Division of the pattern-evoked retinal response by respiratory acidosis

Analysis in the frequency domain divided the pattern-evoked retinal response (PERR) of the cat into two analog components. A mild respiratory acidosis was produced by raising the end-tidal CO2 from 4-6% to 12-14%. The onset of the acidosis (without hypoxia) was highly correlated with marked decreases in the low-frequency PERR component and in the b-wave of the flash-elicited ERG. The high-frequency PERR component was unchanged relative to the labile low-frequency component (p less than 0.01). The literature has established a firm link between local pH and Müller cell function. These data tend to confirm the hypothesis that the cat's PERR to some grating stimuli of less than 100% contrast may contain separable components of different cellular origin(s).

Electroretinal changes in the presence of a carbonic anhydrase inhibitor

A specific carbonic anhydrase activity inhibitor (methazolamide) was injected into one vitreous body each of 4 New Zealand White rabbits. Electroretinograms (ERG) were recorded before and several times after the methazolamide injection. The stimulus levels maximized the rod and cone response characteristics of the rabbit ERG. The effects of methazolamide were followed over 5 h. During this time, the electroretinograms showed a decline in amplitude of both a and b waves at both stimulus levels. The data support the involvement of carbonic anhydrase in the excitatory physiological events in the retina. Preliminary evidence indicates a slow recovery of the carbonic-anhydrase-inhibited ERG.

Degenerative changes in maculas of rhesus monkeys

In the hope of identifying an animal model for age-related macular degeneration (AMD) we undertook a pilot investigation of aged rhesus monkeys. Twenty-nine monkeys from a seminatural colony were examined at the Caribbean Primate Research Center. Macular drusen were found in 74% of the monkey eyes. Alterations of the retinal pigment epithelium within the macula were noted in 45% of the eyes. Fluorescein angiography in selected animals revealed window defects consistent with drusen. None of this sample showed the exudative form of AMD or disciform scarring. One typical monkey underwent special studies including measurement of visual resolution by electrophysiological study of the retinal and visual cortex. Application of human criteria to this animal supported the diagnosis of early AMD. Histopathologic study of one eye by transmission electron microscopy confirmed the presence of drusen with nearly identical ultrastructural features to those found in the human pigment epithelium in AMD.

Retinal cyclic light damage threshold for albino rats

This study determined the minimum cyclic [12L:12D] light intensity which produces retinal damage in albino (Sprague-Dawley) rats raised from birth to 15 weeks of age under a cyclic light intensity of 6 lux. Four experimental light intensities were tested, including: 1345, 270, 130 and 65 lux. Control animals remained under 6 lux. For each of the intensities tested, the retinas of groups of six rats were evaluated after various durations of light exposure for physiological and morphological evidence of light damage. The indices of damage were (a) histological and morphometric changes in the retina and (b) changes in the amplitude of the b-wave of the electroretinogram. The data indicated that light intensities of 1345 or 270 lux severely damaged retinas of albino rats within 3-7 days of the initiation of light exposure. Exposure to 130 or 65 lux produced much less dramatic changes in the responsiveness and morphology of the retina which did not appear to be permanent. Based on these results, a reasonable estimate for the threshold cyclic light intensity which produces damage to retinas of albino rats raised under 6 lux lies between 130 and 270 lux, or approximately 1.3 log units above the light intensity under which the animals were raised.

Cyclic light intensity threshold for retinal damage in albino rats raised under 6 lx

This study determined the minimum cyclic (12L:12D) light intensity which produces retinal damage in albino (Sprague-Dawley) rats raised from birth to 15 weeks of age under 6 lx (12L:12D). Four experimental light intensities were tested, viz. 1345, 270, 130 and 65 lx. Control animals remained under 6 lx. After various durations of exposure to one of the intensities tested, the retinas of groups of six rats were evaluated for physiological and morphological evidence of light damage. The indices of damage were: (1) histological and morphometric changes in the retina, and (2) changes in the amplitude and latency of the b-wave of the electroretinogram. Light intensities of 1345- or 270 lx severely damage retinas of albino rats raised from birth under 6 lx within 3-7 days of the initiation of light exposure. Exposure to 130- or 65 lx produced much less dramatic changes in the responsiveness and morphology of the retina that did not appear to be permanent. Based on these results, a reasonable estimate for the threshold cyclic-light intensity which produces damage to retinas of albino rats raised under 6 lx lies between 65- and 130 lx, or slightly more than 1 log unit above the light intensity under which the animals were raised. The effects of an animal's light history on retinal susceptibility to light damage are discussed.

