Eberhard Dodt

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.

Beitrge zur Elektrophysiologie des Auges: I. Mitteilung. ber die sekundre Erhebung im Aktionspotential des menschlichen Auges bei Belichtung

Post-excitatory inhibition ist im menschlichen Erg bei Auslosung zweier einander in kurzem Abstande folgender Lichtreize an der Hemmung derb-Welle des zweiten Reizes feststellbar. Lange und “Intensitat” der Hemmung sind direkt abhangig von der Starke des sie bewirkenden vorangehenden Reizes.

Simultaneously recorded retinal and cerebral potentials to windmill stimulation

Visual evoked retinal and cerebral potentials were recorded to onset rotation of an isoluminant sectored disc. While the retinal potentials recorded to onset rotation closely resembled the electroretinogram to a checkerboard or stripe pattern of fixed element size, the visual evoked potential changed interindividually and intraindividually from a fast positive wave at high contrasts, velocities and number of windmill segments to a later negative component at low contrasts, velocities and windmill segments. With change in luminance, contrast, speed and extent of rotation field size and number of disc segments, the visual evoked potential was generally less affected than the electroretinogram.

Pattern electroretinogram of the blue cones

In man the electroretinogram to pattern reversal stimuli (P-ERG) represents a cone response of the proximal retina, dominated by the cone mechanisms sensitive to red (R) and green (G). Additionally there is a cone mechanism sensitive to blue (B) which can be studied with and without steady exposure to yellow light. During exposure to a super-imposed uniform yellow background (576 nm) the transient P-ERG of the B cones is represented by potentials of small amplitude (< 1 μV). The latency (peak time) of the response is about 30 ms longer than that of the midspectral (R and G) cones. Furthermore, the P-ERG of the B cones saturates at low luminances and exhibits a maximum amplitude at about 460 nm. Without yellow adaptation, the P-ERG of the B cones can be studied only with low-intensity stimuli of short wavelengths. Near threshold, both the long-latency response of the B cones and the short-latency response of the R and G cones are recorded simultaneously, forming a double-peaked wave shape. At suprathreshold luminances, even of short wavelength (435 nm) the P-ERG of the B cones is concealed by the larger short latency response of the midspectral cone mechanism.

Pattern electroretinogram peak times as a clinical means of discriminating retinal from optic nerve disease

Fifty-two patients with unilateral or bilateral retinal or optic nerve disease exhibited abnormal peak times and/or amplitudes in the pattern electroretinogram. While this abnormality in patients with optic nerve diseases was confined to an amplitude reduction, 40% of the eyes with retinal diseases exhibited additionally a peak time delay of the p and/or q component. We conclude that recording of pattern electroretinogram peak times provides an additional means to distinguish retinal from optic nerve diseases.

Spatial frequency of the human short-wavelength-sensitive (blue) cone mechanism

The interactions of spatial and chromatic processing of the short-wavelength-sensitive cone mechanism were studied in humans with patterned (checkerboard) stimuli of various spatial frequency (10, 22, 44, and 85 min of arc respectively), under steady exposure to yellow light (575 nm, 390 cd/m2). Psychophysical studies and pattern-reversal visual evoked potentials were employed. Parameters of the transient pattern-reversal visual evoked potentials (pattern reversal rate of 2.4 s−1) especially observed were the latencies of P2 (P100) and N3 and the amplitude of P2-N3. It was only with the largest applicable check size (85 min of arc) that both the psychophysical studies and visual evoked potentials could succeed in satisfactorily isolating the short-wavelength-sensitive cone mechanism. Pattern-reversal visual evoked potential latencies are recommended in the evaluation of this cone mechanism because of their smaller variance and higher selectivity in isolating the short-wavelength-sensitive cone mechanism than the amplitude. The peak sensitivity of this cone mechanism was shown to be at about 449 nm at the corneal level. The short-wavelength sensitive cone mechanism represented the characteristics of low spatial resolution and long latencies of the pattern-reversal visual evoked potentials.

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 rim. 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.

Effect of pre-adaptation on pattern electroretinogram (P-ERG).

Up to now light and dark adaptation are considered as of minor importance when recording the pattern electroretionogram (P-ERG) which in man reflects cone vision. In order to test this supposition transient P-ERGs were recorded in six healthy subjects to a reversing checkerboard pattern produced by a pivoted mirror system after two minutes of pre-exposure to darkness (1), to the pattern reversal stimulus (2), and to uniform illumination of 3.3 log cd/ m2 (3). Field size was 18 × 20 degrees, check size 69 min of arc, reversal frequency 3.5/s, contrast 0.95.When recording the P-ERG within the first 30s after pre-exposure, the amplitude/luminance function of the p-q and the q-r component was shifted along the luminance axis without changing the maximum amplitude. For a criterion response of 1.5 μV there was a sensitivity difference of 1.3 log units between measurements after pre-exposure to darkness and after strong illumination. Furthermore, the P-ERG latencies of the q and the r component displayed significantly shorter values after pre-exposure to strong light than after darkness, whereas no change of p-latency was recorded.We conclude that pre-exposure to darkness and strong illumination has no effect on the P-ERG elicited at high luminance levels. However, pre-adaptation should be considered when recording the P-ERG at luminance levels near threshold.

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° × 39° 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°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 checker-boards has to be regarded as a mixture of both pattern- (contrast) and luminance-specific components.

Professor Wolfgang Straub

Professor Wolfgang Straub died on 10 March 1993, at the age of 72 years. The International Society for Clinical Electrophysiology of Vision has lost one of its founding members. In 1959, 2 years before the acceptance of the Societys by-laws, he already served as treasurer of ISCERG and remained in that office until March 1977. Because of a shift in his interests to other important areas of research, such as toxoplasmosis and conjunctival transplants, he left the Society in 1986. I first met Professor Straub in 1951 Professor Straub.