Topodiagnostik von Sehstörungen durch Ableitung retinaler und kortikaler Antworten auf Umkehr-Kontrastmuster

Electrophysiology and glaucoma: current status and future challenges

Visual electrophysiology allows non-invasive monitoring of the function of most processing stages along the visual pathway. Here, we consider which of the available methods provides the most information concerning glaucomatous optic nerve disease. The multifocal electroretinogram (ERG), although often employed, is less affected in glaucoma than two direct measurements of retinal ganglion cell function, namely the pattern ERG (PERG) and the photopic negative response (PhNR) of the ERG. For the PERG, longitudinal studies have been reported, suggesting that this method can be used for the early detection of glaucoma; for the PhNR, no longitudinal study is available as yet. The multifocal PERG can spatially resolve ganglion cell function but its glaucomatous reduction is typically panretinal, even with only local field changes and so, its topographic resolution is of no advantage in glaucoma. The multifocal visual evoked potential promises objective perimetry and shows sensitivity and specificity comparable with standard automated perimetry but has not been established as a routine tool to date.

The multifocal pattern electroretinogram in glaucoma

BACKGROUND

The pattern ERG can be used to detect early glaucomatous change, because the response of cells in the inner retina from (typically) 20 degrees -40 degrees of area is reduced before perimetric abnormality is certain. The multifocal pattern electroretinogram (mfPERG) allows analysis of many local regions within this area. The aim of this study was to investigate whether in patients with presumed glaucoma the mfPERG permits diagnosis and discrimination from normals.

METHODS

Measurements on 25 age-related normal eyes were compared to those on 23 eyes with different stages of glaucoma. A RETIScan system was used to generate a stimulus pattern of 19 hexagons, each consisting of six triangles. The triangles pattern-reversed black to white at 75 Hz. Those 19 hexagons were grouped into three stimulus regions: a central field, a middle, and a peripheral ring. The complete array subtended 48 degrees at the eye. The hexagons alternated between black and white, in a temporal pattern that followed a corrected binary m-sequence (length 512, 10 cycles with 39 s each). The amplitudes and latencies of positive responses at approximately 50 ms (P-50) and negative responses at approximately 95 ms (N-95) were analyzed.

RESULTS

In patients with glaucoma the P-50 and N-95 components of the mfPERG were significantly reduced for the central area and both outer rings compared to normal volunteers (p<0.001, Mann-Whitney-U). The most distinct reduction was observed for N-95 and the central ring. Changes in latencies were not conclusive. The reduction of the components increased with the stage of glaucoma. A predictive model for detecting early glaucomatous changes was designed based on P-50-N-95 with 88% sensitivity and 76% specificity.

CONCLUSION

In glaucoma a marked reduction of components, especially centrally is observed in the mfPERG. This hints to an early involvement of central ganglion cells and may be useful for future functional tests.

Visual motion detection in man is governed by non-retinal mechanisms

It is generally assumed that there is no sizable proportion of motion detectors in the primate retina. To test this specifically for humans, visual evoked potentials (VEPs) and electroretinograms (ERGs) were recorded simultaneously to visual motion onset (9.3 degrees /s) of an expanding or contracting 'dartboard'. The degree of motion-specific responses in cortex and retina was assessed by testing the direction specificity of motion adaptation with three conditions in a fully balanced paradigm: motion-onset potentials were measured after adaptation to: (1) a stationary pattern; (2) motion in the same direction as the test stimulus; and (3) motion in the opposite direction. Motion-onset responses in the VEP were dominated by the typical N2 at 150 ms, in the ERG by a positivity at 70 ms. Onset of contraction or expansion evoked virtually identical VEP and ERG responses (P>0.5). Motion adaptation produced strong direction-specific effects in the VEP (P<0.05), but not in the ERG (P=0.58): In the adapting and non-adapting direction the VEP (N2) was reduced by 75 and 50% (P<0.001), the ERG by 32 and 26% (P<0.01 and 0.05), respectively. The striking difference of the direction-specificity of motion adaptation between cortex and retina suggests that in humans the vast majority of motion-specific processing occurs beyond the retinal ganglion cells.