Pattern electroretinogram: effects of reference electrode position

The pattern electroretinogram has assumed greater clinical and experimental significance because of its inner retinal origins. However, clinical tests may be confounded by an artifact. We tested subjects varying reference electrode position and eye stimulated while employing the Dawson-Trick-Litzkow (DTL) fiber electrode as the active electrode. The presence of a statistically significant artifactual response could not be confirmed. However, the variability of responses elicited with the outer canthus was less and the signal-to-noise ratio greater than with other reference positions.

Infrared. An unseen and unnecessary hazard in ophthalmic devices

The concept of instrument-induced light damage to the retina is well established. Many have argued that ultraviolet (UV) energy is overwhelmingly responsible and that longer wavelength infrared (IR) energy causes thermal change and plays a minimal role. We suggest that infrared may play a primary role in instrument-induced damage. We measured the radiometric and photometric outputs from commonly used ophthalmic devices in normal use and in IR-blocked conditions. We found IR output to represent a significant portion of the total irradiation produced by these instruments. Infrared was easily and inexpensively minimized without compromising usable visible light in these instruments using an efficient IR filter. The instruments' incandescent lamps, glass envelopes, and optics are poor UV emitters or transmitters. Manufacturers' data do not indicate UV emission. Evidence for UV blocking and IR transmission by the intraocular media is presented.

Macular degeneration in a community of rhesus monkeys. Ultrastructural observations

The eyes of 29 aged adult, (mean age, 20 years) rhesus monkeys were examined for the presence of age-related macular degeneration (AMD). This sample represented approximately 25% of the aged population in the seminatural colony at the Caribbean Primate Research Center (CPRC) of the University of Puerto Rico. Approximately 75% of the animals examined had drusen in the posterior pole. Ultrastructural analysis was used to determine whether the pathologic alteration of Bruch's membrane and drusen in the colony resembled those noted in aged or AMD-afflicted human retinas. There were abnormalities in all layers of Bruch's membrane. Deposits of heterogeneous material, comprised of membranous, granular, and cellular components, were seen in both the inner collagenous zone (ICZ) and the outer collagenous zone (OCZ). Accumulation of this drusenoid material in the ICZ produced a scalloping of the basal border of the retinal pigment epithelium (RPE). Dense bodies were seen in both Bruch's membrane and RPE cytoplasm near the basal infoldings. Cytoplasmic processes, as well as whole cells, were seen with regularity within the drusenoid material. In one case there was a cell with a basement membrane crossing the middle elastic layer of Bruch's membrane. These changes are consistent with those reported in human aging and AMD. Aged individuals in this colony appear to be predisposed to macular degenerative changes and may prove to be an invaluable animal model for studying AMD in humans.

Tissue responses of the monkey retina: tuning and dependence on inner layer integrity

Recent technical and conceptual advances have made it possible to experiment with models of local human inner retinal disease and changes in very small, tissue-specific signals. Local retrograde degeneration of the ganglion cells was induced in four rhesus monkeys by 160 degrees microdiathermy fiber layer burns at the nasal or temporal edges of the optic disc. There were no abnormalities of the classical electroretinograms (ERGs) during the following 210 days. With nasal lesions, pattern-evoked retinal (PERR) and cortical responses over a range of grating contrasts and spatial frequencies were largely normal. The authors found a cortical spatial tuning peak near 0.5 cycles/deg (cpd) and a retinal peak at 0.25-0.3 cpd. With temporal lesions, the retinal signals to high frequency stimuli (greater than 1.0 cpd) approached zero between 20-60 days. The cortical evoked signal declined with a course similar to the retinal components. Histological evidence was found for extensive loss of ganglion cells and fibers in a central 30-40 degrees temporal area, including the macula, 210 days after the temporal lesions. This is strong evidence that local ganglion cell-dependent electrical potentials, bearing little relation to the ERG, can be measured in response to selected stimuli.

