The electrical response of the human eye to patterned stimuli: clinical observations

Surrogate electroretinographic markers for assessing therapeutic efficacy in the retina

Visual acuity remains the primary surrogate marker for clinical trials in ophthalmology (and the primary outcome for most US Food and Drug Administration applications) due to its long history, ease of measurement and clear relationship to clinically meaningful characteristics of daily life. However, treatment trials are being planned for diseases that are currently untreatable where visual acuity may not be the most appropriate outcome measure. Specialized electroretinographic tests can be powerful surrogate markers in such trials. The selection of outcome measures and surrogate markers depends in part on whether the goal is to preserve remaining vision in a progressive retinal degeneration or to improve vision in an eye that has already undergone extensive degeneration. Among the electroretinographic tests available are those involving the whole retina (full-field electroretinographic), the posterior pole (pattern electroretinographic) or focal areas within the macula (multifocal electroretinographic). The advantages and disadvantages of each will be discussed along with selected applications of each test to a specific category of disease.

Levodopa-carbidopa and childhood retinal disease

PURPOSE

Our purpose was to determine the influence of levodopa-carbidopa on visual function in children with retinal disease.

METHOD

Two studies were undertaken, a single-dose study and a longitudinal dosing study. A double-masked, placebo controlled single-dose study was undertaken of levodopa-carbidopa (2.08 mg/kg of body weight levodopa with 25% carbidopa) on monocular visual acuity in 14 children with retinal disease. Subjects received two capsules approximately 2.5 hours apart, and monocular visual acuity was measured 2 hours after each capsule ingestion. The second study was a double-masked, placebo-controlled 12-week longitudinal dosing (0.62 mg/kg of body weight) crossover study in which subjects received levodopa-carbidopa for 6 weeks and placebo for 6 weeks.

RESULTS

The single-dose study revealed a small but statistically significant improvement in monocular visual acuity after levodopa-carbidopa ingestion. The longitudinal study revealed a small but statistically significant improvement in binocular visual acuity after levodopa ingestion. In both studies placebo had no significant effect on visual acuity. Six subjects participated in both studies and demonstrated a significant correlation (r = 0.76, p < 0.05) between change in visual acuity in the single-dose study and the longitudinal dosing study.

CONCLUSION

The results are consistent with the hypothesis that dopamine influences the receptive field characteristics of retinal cells. The results also suggest that there may be low retinal dopamine levels in some types of retinal disease, which may be amenable to treatment.

Measurement of the micro-electroretinogram and component analysis

The authors propose a new technique for detecting micro-electroretinograms (muERG) which uses light-emitting diode (LED) stimulation and frequency analysis. The advantage of this method is the applicability of linear system analysis to a muERG obtained by partial stimulation of a computer-controlled waveform. The paper discusses two basic techniques for detecting the muERG. One is Fourier analysis of a synchronised ERG obtained by sine-wave stimulus, and the other is correlation analysis using a random stimulus. The results of the two methods are compared, and the usefulness of the proposed method is indicated; the muERG is analysed by means of a model consisting of five components of a second-order transfer function with delay. Functions of the central and peripheral areas of the retina could be analysed in more detail by this technique.

Scotopic versus photopic pattern onset-offset electroretinograms

We investigated the contribution of rods and cones to the human pattern electroretinogram to onset and offset checkerboards of different spatial frequency and wavelength in a 39 degrees x 39 degrees field. Under strictly scotopic conditions, there was a negative potential at onset and a positive potential at offset, whereas under photopic conditions, there was a positive potential at onset and a negative/positive potential at offset. Thus, the waveform to pattern onset (offset) was that of the luminance electroretinogram to decreasing (increasing) luminances. For pattern onset, the sensitivity difference 486-601 nm under scotopic and photopic conditions closely followed the luminosity function of rods and cones. The amplitude of the scotopic onset response increased with check size up to 3 degrees 30' and that of the photopic onset response, up to 30'. With larger checks, the scotopic and photopic onset response markedly decreased. This indicates antagonistic center-surround organization of the receptive fields under both scotopic and photopic conditions. By contrast, the offset response monotonically increased with check size under scotopic and photopic conditions, which suggests a luminance component in the pattern electroretinogram. Consequently, the pattern electroretinogram to reversing checkerboards has to be regarded as a mixture of both pattern- (contrast) and luminance-specific components.

Fundamental differences between the nonlinearities of pattern and focal electroretinograms

We directly compared nonlinear kernels of normal human pattern electroretinograms (PERGs) and corresponding localized flash ERGs (FERGs). The FERG was triphasic and resembled an adaptive process because it decayed slowly without changing shape over several kernel orders and interpulse intervals. The PERG was biphasic in the slice nearest the diagonal of the second-order kernel, similar to the FERG in slices farther from this diagonal, and without power in higher-order kernels. The unique PERG features were short-term effects that immediately followed a contrast transition. The appearance-disappearance PERG had a triphasic first-order kernel and a biphasic second-order kernel. The latter was similar to, but half the size of, that for the contrast-reversal PERG. When the first off-diagonal slices of the two PERG second-order kernels were analyzed in detail, we found in both that the first positive peak was larger than the FERG at intermediate spatial frequencies. Both PERG peaks in the slice had a low contrast threshold and were linear with contrast. The three FERG peaks of the corresponding FERG slice had a higher threshold and were saturated with increasing contrast. These observations show that the PERG contains substantial pattern specific nonlinear components and cannot be dismissed as merely the nonlinear subcomponents of the corresponding FERG.

