the D.C. recorded ERG with special reference to the c-wave

The main positive component of the c-wave of the ERG is generated by the pigment epithelium-receptor complex, which is supplied from the choroidal circulation. Occlusion of the central retinal artery (OCRA) causes serious morphological damage only to the inner retina. An effect on the c-wave is therefore not primarily to be expected. In this study on five Cynomolgus monkeys with unilateral, laser-induced OCRA the c-wave was decreased, slightly in the early stage and markedly in the late stage. A hypothetical explanation of the reduced c-wave amplitude, involving changes in potassium ion concentration, is discussed. The conflicting results in other reports may be explained by uncontrolled influence of slow c-wave amplitude oscillations, by differences between acute and late stages, and by possible surgical damage to the choroidal circulation in some cases.

Studies on acute and late stages of experimental central retinal artery occlusion in the Cynomolgus monkey. II. Influence on the cyclic changes in the amplitude of the c-wave of the ERG and in the standing potential of the eye

The slow (2-3/h) oscillations of the c-wave amplitude and of the standing potential of the eye (SP) were studied in the Cynomolgus monkey during the acute and late stages after experimentally induced (laser photocoagulation) occlusion of the central retinal artery (OCRA). Whereas the healthy control eyes showed large cyclic variations in both the c-wave amplitude and the SP, no oscillations in the c-wave amplitude were observed in the OCRA eyes at any stage, and the SP oscillations were barely detectable in these eyes. OCRA causes morphological damage to the inner retina but not to the pigment epithelium-photoreceptor complex, which generates the positive component of the c-wave, and where the SP is also believed to originate. The findings of the present study strongly indicate that the SP and c-wave oscillations are related, and that they are either dependent on an intact inner retina or that the pigment epithelium-photoreceptor complex is functionally affected by OCRA. If not taken into consideration, the marked difference in oscillations between the OCRA eye and the healthy eye may be a major source of error when comparing c-waves from the two eyes, and it seems that some of the conflicting results reported by others can be explained in this way.

From basic to clinical research: a journey with the retina, the retinal pigment epithelium, the cornea, age-related macular degeneration and hereditary degenerations, as seen in the rear view mirror

PURPOSE

This Acta Ophthalmologica Award and Gold Medal Honorary Lecture (the Lundsgaard Gold Medal Honorary Lecture) reviews some of the work I have carried out with my mentors and many of my wonderful collaborators and research students over more than 40 years, also including related work by other groups. It concentrates on the basic electrophysiology and ultrastructure of the retina and the retinal pigment epithelium (RPE), as well as covering basic and clinical aspects of the cornea, contact lenses, age-related macular degeneration (AMD) and hereditary diseases.

METHODS

The review describes research performed using light and electron microscopy, basic and clinical electrophysiology, genetics and biochemistry in animal experiments and in research on patients. It also outlines clinically used techniques, such as laser and photodynamic treatment and scanning laser ophthalmoscopy.

RESULTS

The paper reports on the following subjects: the mechanisms behind some of the electrical potentials originating in the retina and the RPE and the use of these potentials in hereditary diseases; corneal receptors for lectins and presumably for bacteria; the turnover of the photoreceptor outer segment and the formation of lipofuscin, including the relation of these processes to AMD; certain treatments for AMD, and hereditary degenerations in animal models, such as the RPE65 gene mutation in Briard dogs, which makes them a model of Leber's congenital amaurosis. The dogs are now treated successfully with gene therapy in the USA, and a clinical trial is in preparation.

CONCLUSIONS

During the last 40 years we have had the good fortune to experience a dramatic growth in knowledge and understanding within ophthalmic science of basic mechanisms. Huge progress has been made in diagnostics and clinical ophthalmological treatments, much to the benefit of our patients. Even a small contribution made by my group to these developments has been well worth the effort, particularly as scientific work is not just deeply satisfying: it is also fun!