The effects of dim cyclic light on pigment epithelial function in the albino rat

Most pathologies of the outer retina include physiological and morphological changes in the pigment epithelium. The question of pigment epithelial involvement in retinal light damage caused by low intensities of light is still unresolved. In the present study, we investigated the effects of low intensity cyclic light on pigment epithelial function in albino rats. The functioning of the pigment epithelium was assessed electrophysiologically from d.c. recordings of ERG c-waves and sodium azide induced changes in the resting potential. Responses obtained from albino rats raised under low intensity cyclic light (0.63 ft cd. 12:12 L:D) were compared to those obtained from albino rats raised under minimal light exposure conditions (dark-reared) and pigmented rats housed under low intensity cyclic light. We report, for the first time, that albino rats raised from birth under low intensity cyclic light possess c-waves. Their responses were comparable in amplitude and latency to those recorded from pigmented rats housed under similar conditions, but were significantly smaller than those recorded from dark-reared albino rats. The reduction in the amplitudes of the c-waves recorded from cyclic light-reared albino rats was probably not due to retinal light damage. Comparisons of the amplitudes and latencies of ERG b-waves recorded from cyclic light-reared and dark-reared albino rats did not suggest that the retinas of the cyclic light-reared albino rats had been damaged by light. Light microscopic examination of these retinas also provided no evidence for light damage. The transient, positive potential changes recorded from cyclic light-reared albino rats in response to bolus injections of sodium azide were significantly smaller than those recorded from either dark-reared albino rats or pigmented rats housed under low intensity cyclic light. The results of these experiments suggest that the pigment epithelium of albino rats is functionally altered by extremely low intensities of cyclic light.

Disease-specific electrophysiological findings in adult ceroid-lipofuscinosis (Kufs disease)

Two adults with mild dementia and a history of memory loss and disequilibrium were seen in the eye clinic following complaints of acuity loss in the 20/30-20/70 (Snellen) range. Results from the fundus examination of one patient were entirely normal; the other showed minimal vascular attenuation and optic atrophy. Electrophysiology was remarkable: (1) Photopic ERG b-waves were reduced, delayed, and showed pronounced oscillations. (2) EOG 'light-rise' potentials were absent or very small. (3) Binocular pattern-VER signals showed addition of the monocular signal. Scotopic ERG signals were normal. Brain biopsy and microscopy showed intercellular, autofluorescent ceroid deposits which provided a clear diagnosis of Kufs disease. Histology of model animal retinal cells show ceroid deposits in cell classes implicated by the human retinal signals. The cluster of electrophysiological results point toward early changes in the pigment epithelium and inner plexiform layer cells as a means of noninvasive diagnosis.

Culture of retinal pigment epithelium from chorio-retinal biopsies

The recently devised methods for surgical sampling of chorio-retinal tissue permits morphological evaluation of retina in various diseases. In the present report it is demonstrated, that the retinal pigment epithelium in such samples can be isolated as a viable cell population, and that the cell mass can be increased sufficiently in culture to permit biochemical as well as morphological studies. This procedure thereby expands the diagnostic potential of the sample tissue.

Retinal pigment epithelial dysfunction in early ovine ceroid lipofuscinosis: electrophysiologic and pathologic correlates

Retinal degeneration is a major finding in the human and ovine ceroid lipofuscinosis. Sequential electroretinographic (ERG) studies in a young, asymptomatic, affected lamb are presented here, which demonstrate a progressive loss of the scotopic b-wave and unrecordable c-waves under halothane anesthesia. Even at this initial stage of disease, lesions were evident in the form of dystrophic retinal pigment epithelial (RPE) villi, and loss of photoreceptor cells and rod outer segments. Cone inner segments were enlarged and scanning electron microscopy emphasized these abnormalities. All cells in tapetal and nontapetal areas contained fluorescent inclusions with similar emission spectra (maximum = 539 nm). By transmission electron microscopy, storage bodies consisted of 'finger-print' profiles and were most prominent in bipolar cells. The pathological features of the retina correlate well with the observed ERG changes, reaffirming the sheep as a useful model to delineate early events in ceroid lipofuscinosis.

Power spectral analysis of visual evoked potentials in multiple sclerosis

Power spectral analysis (PSA) was performed on the visual evoked potentials (VEPs) to counterphased checkerboard stimuli from 98 eyes in 49 patients with multiple sclerosis (MS) and 54 eyes of 27 normal volunteers. Attenuation of high frequency components of the transient visual evoked potential was found in 53% of MS patients and 4% of controls. Loss of high frequency components was poorly correlated with prolonged latency (r = 0.346). The consideration of both PSA and latency of the VEP increased the percentage of MS patients exhibiting visual pathway conduction abnormalities from 61% to 86%. The use of PSA in the diagnosis of MS is useful in increasing detection especially in cases where deformed waveforms preclude a reliable estimation of latency.