Little correlation of the pattern electroretinogram (PERG) and visual field measures in early glaucoma

Pattern-electroretinograms (PERG) to checkerboard reversal at 16/s. 0.8 degrees and 15 degrees check size and visual fields (Octopus G1) were retrospectively analyzed in 40 eyes of 30 patients with early glaucoma. The mean visual field defect was calculated separately for the central 26 degrees x 34 degrees covered by the PERG stimulus (MDc) and the more peripheral area (MDp) surrounding the stimulus. Deeper field loss was correlated with a reduced pattern electroretinogram amplitude (p < 0.01 for both MDp and MDc), indicating that the pattern electroretinogram deteriorates as glaucoma advances. If the analysis was confined to those 18 eyes (16 patients) that had no field defect within the area covered by the PERG stimulus (normal MDc but abnormal MDp), 13 of these had an abnormal PERG amplitude (p < 0.001). The results suggest that the PERG can reveal impairment of ganglion cell function that is not detected by conventional perimetry.

Check size tuning of the pattern electroretingoram: a reappraisal

The pattern electroretinogram was recorded to checkerboard stimuli with a wide range of check sizes and two stimulus field sizes. Check sizes ranged from 0.25 degree to 7 degrees (field size, 16 degrees x 14 degrees) and 0.25 degree to 15 degrees (field size, 32 degrees x 27 degrees) in 14 and seven subjects, respectively. Reversal rate was 4.5/s. For minimal intrusion of blink artifacts the interrupted stimulation technique was employed. The P50 and N95 components of the pattern electroretinogram were evaluated separately. With both stimulus field sizes amplitude of P50 and N95 was maximal between 0.75 degree and 1 degree. With smaller check sizes the amplitude dropped monotonically. With larger check sizes field size played a role: with the 16 degrees x 14 degrees field, P50 gradually dropped to 89% from 1 degree to 7 degrees, which was paralleled by N95 only up to 7 degrees, where N95 dropped to 81% (p < 0.05). With the 32 degrees x 27 degrees field, there was no significant difference in size dependency between P50 and N95 for large check, both components staying constant from 1 degree to 15 degrees. We conclude that there is only minor large-check attenuation of the pattern electroretinogram, especially with a large field. The apparent field-size dependency may explain previous discrepancies in the literature.

The origins of luminance and pattern responses of the pattern electroretinogram

A luminance response is spatially insensitive and dependent upon temporal contrast. The contribution of luminance to the pattern onset electroretinogram (PERG) can be estimated from the temporal contrast of a pattern's illuminance profile on the retina. A profile can be computed for different spatial frequencies by resynthesising the Fourier transform of the pattern after it has been filtered by the eye's MTF. A pure luminance response does not fully account for the amplitude of the PERG. The residual response is called "pattern specific". After the effects of spatial contrast degradation have been corrected for the response is spatially bandpass tuned with a peak that changes with eccentricity. An optimum foveal stimulus is estimated to be 3.5' arc. The pattern specific response component shows a correlation with inner nuclear and ganglion cell volumes. When the relationship of the luminance ERG with temporal contrast was explored empirically it was found to behave in good approximation to the theoretical predictions.

Utility of the color pattern-electroretinogram (PERG) in glaucoma

Pattern-onset electroretinograms (PERGs) with red-green color contrast (CC) and green-"black" luminance contrast (LC) stripe patterns (0.3 c/deg) were recorded in a group of 80 control subjects and in a group of 42 patients having glaucomas of varying etiology and severity. The PERG data were correlated with the results of static perimetry and optic disc morphometry. In the glaucoma group the PERG was reduced significantly and by relatively similar amounts with both CC and LC stimuli. A significant correlation of the PERG reduction with visual field loss was found only with the CC, not with the LC PERG. Correlations between PERG amplitudes and neuroretinal rim areas of the optic disk were similar for the LC and for the CC stimulus. The rather poor percentage of correct classification of controls and patients based on the PERG or the optic disc morphometry alone can be improved by two-dimensional discriminant analysis using both CC PERG and papillometry data.

The different contributions of local luminance decreases and increases to the pattern electroretinogram (PERG)

The typical pattern-onset-offset stimulus (stimulus A) consisting of local luminance increases and decreases was broken down into stimuli presenting only local luminance increases (stimulus B) or only local luminance decreases (stimulus C). With stimulus B the onset ERGs are luminance responses. With stimulus C the onset ERGs are pattern-related responses showing a spatial band-pass function. With stimulus A the response is a linear addition of responses to stimuli B and C. The simultaneously recorded VEP is a pattern-related response with all three stimuli (A-C).