Evoked responses in patients with macular holes

Thirty-two eyes with idiopathic macular holes and one eye with a traumatic macular hole were assessed by pattern-reversal electroretinography, ganzfeld electroretinography and pattern-reversal visual evoked potentials. Results were inspected for qualitative abnormalities and then measured in comparison with fellow eyes and 41 control eyes of similar age. Qualitative abnormalities occurred in some eyes with macular holes, most commonly a reduction in pattern-reversal electroretinogram or pattern-reversal visual evoked potential amplitude; 15' check amplitudes were significantly lower in eyes with macular holes than in control eyes, but no significant difference in latency was found. Control pattern-reversal electroretinogram and pattern-reversal visual evoked potential amplitudes were noted to decline with age, and paired t-tests on an age-matched subgroup of eyes with macular holes and control eyes showed appreciable differences only in the pattern-reversal electroretinogram q-r (N95) amplitude.

Retinal origin of VECP delays as revealed by simultaneously recorded ERG to patterned stimuli

Electroretinal and electrocortical potentials evoked by reversing checkerboards (PERG, PVECP) were simultaneously recorded in diseases of the central retina (10 patients) and in tumor-related optic nerve disorders (11 patients) exhibiting comparable P-100 delays in the PVECP. Retinal diseases showed a peak-time delay both in the PERG and in the PVECP, while optic nerve disorders revealed no delay in the PERG. Thus, simultaneous recording of PERG and PVECP may differentiate retinal from postretinal disorders of the visual system. The method also may provide information about the site of an underlying disorder in cases of visual disturbance without obvious changes of the ocular fundus.

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.

Sensitivity distribution in the central and midperipheral visual field determined by pattern electroretinography and harmonic analysis

To investigate the value of the extrafoveal pattern electroretinogram for evaluation of local retinal defects, electroretinographic responses to contrast reversal stimulation were recorded at various locations in the central and midperipheral retina. Normal values were established in 20 eyes, and spatial selectivity at different retinal eccentricities was determined in four eyes. The response amplitude was found to decrease steeply from the fovea until about 12 degrees eccentricity and to decline only slowly thereafter. For central stimulation a clear attenuation for coarser patterns was observed. At 20 degrees and at 30 degrees eccentricity the spatial tuning function exhibited a bandpass characteristic, with the maximum amplitude shifting to lower spatial frequencies. Despite the relatively low peripheral amplitudes a satisfactory signal to noise ratio of the second harmonic responses can be obtained by Fourier analysis, which improves clinical applicability. The comparison of sensitivity in the upper and lower retina revealed a considerable asymmetry, with responses in the upper retina approximately 20% higher. No significant difference, however, was found when the temporal or nasal retina was stimulated. The reproducibility of these results is high enough to encourage clinical studies. A case of retinal venous branch occlusion exemplifies the applicability of this technique in eccentric fundus lesions.

Hemiretinal stimuli elicit different amplitudes in the pattern electroretinogram

Pattern electroretinograms were elicited in 13 normal eyes by half-field checkerboard stimulation of nasal-temporal and upper-lower retinal areas. With nasal-temporal visual half-field stimulation the p-q component amplitude of the nasal hemiretina was significantly larger than that of the temporal hemiretina. With upper-lower visual half-field stimulation the amplitude was significantly larger for the upper than for the lower hemiretina. No significant differences were found with respect to the component latency. Finding the p-q amplitude of the pattern electroretinogram to be closely related to the distribution of nerve cells in the innermost retinal layer supports the conclusion that the pattern response is generated in the more proximal retinal layers including the retinal ganglion cells.

A review of the clinical applications of the pattern electroretinogram

The pattern electroretinogram (PERG) has recently been introduced as a clinical procedure. It has been thought by many to represent activity of the retinal ganglion cells, although this is still a matter of contention. The exciting prospect of a selective test of ganglion cell function led to the application of the PERG in a variety of ophthalmological conditions. In the course of these investigations the PERG was found to be diminished in cases of maculopathy, optic atrophy, optic neuritis, toxic optic neuropathy, neurotransmitter disorders, glaucoma and ocular hypertension and in retinal vascular disorders such as diabetes. It was also affected in some cases of amblyopia. This paper briefly describes the techniques used to record the PERG and reviews current literature pertaining to its clinical application.

The pattern electroretinogram

Physiological experiments and the exploitation of clinical conditions have provided compelling evidence that retinal ganglion cells and other inner retinal structures generate the pattern ERG (PERG). As an increasing number of clinical reports have been published some contradictory findings have been reported. These may be ascribed to variation in recording and measuring techniques. The PERG consists of two major portions, the early positive and the following negative component which can be investigated separately if the stimulus conditions allow isolated (or "transient") responses to be recorded. Care has to be taken in positioning the reference electrode, maintaining accurate refraction, and the influence of pupil size must be considered. Furthermore the PERG is contaminated by a luminance component which may be generated in the outer retina. The size of this increases with low spatial frequency (large check-sizes) and high mean luminance. The PERG permits the examination of an additional level of the retina and helps the understanding of pathophysiology of various eye diseases, and is of clinical importance in routine diagnosis and assessment. In glaucoma the PERG amplitude is often reduced before it is possible to detect a scotoma and it is therefore an important prognostic indicator in patients with ocular hypertension. In diabetic retinopathy, retinal ischaemia sufficient to lead to the pre-proliferative state can be demonstrated. The PERG also has a major clinical role in examining localised retinal pathology. If combined with VECP recording, it greatly extends the interpretations possible, since not only can damage to the optic nerve be detected by both tests, but the normal PERG in the presence of an abnormal PVECP implies that the losses are confined to the central pathway.