Hereditary retinal degeneration in the Rhode Island Red chicken. I. Histology and ERG

Hereditary blindness in Rhode Island Red chickens was analyzed at various post-hatching stages by light microscopy and electrophysiological recordings. At the time of hatching the retina of affected chicks appeared morphologically normal and identical to that of control, non-affected chicks. Whereas the electroretinographic (ERG) response to light stimulus in normal chicks was near the adult level at the time of hatching, no ERG either under light- or dark-adapted conditions was measurable in affected chicks at any stage examined. Photoreceptor cells of affected animals were seen to undergo degenerative changes after about one week post-hatching. Decrease in number of outer segments, spaces between inner segments and large spaces in the outer nuclear layer were apparent by Day 10. By Day 21, most of the photoreceptor inner segments appeared swollen, and the decrease in number of outer segments and photoreceptor nuclei was noteworthy. By the end of the second month no outer segments were seen and the majority of identifiable inner segments were from cones, a larger proportion than normally present being double cones. By six months, very few photoreceptor inner segments and nuclei remained; most inner segments were deformed and diminutive but usually contained a clearstaining oil droplet characteristic of the principal member of the double cone. In all stages after one week of age, pycnotic nuclei and thinning of inner retinal layers accompanied photoreceptor degeneration. In all specimens examined, degeneration of retinal cells was more pronounced in the superior central retina than in the periphery. Pathological changes were frequently also noted in the pigment epithelium overlying degenerating retina. Because the chick retina is well developed at birth, contains a fovea and a significant cone population and because cones (particularly one specific type) survive rods, we believe that this congenitally-blind chicken may be a useful model for studies on human hereditary retinal degenerations.

Rate-dependent inhibition of pattern-evoked retinal responses in the pigeon

Robust signals of corneal derivation were recorded from the pigeon (a foveate model) in response to alternating gratings. When a rotating windmill-like display was superimposed upon the visual field, there was rate-dependent inhibition of the response to the gratings. Inhibition diminished linearly with grating contrasts (11-40%) and nonlinearly above 50%. The responses to diffuse-light stimuli showed no rate dependence but a predictable, steady reduction at all windmill rates. Rate-dependent inhibition has been reported only in the inner-retinal cells of other animals where intracellular recordings have been made.

Results from a simple laser color perimeter

Stimulus size, chromaticity, adaptation, luminance and local adaptation are more easily controlled when laser light rapidly scans the appropriate portion(s) of the visual field. Adaptation of a Ferree-Rand perimeter to the use of He-Ne laser light gives chromatic and achromatic thresholds of relatively low variance.

Relations between the Human Retinal Cone and Ganglion Cell Distribution

The distribution of human cone cells is well known and is frequently cited. The distribution of human ganglion cells is less well known and was published in a booklet which is less frequently cited in the eye literature. Selected published curves provided by these authors and Snellen acuity data were digitally encoded, transformed to linear vertical axes where necessary, scaled and compared. Comparisons were possible along the temporal hemi-meridian and provided correlations (r = 0.99 for ganglion cells versus cone cells; r = 0.99 for ganglion cells versus acuity, and r = 0.98 for cone cells versus acuity). Integration of densities along the horizontal hemi-meridian out to 50 degrees showed that approximately 50% of the cone cells are within 18 degrees of the foveola and that approximately 50% of the ganglion cells are within 13 degrees of the foveola. Appreciation of the strong interrelations between the cone-ganglion cell distributions may provide for a better understanding of the visual of inner-retinal disease. Presently it accounts for aspects of the production of pattern-elicited retinal signals which are under intense investigation in many centers.

Canine visual acuity: retinal and cortical field potentials evoked by pattern stimulation

The visual acuities (i.e., visual thresholds for pattern detection) of four dogs under neuromuscular block were measured using visually evoked cortical potentials (VECP) and/or pattern-evoked retinal potentials (PERR). Stimuli were phase-reversing square-wave gratings with a mean luminance of 86 cd/m2 and 70% contrast. The mean of the VECP thresholds of two dogs tested was 12.59 cycles per degree (cycles/deg). The mean of the PERR thresholds of four dogs tested was 11.61 cycles/deg. The difference between VECP and PERR thresholds was not statistically significant. VECP acuities appear to be determined at or before the last stage of retinal processing (PERR). Our estimates of canine acuity are 1.3-2 times those reported for cats and 0.2-0.4 times those reported for primates when tested under comparable luminance and contrast conditions.