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.

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.

Pattern electroretinograms in optic neuritis during the acute stage and after remission

A total of 20 patients with unilateral acute optic neuritis were studied. Each patient had experienced the recent onset of a decrease in visual acuity, a relative afferent pupillary defect, a relative or absolute central scotoma and a colour-vision defect. The pattern-reversal electroretinogram (PERG) of each patient was analysed with regard to the amplitude of the positive and negative components. During the acute stage the amplitude of the positive component was reduced in all patients and that of the negative, in 18 of 20 cases. Parallel to clinical recovery, a steady increase was observed in the amplitude of the positive component to normal values; no statistical differences between affected and fellow eyes was found. In contrast, the amplitude of the negative component remained significantly reduced after clinical recovery.

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.

Pattern electroretinogram in glaucoma and ocular hypertension

We recorded the pattern electroretinogram (PERG) to small (0.8 degree) and very large (15 degrees) check sizes in normal subjects, in patients with early-stage glaucoma, and in patients with ocular hypertension. In glaucoma, the PERG amplitude was reduced. This reduction was more prominent for a check size of 0.8 degree as compared with 15 degrees stimuli and for high (16/s) as compared with low (7.8/s) reversal rates. Using a discriminant analysis of the amplitudes for two different check sizes, we could distinguish the normal and the glaucoma groups with a specificity of 96% and a sensitivity of 91%. Of the ocular hypertension patients, 43% were classified as pathologic by the discriminant analysis. Thus multivariate analysis of the PERG may increase its diagnostic value.

Separable evoked retinal and cortical potentials from each major visual pathway: preliminary results

Single cell experiments in primates show that there are two major parallel pathways named after the lamination in the lateral geniculate nucleus. Each of these systems can be preferentially excited by appropriate stimuli. Here we report that in man the polarity of the evoked potentials both in retina and in cortex depends on which of these pathways is stimulated. The identification of the resulting waveforms is thereby simplified--a matter of practical importance. The fact that at retina and cortex there are characteristic potentials may reflect the different cell biology of the two pathways.

Differences between pattern onset and pattern reversal retinal responses

The pattern-evoked electroretinogram was recorded to pattern onset-offset and pattern reversal stimuli in two color-normal subjects with either luminance contrast of black-red (600 nm) and black-green (526 nm) square-wave stripe patterns or color contrast red-green patterns. The size of the onset response shows a spatial tuning with luminance contrast patterns and only a simple low-pass filter function with color contrast patterns. The peak latency of the response to luminance contrast increases with increasing spatial frequency but stays constant with color contrast patterns. The size of the reversal responses, however, shows only a low-pass filter function under both contrast conditions. The peak latency to luminance contrast shows a slight increase and to color contrast it remains constant with increasing spatial frequency. The differences noted under the various stimulus conditions must take into account the possible effects of different luminance modulation depths of onset and reversal stimuli, the modulation transfer function of the eye, and the activity of luminance-antagonistic and color-antagonistic receptive fields.

The pattern-evoked electroretinogram (PERG): age-related alterations and changes in glaucoma

Pattern-onset electroretinograms (PERGs) were studied in 147 normal subjects of different ages (14-79 years) and in 110 eyes of 65 patients with glaucoma or ocular hypertension. The responses showed an increase (P less than 0.001) in peak latency with increasing age and a decrease (P less than 0.001) in amplitude which approximately parallels the loss of ganglion cells estimated by other authors. Many glaucoma eyes showed a loss of the normally present spatial tuning. In the age group above 50 years 50% of the onset responses were significantly diminished and the peak latencies were not significantly different. A negative correlation (P less than 0.001) was found between the size of the PERG and the cup/disc ratio and a positive correlation (P less than 0.001) with the area of the neuroretinal rim of the optic disc. The PERG decreased (P less than 0.01) with increasing visual field losses.

Electrophysiological findings of neuronal ceroid lipofuscinosis in heterozygotes

Nineteen obligate heterozygotes, 8 individuals at risk of being heterozygote, and 10 patients afflicted with four different forms of neuronal ceroid lipofuscinosis were examined electrophysiologically. The group of obligate heterozygotes was compared to age-matched control groups. Statistically significant differences were found between scotopic b-wave amplitudes, P-ERG amplitudes, and EOG light peaks of the obligate carriers of the juvenile type and the control subjects. The photopic L-ERGs and the latencies of the VEPs were mostly within the normal range. The findings represent the first evidence of functional ophthalmological changes in obligate carriers of neuronal ceroid lipofuscinosis and demonstrate that heterozygotes with certain hereditary autosomal recessive diseases may manifest subtle functional signs.