Oscillatory potentials in the retina: what do they reveal

This chapter is an overview of current knowledge on the oscillatory potentials (OPs) of the retina. The first section describes the characteristics of the OPs. The basic, adaptational, pharmacological and developmental characteristics of the OPs are different from the a- and b-waves, the major components of the electroretinogram (ERG). The OPs are most easily recorded in mesopic adaptational conditions and reflect rapid changes of adaptation. They represent photopic and scotopic processes, probably an interaction between cone and rod activity in the retina. The OPs are sensitive to disruption of inhibitory (dopamine, GABA-, and glycine-mediated) neuronal pathways and are not selectively affected by excitatory amino acids. The earlier OPs are associated with the on-components and the late OPs with the off-components in response to a brief stimulus of light. The postnatal appearance of the first oscillatory activity is preceded by the a- and b-waves. The earlier OPs appear postnatally prior to, and mature differently from, the later ones. The second section deals with present views on the origin of the OPs. These views are developed from experimental studies with the vertebrate retina including the primate retina and clinical studies. Findings favor the conclusion that the OPs reflect neuronal synaptic activity in inhibitory feedback pathways initiated by the amacrines in the inner retina. The bipolar (or the interplexiform) cells are the probable generators of the OPs. Dopaminergic neurons, probably amacrines (or interplexiform cells), are involved in the generation of the OPs. The earlier OPs are generated in neurons related to the on-pathway of the retina and the later ones to the off-channel system. Peptidergic neurons may be indirectly involved as modulators. The individual OPs seem to represent the activation of several retinal generators. The earlier OPs are more dependent on an intact rod function and the later ones on an intact cone system. Thus, the OPs are good indicators of neuronal adaptive mechanisms in the retina and are probably the only post-synaptic neuronal components that can be recorded in the ERG except when structured stimuli are used. The last section describes the usefulness of the oscillatory response as an instrument to study the postnatal development of neuronal adaptation of the retina. In this section clinical examples of of the sensitivity of the OPs for revealing early disturbance in neuronal function in different retinal diseases such as pediatric, vascular and degenerative retinopathies are also given.

A cyclic adenosine monophosphate agonist elevates the b- and c-waves of the rabbit direct-current electroretinogram

The effects of the stable cyclic adenosine monophosphate analogue adenosine 3',5'-cyclic monophosphorothioate Sp-isomer (Sp-cAMPS) on the direct-current electroretinogram and the standing potential of the eye were studied. Corneal recordings were obtained from unilaterally vitrectomized albino rabbit eyes during alternating intravitreal perfusions with Sp-cAMPS and a control solution (Pharmacia eye irrigating solution). The contralateral eye was used as a control. To evaluate further the effects on the c-wave, in vivo intraretinal microelectrode measurements were made during simultaneous intravitreal perfusion of Sp-cAMPS and irrigating solution, respectively. Sp-cAMPS in concentrations of 1, 10 and 100 microM was tested by corneal direct-current electroretinography. There was no significant effect on the a-wave amplitude. The b-wave amplitude was reversibly elevated at an Sp-cAMPS concentration of 100 microM was tested by corneal direct-current electroretinography. There was no significant effect on the a-wave amplitude. The b-wave amplitude was reversibly elevated at an Sp-cAMPS concentration of 100 microM (p < 0.01, n = 7). The c-wave amplitude was reversibly elevated at an Sp-cAMPS concentration of 100 microM (p < 0.01, n = 7). The c-wave amplitude was reversibly elevated at a concentration of 10 microM (p < 0.001, n = 8), and this effect was more pronounced at 100 microM (p < 0.001, n = 7). The SP increased reversibly at a concentration of 100 microM (p < 0.001, n = 7). Microelectrode recordings were performed with Sp-cAMPS at a concentration of 100 microM. The recordings showed significant increases in both the transepithelial potential (p < 0.01, n = 3) and the slow PIII (p < 0.01, n = 3). The effects of Sp-cAMPS on the b-wave as well as the two components of the c-wave suggest influences on both the inner retina and the retinal pigment epithelium of the rabbit eye.