Retinal and cortical pattern responses: a comparison of infants and adults

Retinal and cortical responses to pattern reversal stimulation were recorded simultaneously in three human infants of average age 3.5 months and two adults of average age 28.5 years. The relative power of retinal signals as a function of spatial frequency for both adults and infants was very similar and extrapolated to a threshold of about 30 c/deg [6/6 (20/20)] as did the relative power of the adult cortical signal. The infant cortical signal was relatively more attenuated at higher spatial frequencies and extrapolated to a threshold of 8.5 c/deg [6/22.2 (20/74)]. A greater relative maturity of retinal as compared with cortical neural function in 3.5-month-olds is inferred.

Human pattern-evoked retinal responses are altered by optic atrophy

Electrical signals in response to both diffuse flashes of light and phase-alternating spatial patterns were recorded from the eyes and from the occipital scalp of a subject with a traumatic unilateral (right) optic nerve section. Clinical examination disclosed a grossly normal right eye with no light perception, a Marcus Gunn pupil, and an atrophic optic disc. The left eye was normal. The electroretinogram responses on the lesioned side (OD) were normal, but there was no pattern-evoked retinal response (PERR) and no recordable visual-evoked response present. The conclusion is that the optic nerve and ganglion cells appear to be selectively responsible for the PERR potential.

Dichoptic interaction of harmonically related spatial and temporal frequencies

Steady-state visual evoked potentials were recorded in response to contrast Reprint requests: Dr. Elmar T. Schmeisser, Division of Ocular Hazards, Letterman Army complex (sum of two spatial frequencies) suprathreshold sinusoidal gratings of two and six cycles/degree in a 5 degrees field. Sine mode counterphasing enhances the second temporal harmonic component of the evoked waveform; square mode counterphasing generates a response with essentially all the power at the fundamental temporal frequency. Digital sums of observed responses to simple gratings presented alone had significantly greater amplitudes than any observed response to presented complex gratings. Response amplitudes ranked significantly by condition: binocular greater than dichoptic greater than monoptic. It is concluded that (a) sinusoidal counterphasing stimuli lead to two stimulus events per shift (an observed grating onset and offset), and (b) spatial frequency channels that are harmonically related in 1:3 ratio do not destructively interfere with each other in either monoptic or dichoptic presentation.

The contribution of the central retina to the laser speckle visual evoked response

Laser speckle stimuli were used to examine the contribution of the central retina to the visual evoked response (VER). The foveally fixated test stimuli were either small circular spots or annular targets with dark centers. The speckle elements were shifted seven times per second. Reliable responses could be recorded for even the smallest (1 degree) target size. In addition, it was clearly evident that for spots up to 5 degree in diameter, VER amplitude increased linearly with stimulus area. However, data from annular targets revealed that this result was not strictly dependent upon stimulus area. In many instances, large annular stimuli evoked responses with smaller amplitude than spot stimuli with less area. This result may be due to a center-surround antagonistic interaction. These experiments suggest a potential value for laser speckle VER techniques in the assessment of discrete lesions of the macula.

Interocular luminance differences and the binocular pattern-reversal visual-evoked response

Although the visual-evoked response (VER) is frequently used to assess binocularity, the contribution of the monocular components to the binocular VER is poorly understood. To more fully elucidate this relationship, we examined checkerboard (14 min arc checks) pattern-reversal (3.75 Hz) VERs evoked from observers with normal binocularity, with conditions in which interocular luminance differences (from 0.3 to 2.0 log units) were established. When compared with monocular VERs obtained with similar luminances, the binocular response was always less than the sum of the component monocular responses. In addition, the amplitude of the binocular signal was dependent on the amount of interocular luminance the difference. For interocular luminance differences of less than 0.6 log units the amplitude of the binocular response was consistently greater than either corresponding monocular VER. When the interocular luminance difference was 1.3 log units or greater the amplitude of the binocular response fell below the level of either corresponding monocular response. Furthermore, it does not appear that these results can be attributed to a passive spread of electrical potentials from monocular cortical cell populations. We therefore suggest that these results indicate the activity of a binocular neural process.