Electrical responses of the human eye to changes in saturation of the stimulating light

Previous studies on color vision using the electroretinogram or the evoked potential have concentrated only on brightness and hue but not on the saturation of color. In the present investigation patterns of alternate stripes of a highly saturated color and a totally desaturated color (= white) of equal brightness were presented in the pattern-reversal mode. It can be demonstrated that the reading obtained is in agreement with psychophysical saturation scale data of earlier investigators.

Pattern electroretinogram: effects of miosis, accommodation, and defocus

The effect of miosis, lens accommodation, and defocusing on the various components of the pattern electroretinogram elicited by checkerboard reversal was investigated by employing aperture stops of different size, varying the mean luminance of the target, and inserting spherical lenses of various optical diopters in front of the eye. After topical administration of pilocarpine (1%) the following changes in the pattern electroretinogram were observed: (i) a decrease of the pupillary diameter leading to a lowered target luminance, which produced attenuation of PERG amplitudes and prolonged latencies, and (ii) defocusing by accommodation of the crystalline lens, which produced attenuation of PERG amplitudes no effect on latencies.

Pattern electroretinogram plus visual evoked potential: a decisive test in patients suspected of malingering

Along the processing chain in the visual pathway the pattern electroretinogram (PERG) is a better indicator of the peripheral function than the visual evoked potential (VEP). Therefore the PERG and the VEP will be impaired equally by disturbances before the ganglion cell layer (e.g., blurred image or retinal disease) and differently by further centrally located diseases (e.g., tumor compression of the optic nerve). Thus in patients complaining of reduced visual acuity who show disturbed VEP but a normal PERG, malingering can be definitely ruled out. Representative combinations of PERG and VEP findings are described.

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

Following the first recording of electroretinographic responses in man to a barred pattern by Riggs and associates (1964) in normal and by Lawwill (1973, 1974) in clinical cases, the first striking observation of a complete loss of pattern electroretinogram (PERG) after injurious section of the optic nerve by Groneberg & Teping (1980) has led to the conclusion that the PERG originates from proximal retinal structures different from those responsible for the luminance electroretinogram (LERG). Typical changes of the PERG are seen during branch occlusion of the central retinal artery and vein. In ocular hypertension without visual field loss and glaucoma-related papillary changes the PERG is decreased at intraocular pressures above 26 mm Hg. In cases of primary glaucoma with regulated intraocular tension and without using miotics the amplitude of the PERG reflects the damage to the inner retinal layers. This favorably compares with the P100 latencies of the visual evoked cortical potential (VECP) which in primary glaucoma were partly within, partly outside the normal range. Other retinal diseases showing amplitude changes in the PERG are primary macular dystrophy, diabetic retinopathy, and the acute stage of optic neuritis. In all these cases the Ganzfeld LERG may be normal or nearly normal, whereas the PERG undergoes typical changes. On the contrary a highly preserved PERG can be recorded in cases of retinitis pigmentosa where the electrooculogram light rise and the LERG are already missing. In light of these findings the recording of PERG constitutes a new promising method of clinical electroretinography reflecting the activity of the hitherto omitted innermost retinal layers. It thereby contributes essentially to the location of disturbances within the visual system.

Retinal receptive fields under different adaptation levels studied with pattern-evoked ERG

The pattern-evoked electroretinogram (PERG) was studied in three normal subjects under different levels of adaptation using pattern onset-offset stimulation. The occurrence of a response maximum (spatial selectivity) in the onset response at a certain spatial frequency can be interpreted as reflecting the activity of neurons having antagonistic center-surround receptive fields and allows an estimation of receptive-field center sizes. The present data suggest that in the light-adapted state, the majority of receptive fields, dominating the response can be estimated to have center diameters of 6-13 min of arc. With increasing dark adaptation, the spatial selectivity shifts from high to low spatial frequencies over 1.3-2.4 octaves, indicating that large receptive fields are active in the dark and small fields in the light. The physiological mechanisms of these changes are discussed on the basis of findings obtained from single-unit studies.

