The A-wave of the human electroretinogram and rod receptor function

The amplitude of the leading edge of the a-wave of the human electroretinogram (ERG) was compared with predictions from a computational model of the light-induced responses of rod mammalian receptors. According to this model, a linear process describes the amplitude and time course of the response to relatively low flash intensities and at brief times after the onset of the flash. At higher flash intensities, a nonlinear process, described by the Naka-Rushton function or a saturating exponential, is involved. The primary focus here is on intensity-response data recorded with a clinical ganzfeld apparatus. The leading edge of the rod a-wave recorded from normal observers and patients with congenital stationary night blindness (CSNB) was described by a linear process for flash intensities up to the maximum available flash intensity, 2.0 log scot td-sec. This finding is consistent with the model of the rod's response. It suggests, however, that when ERGs are recorded with clinical systems limited to 2.0 log scot td-sec, these data cannot be used to distinguish between changes in the parameters (eg, semisaturation intensity versus maximum response) of the human rod receptors. Responses to flash intensities up to 3.4 log scot td-sec were recorded using a custom, high-intensity ganzfeld system. Both the linear and nonlinear components of the model were needed to fit the ERGs recorded with this system. This suggests that changes in different receptor parameters can be distinguished with higher intensity flashes.

Diagnostic clinical findings of a new syndrome with night blindness, maculopathy, and enhanced S cone sensitivity

We studied eight patients who had night blindness, maculopathy (often cystoid), degenerative changes in the region of the vascular arcades, relatively mild visual field loss, and an unusual but characteristic electroretinogram. The dark-adapted electroretinogram showed no response to low-intensity stimuli that normally activate the rods, but large, slow responses to high-intensity stimuli. These large, slow waveforms persisted without change under light adaptation, and showed a striking mismatch to photopically balanced short and long wavelength stimuli (with sensitivity much greater to short than long wave-lengths). Since there is evidence from other studies that the electroretinogram and psychophysical responses represent hypersensitivity of short wavelength-sensitive (S or blue) cones, we propose that this disorder be called the enhanced S cone syndrome. There can be different degrees of severity in this syndrome, and progression appears to be slow.

Dark adaptation in locally detached retina

Nonrhegmatogenous retinal detachments were formed in the eyes of Dutch rabbits by subretinal injection of Hanks' balanced salt solution. The electroretinogram (ERG) was recorded locally from the acutely detached retina, and simultaneously from the surrounding attached retina (vitreal ERG [VERG]), before and after exposure to diffuse intense irradiation. Light adaptation elevated b-wave threshold for both the local ERG (LERG) and VERG by about 3 log units; thresholds for both responses recovered fully within 60-90 min after the irradiation. The normal time course of dark adaptation of the LERG suggests the occurrence of substantial rhodopsin regeneration in the rod photoreceptors of nonrhegmatogenously detached retina. These results differ from reports that visual pigment regeneration is slow in central serous chorioretinopathy, possibly because our detachments were studied within hours of formation, whereas some photoreceptor degeneration may be present in older clinical detachments.

Patterns of rod and cone dysfunction in Bardet-Biedl syndrome

We studied visual function in 16 patients with the Bardet-Biedl syndrome. Visual acuity, kinetic perimetry, and electroretinography results indicated a severe loss of central and peripheral vision and rod and cone function by the second or third decade of life. Light- and dark-adapted static perimetry in patients 10 to 15 years of age with early involvement showed a parallel and marked loss of rod and cone sensitivity across the visual field. Patients with more advanced disease and no measurable peripheral visual field showed different patterns of central visual dysfunction: an island of only cone function centered in a bull's-eye lesion; patches of rod function surrounding geographic atrophy; or a central island of excellent rod sensitivity but severely impaired cones. In the two least-affected patients, a 13-year-old boy and the asymptomatic 45-year-old sibling of a patient, there were more rod than cone abnormalities as determined by electroretinography and static perimetry.