Adrenergic effects on the corneal and intraretinal direct-current electroretinogram and on the standing potential of albino rabbit eyes

This study was undertaken to investigate further the responsiveness of the albino rabbit retinal pigment epithelium and the inner retina to adrenergic agents as reflected in changes of the direct-current electroretinogram and of the standing potential of the eye. After unilateral vitrectomy on albino rabbits, a continuous intraocular perfusion with a reference solution was established. The reference solution was then alternated with the test solution. The direct-current electroretinogram and the standing potential were recorded from both eyes with a scleral contact lens and a reference electrode connected to matched calomel half-cells. An in vivo experimental technique that allows intraocular perfusion of a test substance and simultaneous intraretinal microelectrode measurements was also used. The alpha-adrenergic agonist phenylephrine (0.04 microM, n = 8) produced a reversible increase in c-wave amplitude (48%, p < 0.001) and also a small increase in b-wave amplitude (12%, p < 0.002). There was no significant influence on the a-wave amplitude. The standing potential was elevated at 1694 +/- 362 microV (mean +/- SEM) (p < 0.002). The alpha 2-adrenergic agonist clonidine caused similar effects on the electroretinogram, although at a higher concentration (40 microM, n = 5), with an elevation of the c-wave (25%, p < 0.001) and a small b-wave increase (12% p < 0.002). No significant influence on the a-wave or on the standing potential was found. Intraretinal direct-current electroretinogram-recordings during intraocular perfusion with phenylephrine showed an increase in transepithelial potential (p < 0.004; n = 6), accompanied by a reduction of the slow PIII (p < 0.0035; n = 6). The c-wave increase resulting from alpha-adrenergic stimulation seems to be generated partly across the retinal pigment epithelium, with an increase in transepithelial potential, combined with a reduction of the slow PIII. The elevation of the b-wave amplitude, together with the influence on the slow PIII, suggests alpha-adrenergic effects also on the inner retina. The experimental technique used in this study with intraocular perfusion after vitrectomy and simultaneous intraretinal direct-current recordings seems to be a practicable method for studies of the influence of pharmacologic agents on the retina and the retinal pigment epithelium.

Electro-oculogram changes in patients with ocular hypertension and primary open-angle glaucoma

Recent evidence suggests that retinal hypoxia and ischemia affect the standing potential of the eye and the activity of the photoreceptors. To test whether chronically elevated intraocular pressure would produce similar effects, we measured electro-oculograms in two groups of patients: ocular hypertensive patients and patients with primary open-angle glaucoma. There were significant differences among the average electro-oculogram ratios of these groups compared to age-similar controls. The control observers had an average light-peak/dark-trough ratio of 2.86, the ocular hypertensive patients had an average ratio of 2.44, and the patients with primary open-angle glaucoma had an average ratio of 2.07. This indicates that long-term elevations in intraocular pressure can decrease the light peak of the electro-oculogram, even in patients with no other evidence of glaucomatous damage. This deficit may have its origins in the sensitivity of the outer retina to choroidal ischemia.

Electrophysiological consequences of retinal hypoxia

Experiments on cats show that electrical activity of the inner (proximal) retina is unaffected during systemic hypoxia as long as arterial oxygen tension (PaO2) is above 40 mm Hg. This is due to effective regulation of inner retinal tissue PO2 by the retinal circulation. In contrast, some electrical signals generated in the outer (distal) retina begin to change when PaO2 falls below 70-80 mmHg. The outer retinal responses are generated by the retinal pigment epithelium, but their susceptibility to hypoxia results primarily from their dependence on photoreceptors. Photoreceptor metabolism is sensitive to hypoxia because of the high oxygen consumption of photoreceptors and their reliance on the choroidal circulation, which cannot regulate PO2 in the outer retina. Retinal electrophysiology and oxygen distribution are altered by acutely elevated intraocular pressure just as by hypoxia. These results raise the question as to how inner retinal function can be preserved when outer retinal function is altered. The explanations proposed relate to (1) differences in conditions of light adaptation in different studies, (2) the possible inappropriateness of the previous measurements in the inner retina for revealing photoreceptor dysfunction, and (3) a possible preservation of photoreceptor electrical responses when their metabolism is altered. Comparison of cat and human studies suggests that the human retina is affected in much the same way during hypoxia as the cat retina, but further experiments are required for an understanding of the role of hypoxia in human disease.