The pattern ERG in response to colored stimuli

The pattern-evoked electroretinogram (PERG) was studied in response to square-wave stripe patterns contrasting either between dark and colored stripes ("red-black" or "green-black" luminance contrast pattern) or between the two colors (red-green chromatic contrast pattern). All lights were matched in photopic luminance. A two-channel Maxwellian view system was used to present the stimuli in the onset-offset mode. When no pattern was present a mixture of both colors was seen. Different spatial frequencies were studied and the amplitudes of the onset response were evaluated. When the two luminance contrast patterns were presented, the responses showed a spatial selectivity. However the combination of the two colors (red-green contrast) resulted in a monotonic decrease of the response with spatial frequency. The spatially selective behavior of the response to luminance contrast patterns could be associated with the on-off center-surround organization of retinal receptive fields. The behavior of the response to chromatic-contrast patterns, on the other hand, could be explained by the action of color-opponent center-surround receptive fields as described in the primate.

The spatial organization of retinal receptive fields in light and darkness as revealed by the pattern electroretinogram

The spatial selectivity of the electroretinogram in response to pattern onset-offset stimuli was studied in man at several levels of adaptation ranging from scotopic to photopic levels. Under conditions of rod function the peak of the spatial selectivity based on amplitude measurements of the pattern-onset response occurs at a low spatial frequency. With increasing light adaptation a gradual shift of the selectivity to higher spatial frequencies occurs. This change in the character of the response can be explained by the assumption that antagonistic center-surround retinal receptive fields contribute to the response, which are larger under scotopic than under photopic levels of stimulation.

Changes in spatial selectivity of pattern-ERG components with stimulus contrast

Electrical mass responses of the visual system to stripe patterns of varying fineness (spatial frequency) can show either an amplitude maximum at a medium spatial frequency, a behavior termed "spatial selectivity," or a monotonic decrease in amplitude with increasing spatial frequency. The former behavior is probably mediated by neurons having a center-surround receptive field structure and the latter by neurons lacking this antagonism. The pattern-evoked human electroretinogram was studied in this report using different spatial frequencies and pattern contrasts. The positive component of the response showed a spatial selectivity only at low contrast but was not spatially selective at the highest contrast. The negative component showed a spatial selectivity at all contrast levels. The data indicate that if pattern-related responses activated by antagonistic receptive fields are to be studied, low contrast values should be employed and attention should be paid to the negative component of the response.

The pattern evoked electroretinogram associated with elevated intraocular pressure

Electroretinographic responses to pattern-reversal stimuli (P-ERG) were recorded in eight patients with protracted elevation of intraocular pressure. Pressures of bigger than 30 mm Hg result in marked amplitude reductions in the P-ERG. The observed changes probably reflect the impaired function of retinal ganglion cells caused by decreased oxygen supply.

Pattern electroretinogram and visually evoked cortical potentials in glaucoma

Electroretinograms (P-ERG) and cortical potentials (P-VECP) evoked by checkerboard patterns were examined in patients with defects of the ganglion cell and nerve fiber layers due to glaucoma. Only a few patients exhibited a prolonged latency in the P-VECP, whereas in the P-ERG all patients with papillary and visual field defects revealed a significantly attenuated amplitude. Since there is a substantial fluctuation in the assessment of papillary excavation and visual field, the P-ERG offers a further means of evaluating and follow-up of retinal function in glaucoma patients.

Binocularity in comitant strabismus: II. Objective evaluation with visual evoked responses

Campos (1982a) reported on his psychophysical studies on the binocularity of patients with comitant strabismus. With binocular visual field techniques, it was possible to show that patients with small-angle eso- and exotropia exhibit a binocular vision, without suppression of the deviated eye. In the present paper visual electrophysiology is used objectively these findings and to provide more insight in the problem. First, the studies on binocularity in normals and strabismics, done by using visual evoked responses (VER) are reviewed. This type of investigation is relatively new and the results of the literature are still conflicting. Then personal results of the authors are reported. It is shown that with VER it is possible to objectively assess the presence of anomalous binocular vision (ABV) sustained by anomalous retinal correspondence (ARC) in small-angle strabismus. In patients with large angle deviations this type of binocularity is absent. A correlation between psychophysical and electrophysiological data is provided. Lastly a simple method is described for differentiating the binocularity of normals from that of Strabismics with ARC. This method is based on the recording of binocular VER with the anteposition in front of the fixing eye of neutral filters of increasing density.