Hue discrimination and S cone pathway sensitivity in early diabetic retinopathy

Measures of hue discrimination and M (green) and S (blue) cone pathway sensitivities were compared in a group of 24 diabetics with either early background retinopathy or no retinopathy. The Farnsworth-Munsell 100-hue test was used to measure hue discrimination, and a two-color increment threshold technique was used to measure S and M cone pathway sensitivities. The results were compared to the level of diabetic retinopathy, to the degree of macular edema, and to the duration of the disease. No significant correlation was found between the Farnsworth-Munsell 100-hue error scores and the level of retinopathy; S cone pathway sensitivity loss, however, correlated significantly with both the level of retinopathy and the degree of macular edema. Our results indicate that measurements of S cone pathway sensitivity using an increment threshold technique provide a more sensitive method than hue discrimination for detecting color vision deficits in early diabetic retinopathy.

Aland eye disease (Forsius-Eriksson-Miyake syndrome) with probability established in a Danish family

A reinvestigation of a Danish family with X-linked inherited congenital nystagmus through 6 generations revealed a congenital stationary retinal dysfunction syndrome with characteristics of both incomplete congenital stationary night blindness and Aland Eye Disease. In spite of rather uniform electrophysiological findings in our patients, this retinal disorder which affects both cones and rods demonstrated considerable intrafamilial diversity with respect to visual acuity, nystagmus, refractive state and fundus pigmentation.

SWS (blue) cone hypersensitivity in a newly identified retinal degeneration

Photoreceptor-mediated mechanisms were studied in patients with a recently identified retinopathy typified by night blindness, cystoid maculopathy, and similar scotopic and photopic electroretinograms (ERGs). Dark-adapted spectral sensitivity functions were only partly explained as composites of rod and cone curves shifted to lower sensitivities; there was unusually high sensitivity from 400-460 nm. A rod mechanism, reduced in sensitivity by at least 3 log units, was detectable with dark adaptometry. No measurable rhodopsin was found with fundus reflectometry. Light-adapted spectral sensitivities were subnormal for wavelengths greater than 500 nm but supernormal from 420-460 nm. On a yellow adapting field, the supernormal spectrum approximated that of the short-wavelength-sensitive (SWS) cone system. With spectral ERGs, two mechanisms were demonstrated. Dark- and light-adapted ERGs to green, orange-yellow, and red stimuli had similar waveforms and coincident intensity-response functions on a photopic intensity axis. ERGs to blue and blue-green stimuli were similar, and intensity-response functions coincided on a SWS cone intensity axis. Patients varied in the degree to which rod and midspectral cone function were decreased and SWS cone function was increased.

Changes in ERG amplitude following laser induced damage to the primate retina

The flash evoked ERG is used extensively as a measure of retinal function. To understand better the relationship between extent of retinal damage and the ERG we created multistage laser lesions in one eye each of three Macaca fascicularis monkeys. An argon-dye laser was used to deliver 630 nm energy to cumulative quarter sectors of the temporal hemiretina and the foveal region at two week intervals. Latencies of the a- and b-waves and amplitude from the trough of the a-wave to peak of the b-wave were measured preoperatively and one week after each new lesion. Following the last recording session the eyes were removed and prepared for histological examination. In general, but depending on stimulus condition, the maximum decrease in ERG amplitude (50-70% of normal) occurred following the third or fourth sector lesions. These lesions accounted for a loss of the outer nuclear layer in 18-25% of the entire retinal surface area extending from the edge of the fovea into the anterior equatorial region. Surprisingly, subsequent lesions of the remaining sector and fovea did not result in further reduction of ERG amplitudes. Significant increases in amplitudes were even observed for some stimulus conditions following these final lesions. Latencies of the a- and b-waves were not affected. The fact that in the present study ERG amplitudes recovered, suggest that such values may be tenuous evidence of amount of functional retina in laser-lesioned eyes.