Influence of isobutylmethylxanthine on the direct current electroretinogram of albino rabbit eyes

The direct current electroretinogram and the standing potential of the eye were recorded from both eyes of unilaterally vitrectomized albino rabbits. The effect of intraocular irrigation with the nonselective phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) was compared with that of a balanced salt control solution. IBMX (0.5 mM) produced a reversible increase in the amplitudes of the a-wave (19%, p less than 0.02), b-wave (21%, p less than 0.001) and c-wave (12%, p less than 0.01) of the electroretinogram. A small elevation of the standing potential was found (0.4 +/- 0.2 mV), but this increase was not statistically significant (p greater than 0.05). The results indicate a primary and principal influence of IBMX on the photoreceptors.

Effects of intravitreal perfusion with dopamine in different concentrations on the DC electroretinogram and the standing potential of the albino rabbit eye

The direct current electroretinogram and the standing potential were recorded from both eyes of 23 albino rabbits during intraocular perfusion of one of the eyes, which was vitrectomized, with a physiologic reference solution (PHS). PHS was then replaced by a test solution containing dopamine dissolved in PHS. The fluids were subsequently alternated (PHS-dopamine-PHS). During irrigation with 0.25-0.5 mM dopamine (11 rabbits) the c-wave amplitude was 140% higher (p less than 0.001) and during irrigation with 25 mM dopamine (6 rabbits) 85% lower (p less than 0.01) than it was during the corresponding initial perfusion with PHS. The simultaneously recorded b-wave amplitude was reduced (0.25-0.5 mM: -22%, p less than 0.001; 25 mM: -69%, p less than 0.001) and the SP level increased (0.25-0.5 mM: +2375 microV, p less than 0.01; 25 mM: +2530 microV, p less than 0.05) compared with the values obtained during the corresponding preceding irrigation with PHS. Thus the changes in the b- and c-wave amplitudes during perfusion with dopamine were dependent on the concentration of the drug. In the contralateral control eye (23 rabbits) the c-wave amplitude was 21% higher (p less than 0.001), the b-wave amplitude 14% higher (p less than 0.001) and the standing potential 1007 microV higher (p less than 0.001) during intravitreal perfusion with dopamine in the other eye than during the preceding irrigation with PHS in that eye, possibly as a result of increasing dark adaptation.

Corneal D.C. recordings of slow ocular potential changes such as the ERG c-wave and the light peak in clinical work. Equipment and examples of results

A set-up for D.C. recordings of slow ocular potentials such as the c-wave of the electroretinogram (ERG) as well as the fast oscillation (FO), the light peak (LP) and the dark trough (DT) in both clinical and experimental work is described. It includes matched calomel half-cells connected by saline-agar bridges to a corneal contact lens on the eye and a reference chamber on the forehead, a low-drift differential-input D.C. amplifier, an A/D converter, a computer, a thermoprinter, a flexible disc memory, a plotter, and a device for light stimulation controlled by the computer. Examples of the usefulness of the set-up in clinical work are shown in the form of D.C. c-wave ERGs of normal subjects as well as of patients with vitelliform macular degeneration, choriocapillaris atrophy, and retinitis pigmentosa. The direct corneal recording of the FO and LP is demonstrated as well. The different origins of the standing potential (SP) of the eye, the ERG c-wave, the FO and the LP are reviewed briefly.

