Pattern electroretinogram can be more than the sum of local luminance responses

Pattern electroretinography and visual evoked potentials in optic nerve diseases

BACKGROUND

To evaluate transient pattern electroretinography (PERG) and pattern visual evoked potential (VEP) for the diagnosis, differential diagnosis and follow-up of optic nerve diseases.

METHODS

Twenty-nine consecutive patients (14 female, 15 male) with the diagnosis of ischaemic optic neuropathy (n=14) and optic neuritis (n=15) were included in this study. Mean age of the patients with ischaemic optic neuropathy was 63.3+/-3.3 (60-78) years and the mean age of the patients with optic neuritis was 28.3+/-8.4 (19-43) years. In each patient ophthalmological examination and systemic evaluation were done and VEP and PERG were recorded. As a control group, VEP recordings of 35 healthy subjects were included.

RESULTS

In the ischaemic optic neuropathy group (group 1), mean VEP amplitude (+/-SD) (1.96+/-0.95 microV) was found to be decreased significantly in the affected eyes in comparison to the control group and the unaffected eyes. The delay in latency (116.3+/-20.14 msec in the affected eyes compared with 101.31+/-6.19 msec in unaffected eyes) was statistically significant when compared with the healthy subjects. In the optic neuritis group (group 2), VEP amplitude was decreased (4.13+/-4.04 microV vs 6.97+/-3.35 microV and 6.97+/-4.43 microV) and latency was increased (122.59+/-20.09 msec vs 101.31+/-6.19 msec and 108.76+/-13.57 msec) in affected eyes significantly in comparison to the unaffected eyes and control group, respectively. Even though there were no significant differences for P50 latency and N95/P50 ratios between affected and unaffected eyes in both groups, N95 amplitude decreased significantly in the affected eyes of the ischaemic optic neuropathy patients and N95 latency was found to be decreased in optic neuritis patients. There was no correlation between VEP and PERG findings in both groups.

CONCLUSION

VEP amplitude decreased significantly in ischaemic optic neuropathies while latency delay was more significant in patients with optic neuritis. PERG findings showed decreased N95 amplitude in ischemic optic neuropathy without associated latency changes.

Contributions from lateral interaction mechanisms to the human ERG can be studied with a two-frequency method

We present a two-frequency method to investigate lateral interaction components (liERGs) in the human electroretinogram. Adjacent half cycles of sinusoidal gratings were modulated sinusoidally with different temporal frequencies f1 and f2. The liERGs were defined by the Fourier components at the intermodulation frequencies /f1 - f2/ and f1 + f2 which indicate nonlinear interactions between the half cycles. Significant liERGs were found in all subjects with a monotonic increase of the liERG magnitude in the spatial frequency range from f(s)=0.07 to 2.4 cpd. When /f1 - f2/ was below 5 Hz, liERGs were masked by noise intrusions. In a control experiment we demonstrated that the liERGs were not evoked by stray light artifacts. The liERGs may help to further differentiate the responses that are evoked by patterned stimuli within the retina.

Pattern electroretinography (PERG) and an integrated approach to visual pathway diagnosis

The pattern electroretinogram (PERG) provides an objective measure of central retinal function, and has become an important element of the author's clinical visual electrophysiological practice. The PERG contains two main components, a positivity at approximately 50ms (P50) and a larger negativity at approximately 95ms (N95). The P50 component is affected by macular dysfunction with concomitant reduction in N95. The PERG therefore complements the Ganzfeld ERG in the assessment of patients with retinal disease. In contrast, the ganglion cell origins of the N95 component allow electrophysiological evaluation of ganglion cell function both in primary disease and in dysfunction secondary to optic nerve disease, where selective loss of N95 can be observed. Both macular dysfunction and optic nerve disease can give abnormalities in the visual evoked cortical potential (VEP), and the PERG thus facilitates more meaningful VEP interpretation. This review addresses the origins and recording of the PERG, and then draws on extensive clinical data from patients with genetically determined retinal and macular dystrophies, other retinal diseases and a variety of optic nerve disorders, to present an integrated approach to diagnosis.