Electroretinograms in microcephaly with chorioretinal degeneration

We recorded full-field electroretinograms from a family with two daughters with microcephaly and chorioretinal degeneration and a third daughter and mother with microcephaly without chorioretinal degeneration. The two siblings with inferior chorioretinal degeneration showed electroretinographic responses to 0.5-Hz white light that were reduced 60% to 70% below normal, suggesting that the loss of photoreceptor function exceeded the areas of visible atrophy. The mother and third daughter had normal electroretinograms. The two siblings, ages 12 and 21 years, had virtually the same electroretinographic amplitudes. In a second family, a man with microcephaly and inferior chorioretinal degeneration, examined at ages 9 and 23 years, also showed 60% to 70% reduction in electroretinographic responses to 0.5-Hz white light and showed no change in amplitudes over the 14-year interval. These findings suggest that the chorioretinal degeneration sometimes associated with microcephaly is stable in young adult life, although the long-term prognosis remains to be defined.

Cone dysfunction in a subgroup of patients with autosomal dominant cerebellar ataxia

Four patients with autosomal dominant cerebellar ataxia and retinal degeneration underwent neurologic and ophthalmologic examinations and computer-assessed corneal electroretinography. Previous reports described progressive panretinal degeneration initially involving the cones and subsequently spreading to the rods. By contrast, all our patients displayed evidence of selective dysfunction of the cone system in one or two successive electroretinograms, irrespective of their age or duration of visual symptoms. Color vision testing, funduscopy, and fluorescein angiography suggested a cone dystrophy. Within the period of observation only one patient showed progression of cone dysfunction. In patients with hereditary ataxias, quantitative electroretinography provides an objective and sometimes early indication of retinal degeneration and helps to characterize it.

A form of congenital stationary night blindness with apparent defect of rod phototransduction

We report findings obtained from an individual with an unusual form of congenital stationary night blindness (CSNB). Although the rhodopsin density difference of this subject was normal, there was no evidence of rod-mediated visual function. Dark-adapted thresholds were cone-mediated, and dark-adapted electroretinograms (ERGs) represented activity of the cone system exclusively. ERG a- and b-waves obtained under light-adapted conditions were normal. The absence of a rod a-wave but the presence of normal rhodopsin density, in combination with normal cone function, indicates that this form of CSNB likely involves a defect of phototransduction that is limited to the rods. In addition, light-adapted b-wave responses to high luminance flashes were larger than dark-adapted responses, whereas a-wave amplitudes were reduced by light adaptation. These ERG results address proposed mechanisms by which light adaptation might enhance cone system responses.

Rod-cone dystrophy of the retina. Continuation of a family study described in 1923

In 1923 Alkio described a sibship with 4 children showing macular dystrophy. In the literature this has been classed among the cone dystrophy group. However, Alkio's patients later became completely blind by the age of 50. According to the hospital records one of them showed marked destruction and pigment degeneration throughout the fundus. The daughter of one of those affected has healthy eyes, but two of her three sons are affected in the same way as their grandfather. They were studied by us in 1987. These two males showed the bull's eye type of macular dystrophy with visual acuity of 20/30 resp 20/50, marked dyschromatopsia, central scotoma in the visual field for weak markers, extinguished ERGs and subnormal EOGs. Dark adaptation was considered pathological especially during the cone phase. The peripheral fundus was within normal limits. The parents of the two patients have a common ancestor born in 1720. Autosomal recessive inheritance is therefore very likely. We think that the disorder is closer to the rod-cone dystrophy group than to cone or cone-rod degenerations.

Models of the normal and abnormal rod system

A framework is presented for using threshold data to test hypotheses about the action of a disease, a chemical agent, or a developmental process. A model of the normal rod system, based on models from the physiological and psychophysical literature, is presented. Hypotheses about the alteration of the rod system are specified in this model. The approach is illustrated with a class of hypotheses that places the decrease in sensitivity with retinal disease at the rod receptors and with data from patients with retinitis pigmentosa and congenital stationary night blindness. The implications for models of the normal rod system are considered.