Effects of intraocular irrigation with melatonin on the c-wave of the direct current electroretinogram and on the standing potential of the eye in albino rabbits

The direct current electroretinogram (ERG) and the standing potential (SP) were recorded from both eyes of 14 albino rabbits during intraocular perfusion of one of the eyes, which was vitrectomized, with a recently developed eye irrigation solution (PHS) produced by Pharmacia Ophthalmics. PHS was then replaced by a test solution containing melatonin dissolved in PHS (0.002 microM-200 microM). The fluids were subsequently alternated (PHS - melatonin - PHS). During uniocular irrigation with melatonin the mean c-wave amplitude and SP level of the intact control eye were increased (c-wave +24%, p less than 0.01; SP +0.65 mV, p less than 0.05) compared with the values during the initial perfusion with PHS. In contrast, the c-wave amplitude of the irrigated eye was markedly decreased in many rabbits during perfusion with melatonin compared with the initial PHS, but the mean reduction was small and not statistically significant. The mean SP level was reduced (-1.54 mV, p less than 0.001). This difference between the eyes probably depends on the route by which melatonin reaches the retinal pigment epithelium and thus whether it primarily affects the apical (as in the irrigated eye) or the basal (as in the control eye) pigment epithelial membrane. A peak in the b-wave amplitude was observed in both eyes during uniocular irrigation with melatonin when compared with the amplitude measured during the initial perfusion with PHS (irrigated eye: +27%, p less than 0.001; control eye +18%, p less than 0.002).

Progressive retinal atrophy in the Abyssinian cat: studies of the DC-recorded electroretinogram and the standing potential of the eye

DC-recorded electroretinography (ERG) and direct recordings of the standing potential (SP) were performed on a group of normal cats and Abyssinian cats affected by a hereditary retinal degenerative disease with similarities to human retinitis pigmentosa. A significant reduction of a- and b-wave amplitudes was found at an early stage of disease at a time when there were no major alterations in the c-wave and SP. At later stages both the c-wave and the SP oscillations were significantly reduced or absent. These findings indicate a primary photo-receptor disorder. Threshold studies for the scotopic b-wave showed a loss of retinal sensitivity early in the disease at a time when 30 Hz flicker responses were normal, which could indicate an earlier involvement of the rods than of the cones. There were no major alterations in the timing of the ERG in the affected animals tested.

Delayed basal hyperpolarization of cat retinal pigment epithelium and its relation to the fast oscillation of the DC electroretinogram

Previous work has shown that the cat retinal pigment epithelium (RPE) is the source of two potential changes that follow the absorption of light by photoreceptors: a hyperpolarization of the apical membrane, peaking in 2-4 s, which leads to the RPE component of the electroretinogram (ERG) c-wave, and a depolarization of the basal membrane, peaking in 5 min, which leads to the light peak. This paper describes a new basal membrane response of intermediate time course, called the delayed basal hyperpolarization. Isolation of this response from other RPE potentials showed that with maintained illumination the hyperpolarization begins approximately 2 s after light onset, peaks in 20 s, and slowly ends as the membrane repolarizes over the next 60 s. The delayed basal hyperpolarization is very small for stimuli less than 4 s in duration and grows with duration, becoming approximately 15% as large as the preceding apical hyperpolarization with stimuli longer than 20 s. Extracellularly, this response contributes to the transepithelial potential (TEP) across the RPE. In response to light the TEP first rises to a peak, the c-wave, as the apical membrane hyperpolarizes. For stimuli longer than approximately 4 s, the decline of the TEP from the peak of the c-wave results partly from the recovery of apical membrane potential and partly from the delayed basal hyperpolarization. For long periods of illumination (300 s) the delayed basal hyperpolarization leads to a trough in the TEP between the c-wave and light peak. This trough is largely responsible for a corresponding trough in vitreal recordings, which has been called the "fast oscillation." The term "fast oscillation" has also been used to denote the sequence of potential changes resulting from repeated stimuli approximately 1 min in duration. In addition to the delayed basal hyperpolarization, such responses also contain a basal off-response, a delayed depolarization.

Toxic ocular effects of two fibrinolytic drugs. An experimental electroretinographic study on albino rabbits

The toxic effects on rabbit eyes of 2 intravitreally injected fibrinolytic substances at different concentrations were studied with repeated clinical observations and registrations of the DC ERG. The fellow, control eye of each animal was injected with saline. Urokinase (Ukidan, Serono) (13 rabbits) initially produced aqueous flare (64%), iris hyperaemia (36%) vitreous opacities (27%) and small retinal haemorrhages (18%). 2-3 months after the injection cataract (50%), vitreous opacities (25%) and retinal changes (13%) were observed. The highest dose (10 000 Ploug units) caused reduction of the ERG b-wave, as a sign of retinal toxicity. Tissue activator (D-44, Centre d'immunologie et de biologie Pierre Fabre) (10 rabbits) produced marked aqueous flare (initially 100%, after 2 weeks 50%) and pronounced, persistent vitreous opacities (25% after 2-3 months). At the late stage corneal blood vessels (38%) and cataract (38%) were also found, but only in eyes injected with the highest dose (1000 units), which was retinotoxic as judged by the ERG (reduced b- and c-waves).

Three light-evoked responses of the retinal pigment epithelium

This paper summarizes our findings on light-evoked changes in retinal pigment epithelial cell (RPE) membrane potentials. Experiments were performed on the eye of the anesthetized or decerebrate cat and on isolated tissues from the eyes of a lizard, Gekko gekko, and a frog, Rana catesbeiana. In cat, as was previously shown, the RPE apical membrane potential responds to changes in [K+]0 in the subretinal space. At the onset of illumination it hyperpolarizes to a peak at 4.0 sec as [K+]0 decreases. The next RPE response is a hyperpolarization of the basal membrane that peaks at 20 sec and is also dependent on the decrease in subretinal [K+]0. The last and slowest response is a depolarization of the basal membrane that peaks at 300 sec, and is not obviously associated with K+ changes. The same responses also appear in gecko at a slower time-course, but only the apical-membrane K+-response is present in frog. The three responses also are associated with changes of the opposite polarity at the offset of illumination. These changes in membrane potential are the origin, respectively, of the RPE component of the ERG c-wave, the fast oscillation, and the light peak (slow oscillation).

Origin of the light peak: in vitro study of Gekko gekko

1. The light peak is a large, light-evoked increase in standing potential recorded in mammals, birds and reptiles. We have studied the cellular origin of the light peak in an in vitro preparation of neural retina-pigment epithelium (r.p.e)-choroid from the lizard, Gekko gekko. The tissue was mounted between two separate bathing solutions; the trans-tissue potential was recorded retinal-side positive; micro-electrodes were introduced to measure the trans-epithelial potential (t.e.p.) and to record intracellularly from the r.p.e.2. A 10 min stimulus of diffuse white light evoked an increase in trans-tissue potential that reached maximum amplitude, the light peak, about 15 min after stimulus onset. Since the light peak is present in vitro, it must originate in either the neural retina or the r.p.e.3. A micro-electrode was positioned in the subretinal space and the trans-retinal potential and t.e.p. were measured simultaneously. A 10 min stimulus produced an increase in t.e.p. equal in magnitude and time course to the trans-tissue light peak; no potential was present across the retina. The light peak is therefore generated solely across the r.p.e.4. Intracellular r.p.e. recordings were made to determine whether the light peak was generated at the apical or basal membrane or across the paracellular shunt. A 10 min stimulus first caused a hyperpolarization of both membranes with a time course similar to the r.p.e. c-wave followed by a depolarization of both membranes with the time course of the light peak. We conclude that whereas the r.p.e. c-wave results from a hyperpolarization of the apical membrane, the light peak is generated by a depolarization of the basal membrane of the r.p.e.5. Changes in tissue resistance, R(t), and the ratio of apical to basal membrane resistances, a, were monitored during the light peak by passing current across the tissue and measuring the appropriate current-induced voltages. R(t) decreased and a increased with the time course of the light peak. Assuming that the paracellular shunt resistance is constant, we conclude that the light peak is accompanied by an increase in basal membrane conductance.6. This and the following paper present the first direct demonstration of an interaction between the neural retina and the basal membrane of the r.p.e. The light peak, initiated by absorption of light by photoreceptors, results in a depolarization and conductance increase of the basal membrane.