Retinal detachment prevents normal assembly of disk membranes in vitro

The effects of retinal detachment upon disk membrane assembly in rod outer segments were assessed in Xenopus laevis retinas that had been maintained in eyecup cultures for up to 4 days. In these cultures, assembly of disk membranes occurred at a normal rate in regions of the retina that remained attached to the retinal pigment epithelium. In regions of the retina that were detached from the pigment epithelium, the assembly of new disk membranes either was abnormal or was inhibited. This result cannot be attributed to reduced access of cells in the detached retina to oxygen and metabolites. The experiments described here suggest that the apposition of the retina with the pigment epithelium is a necessary condition for normal disk membrane assembly in Xenopus retinas. This effect may be mediated by contact between the rod outer segments and the pigment epithelium, or by trophic factors in the subretinal space.

[Changes in the B wave of cones and rods in diabetes mellitus induced by streptozotocin in rats]

The purpose of the present study was to evaluate the electroretinophysiological effects of streptozotocin-induced diabetes in the rat. More specifically, both B wave amplitudes and implicit times generated with various intensities of light stimulation were examined in diabetic animals adapted to either scotopic or photopic conditions. Results indicate that B wave amplitudes, but not implicit times, were differentially affected by these two conditions of light adaptation: amplitudes were significantly enhanced but markedly decreased under photopic and scotopic conditions respectively. The possible functional relationships between these specific ERG changes and our previous observations of selective alterations in retinal dopaminergic systems in this animal model of diabetes are discussed.

Cone interaction and color substitution as revealed by pattern ERG

Transient electroretinograms to a reversing color-contrast checkerboard pattern (P-ERG) were recorded in a protanomalous, a deuteranomalous, and a normal observer. Alternate monochromatic checks were of constant wavelength (630 nm red-531 nm green), while the relative energies were varied systematically. When changing the radiance ratio 630 nm-531 nm of the stimulus, the normal subject exhibited a P-ERG to all stimuli with only a relative amplitude minimum at a distinct radiance ratio, whereas the color-deficient observers failed to show a P-ERG at some color contrast 630 nm-531 nm, the radiance ratio of which was different in the protan and deutan. From the radiance ratio of color contrast for the smallest potential in the normal observer, we conclude that the green- and red-sensitive cone mechanism provides a difference signal which generates the response. The data from the color-deficient observer support the view that color discrimination in protans and deutans is reduced because the input of one type of photoreceptor is missing.

LWS cone effects on rod threshold and saturation in achromats with residual cone function

Rod saturation on flashed and steady red backgrounds was investigated in normals and three achromats, two of whom were found to have some residual cone function. LWS cones selectively reduce the background level at which rod saturation occurs and elevate rod thresholds at flashed background levels well below saturation. Both of these LWS cone actions are also present in eyes with greatly reduced LWS cone function. In normal eyes LWS cones also elevate rod thresholds on steady backgrounds. We thus conclude that LWS cones influence rods through different mechanisms under transient (flashed) and steady-state background stimulation and that the increase in rod visual sensitivity observed during prolonged presentation of a background is due to a time-dependent reduction of LWS cone influence on rods. Finally, the finding that rod-cone interactions of the same magnitude found in normals can be seen in individuals where the cones' ability to mediate vision is severely reduced suggests the rod saturation paradigm as a sensitive technique for revealing residual LWS cone function.

An electroretinographic and molecular genetic study of X-linked cone degeneration

We evaluated full-field electroretinograms from members of a family with X-linked cone degeneration. The 15-year-old propositus had near normal visual acuity and a protan deficiency. His maternal grandfather and great uncle had a visual acuity of 20/200, a deficiency in color vision, and signs of macular degeneration; all had normal rod electroretinographic responses and diminished cone electroretinographic responses. The mother and maternal aunt of the propositus had normal visual acuity and diminished cone electroretinograms with predominant loss of red cone function. Their cone responses were greater than those of affected males. Genomic DNA isolated from these patients was analyzed with a red cone pigment gene cDNA probe that disclosed a 6.5-kilobase deletion in the red cone pigment gene. These findings substantiate that a defect in a gene encoding for a cone photoreceptor protein can lead to a cone photoreceptor degeneration.