Intraretinal recordings of slow electrical responses to steady illumination in monkey: isolation of receptor responses and the origin of the light peak

The onset of steady illumination of the mammalian retina initiates a series of complex electrical responses. Corneal recordings in man show a relatively fast ERG, followed by a much slower "light response": a "light peak" followed by a series of damped slow oscillations that may last for 1 or 2 hr. The latter oscillations are a prominent feature of the EOG. We recorded the response to steady illumination with micro electrodes in the intact rhesus monkey eye. The intraretinal "light response" was analyzed by simultaneous recordings at different depths, using a bipolar microelectrode. We found that a steady photoreceptor component can be isolated by fractional recording across the cone outer segment layer in the fovea. Further, through simultaneous recordings at the retinal and choroidal sides of the retinal pigment epithelium, we found that this structure is most probably the generating site of the "light peak" and subsequent oscillations of the standing potential.

Early effects of sodium iodate on the directly recorded standing potential of the eye and on the c-wave of the DC registered electroretinogram in albino rabbits

The early effects of intravenously administered sodium iodate (NaIO3) on the directly recorded standing potential (SP) of the eye and on the c- and b-waves of the DC registered ERG were studied in 8 anaesthetized albino rabbits. In 5 of 6 animals obtaining 40 mg NaIO3/kg bwt. the SP decreased immediately following the injection, and had attained a level 3.5 - 4 mV below the original one after 1 h. The c-wave declined rapidly and 6 min after the injection it was replaced by a large cornea-negative potential. The b-wave was relatively unchanged except in one animal. In 2 rabbits treated with 30 mg NaIO3/kg btw. and in the 6th animal obtaining 40 mg NaIO3/kg bwt. an SP increase instead of a decrease was seen, and the c-wave was more slowly (about 22 min after the injection) replaced by the cornea-negative potential. The b-wave was somewhat increases. These results demonstrate the dose-related and inter-individual variability in the SP reaction to NaIO3 and are in good agreement with the well-known ultrastructural pigment epithelial injury and c-wave changes produced by this substance.

The ERG c-wave in vitelliruptive macular degeneration (VMD)

D.c. ERG registrations and EOG recordings were obtained from six patients with vitelliruptive macular degeneration (VMD). In all cases the EOG was highly pathological, but the alpha- and beta-waves of the ERG were normal. This is typical of VMD, which starts as a generalized defect of the pigment epithelium. Four patients showed no evidence of a c-wave. The other patients demonstrated small c-waves, but only under certain stimulus conditions. Thus, varying stimulus durations, intensities and frequencies are sometimes needed to decide whether or not ERG c-waves can be elicited in different diseases or suspected disorders. The findings are in agreement with the presence of a generalized pigment epithelial defect in VMD, since the major positive component of the c-wave is generated by the pigment epithelium-receptor complex.

The c-wave of the human electroretinogram in central retinal artery occlusion

The pigment epithelium-receptor complex, which is supplied from the choroidal circulation, is the main source of the c-wave of the ERG. Occlusion of the central retinal artery (OCRA), which causes serious damage to the inner retinal layers, should therefore not primarily be expected to affect the c-wave amplitude. Nevertheless, conflicting reports of increased, decreased and unchanged c-waves have appeared in the literature. In the present study on four patients with OCRA the affected eyes showed diminished b- and c-waves. On the other hand, both the a-wave and the trough between the b- and c-waves were accentuated in the diseased eyes. Experiments in progress on Cynomolgus monkeys with experimentally induced unilateral OCRA show similar results.