Rhodopsin levels and rod-mediated function in Abyssinian cats with hereditary retinal degeneration

Abyssinian cats with different stages of a slowly progressive autosomal recessively-inherited retinal degeneration were studied with imaging fundus reflectometry (IFR) and electroretinography (ERG). Maps of the visual pigment distribution were made in an area of retina extending from the posterior pole to the midperiphery. Rhodopsin levels in the midperipheral retina of a 6-month-old affected cat (stage of suspected disease) were reduced about 20% relative to the mean normal value. The same cat, tested at 2.5 yr of age (now moderately advanced stage), showed a 60% reduction. A 3-yr-old affected cat (also moderately advanced) had a reduction in rhodopsin of about 60%. There was no measurable rhodopsin in a 7-yr-old affected cat (advanced stage). Rhodopsin regeneration kinetics at the different stages of disease were found to be similar to those of normal cats. The rod ERG b-wave threshold in the 6-month-old cat was elevated by 0.26 log units; at 2.5 yr of age, the threshold was elevated by 0.48 log units. A 0.34 log units threshold elevation was found in the 3-yr-old cat. There was no detectable ERG in the 7-yr-old cat. The relationship between the rod ERG threshold elevations and the rhodopsin levels was close to that expected if the dysfunction was caused by decreased quantal absorption.

The infant with nystagmus, normal appearing fundi, but an abnormal ERG

Many retinal disorders present during infancy with nystagmus, decreased vision, and normal-appearing fundi, but an abnormal ERG. The most common of these disorders are Leber's congenital amaurosis, achromatopsia, and congenital stationary night-blindness. Other disorders with similar ocular manifestations may be associated with a variety of life-threatening systemic abnormalities. This review describes the clinical, electrophysiological, and laboratory findings that can be used to distinguish among these conditions.

The photoreceptors in atypical achromatopsia

1. The receptoral mechanisms underlying the vision of two atypical achromats of the complete variety were studied with standard psychophysical procedures. 2. Under scotopic conditions the spectral sensitivity of each achromat was well described by the CIE (Commission Internationale de l'Eclairage) scotopic sensitivity function and the recovery of sensitivity after a retinal bleach showed characteristic duplex behaviour with the time constant of recovery of the slower phase matching that of normal rod vision for both foveal and peripheral stimulation. 3. Their spectral sensitivity was measured under conditions of chromatic adaptation in order to reveal any residual middle or long wavelength cone activity. Only one photopic spectral responses was found and this was adequately described by the spectral sensitivity function of Stiles pi 3 mechanism of normal vision. 4. Increment threshold measurements as a function of background intensity revealed a double-branched function in the fovea. The lower branch was found to have the spectral sensitivity of the rods; the upper branch that of Stiles' pi 3 mechanism. Stiles-Crawford measurements of directional sensitivity confirmed that the branch with the rhodopsin action spectrum had the directional sensitivity of rods and that the branch with the action spectrum of pi 3 had the directional sensitivity of cones. 5. These was no evidence for hue discrimination under photopic conditions. Regions of apparently normal performance on hue discrimination tests on more careful examination could be explained by luminosity judgements mediated by short wavelength-absorbing receptors. 6. We reject the notion of there being rhodopsin-filled cones in the fovea of these subjects. The foveal and peripheral vision of each of these achromats can be adequately described in terms of the participation of only two types of receptor, namely normally functional rods under scotopic conditions and normally functioning short wavelength-absorbing cones under photopic conditions. They are therefore functional blue mono-cone monochromats, an explanation which was originally proposed by Blackwell & Blackwell (1957) over thirty years ago.

Are rods and cones of dyslexics anomalous?

Grosser and Spafford (in this journal, 1989) have advanced an hypothesis and presented measurements which they believe support the idea of an excess of cones in the peripheral retinae of dyslexics. This note points out that their hypothesis is based on the erroneous assumption that normals have no peripheral cones. Further, their data can be explained by at least two alternative, though uninteresting, methodological hypotheses, that uncontrolled eye movements or experimenter suggestion (or both) could have produced their results. Finally, the requisite methods for assessing color vision, and the cones, were not met in the study.