[Chromatic adaptation produces opposite effects on the on and off responses of the photopic electroretinogram (author's transl)]

The electrical response of the light adapted human eye to onset (a-wave) and offset (d1-wave of the positive off-effect) of monochromatic test stimuli between 448 and 654 nm (Ganzfeld stimulation, duration of test flash 0.3 s) was recorded by means of the electroretinogram in 10 normal observers during steady illumination of various wavelengths of 2.2 log Troland. During exposure to white light the spectral sensitivity of the a-wave and off-effect as determined by a criterion amplitude of 25 micronV were similar, the sensitivity of the off-effect being 0.4 log units lower as compared to the a-wave. Blue adaptation (467 nm) decreased the sensitivity of the a-wave and increased the sensitivity of the off-effect within the short wavelengths region of the spectrum. Red adaptation (630 nm) decreased the sensitivity of the a-wave and increased the sensitivity of the off-effect within longer wavelengths. Green adaptation (530 nm) produced only a small decrease of sensitivity of the a-wave for green test light, the sensitivity of the off-effect remained constant. The opposite changes of the a-wave and off-effect during chromatic adaptation demonstrate different participation of these components in the generation of color mechanisms in the human retina.

The influence of stimulus duration on the human d.c. registered c-wave. A quantitative study

The c-wave of the human ERG was studied at different stimulus durations with a d.c. technique which permitted stable and reproducible recordings. With increasing stimulus lengths the implicit time increased up to a maximum of about 5.5 s. Also the amplitude of the c-wave rose. However, it was influenced by positive and negative off-effects, seen in most volunteers, and at several stimulus lengths superimposed upon the peak of the c-wave. This fact must be considered when developing a standardized method for measuring the c-wave amplitude proper.

A method for DC-ERG recording of alert humans

We developed a technique for DC-coupled ERG recording of the alert human. The equipment consists of three units: a new cornea suction glass, a separate electrode, and a vacuum control device. We get stable recordings of more than one-half hour. By comparison with the EOG the identity of the EOG "on"-peak and ERG c-wave is assured. A fine structure of DC-ERG recording of 100 muV amplitude is closely correlated to the variation of the blood pressure in the ophthalmic artery. It may either be caused by the variable electric conductivity of the eyeball due to the blood pressure or it may reflect a variation of the retinal potential itself.

Effects of ethyl alcohol on the directly recorded standing potential of the human eye

The effects of ethanol on the human standing potential (SP) were studied with a recently developed method, which allows direct SP recordings by means of a suction contact lens, temperature stabilized calomel electrodes and d.c. amplification. It is well known that the human SP oscillates with a frequency of about 2/hour in response to a sudden change in illumination. In the present paper marked cyclic variations of the SP, resembling damped oscillations, were provoked by a small oral dose of ethyl alcohol. A first maximum was reached after about 10 min. The difference in amplitude between the peak and the trough of the first oscillation was of the order of 4 mV. The oscillatory frequency was about 2/hour. The length of a cycle varied between 25 and 34 min in different volunteers, being fairly constant in the same subject on different occasions. The SP response to ethanol was similar both under scotopic and photopic conditions. The results correlate well with earlier findings of 2/hour oscillations in c-wave amplitude in response to ethanol, as may be expected considering the partly common origin of the c-wave and the SP.

Covariation of the simultaneously recorded c-wave and standing potential of the human eye

The c-wave and the directly recorded standing potential (SP) of the human eye were studied with the aid of a recently developed method including matched temperature stabilized calomel electrodes, d.c. amplifiers and a suction contact lens. This technique, which does not require general anaesthesia, permits simultaneous direct d.c. recordings of the SP and the c-wave in human volunteers during long-term experiments. Upon repetitive light flashes (stimulus duration 1 sec, interval 20 sec and flash intensity 4.5 rel. log units above b-wave threshold) both variables responded with slow amplitude oscillations with a frequency of about 2/hour. The oscillations were similar as to phases and frequencies. Both the potentials are held to be generated mainly in the pigment epithelium. Considering this partly common origin the observed covariation was an interesting finding.