The pattern electroretinogram in anterior visual pathway dysfunction and its relationship to the pattern visual evoked potential: a personal clinical review of 743 eyes

The pattern electroretinogram (PERG) has now been in routine clinical use for sufficiently long to allow a personal clinical review of its relationship to the cortically generated pattern visual evoked potential (PVEP). The PERG and PVEP findings are presented from 520 eyes with optic nerve demyelination (382 eyes), optic nerve compression (90 eyes) or heredofamilial optic atrophy (48 eyes), and these are compared with the findings obtained in 223 eyes with dysfunction anterior to the retinal ganglion cells. Dysfunction anterior to the retinal ganglion cells gives a reduction in the P50 component of the PERG, but this component is usually spared in optic nerve disease where selective loss of the N95 component is by far the most frequently occurring abnormality. A diagnostic strategy is presented.

Temporal analysis of the topographic ERG: chromatic versus achromatic stimulation

The topographic electroretinogram evoked by multi-focal exchange of black and white or red and green stimuli was analysed into linear and non-linear Wiener kernels. The first-order (temporally linear) response showed a biphasic waveform which inverted as the luminance ratio of the exchanged colours passed through unity (established both psychophysically and photometrically). A short latency non-linearity which was dependant on luminance contrast was observed in both chromatic and achromatic ERG. However, in the chromatic second-order response, a long-latency non-linearity, foveally prominent, with a distinct skew in power towards the nasal retina, appeared around the isoluminant point, between the points of silent substitution for the L and M-cone types. Modelling of the second-order responses showed that over a wide range of luminance ratios, the chromatic ERG is well described by a linear combination of the achromatic (contrast-dependent) component and the response at isoluminance. The difference in second-order response between coloured and black and white stimulation, at the same luminance contrast, showed that the long-latency non-linearity is recorded when the red and green cone types are operating out of phase and peaks in amplitude at a green/red luminance ratio of 0.8. This interpretation was confirmed by the lack of the long-latency non-linearity in colour-anomalous subjects (whether deficient in the L or the M-cone type). A marked similarity exists between the properties of the long-latency non-linearity and the frequency-doubled response generated in the ganglion cells of the magnocellular pathway.

Invariance of the pattern electroretinogram evoked by psychophysically equivalent stimuli in human ageing

1. The aim of this study was to assess the contribution of retinal ganglion cells to the decline in contrast sensitivity during human ageing. 2. After determination of the appropriate refraction for each subject, younger subjects were arranged to be exposed to a display luminance which was suprathreshold by the same amount as in older subjects wearing a 4.0 mm diameter artificial pupil with a neutral density filter. 3. In fifty-four subjects, aged 20-99 years, contrast sensitivities measured in response to phase-reversed grating patterns of 2, 5 and 8 cycles per degree declined significantly with increasing age at each spatial frequency studied. 4. Subjects were made psychophysically equivalent by setting the display contrast at x5 and x10 contrast threshold for each subject. The pattern electroretinogram (PERG) was recorded with a sterile silver thread (DLT) electrode placed in the lower canthus of one eye, with the indifferent electrode on the temple and the earth on the forehead. 5. For each contrast multiple at each spatial frequency, the PERG implicit time showed no significant change with age, indicating equivalence of the response across the age range. 6. Control experiments with two young and two elderly subjects established that the PERG implicit time decreased appreciably with increasing contrast, over a range of x2 to x20 contrast threshold. 7. Since the psychophysically equivalent stimulus displays had generated equivalent PERGs in terms of implicit time in young and elderly subjects, this was consistent with the equivalence of retinal ganglion cell function under these conditions. 8. Adverse changes within the retina were therefore inferred to play a major role in the decline in contrast sensitivity with age.

Effect of kainic acid and NMDA on the pattern electroretinogram, the scotopic threshold response, the oscillatory potentials and the electroretinogram in the urethane anaesthetized cat

Kainic acid (KA, 12.5-100 nmol) or N-methyl-D-aspartate (NMDA 25-250 nmol) was injected into the vitreous of one eye of urethane anaesthetized cats. Pattern electroretinograms (PERGs) were recorded to transient contrast reversing bars. Scotopic luminance electroretinograms (ERGs) were recorded to blue flashes. All doses of KA reduced the oscillatory potentials (OPs), PERG and focal ERG (FERG). At 50 nmol KA, the b-wave and scoptic threshold response (STR) were normal. At 100 nmol KA, the STR was absent and the b-wave reduced by over 50%. OPs and STRs were reduced in all NMDA injected eyes. NMDA at 25 nmol enhanced the FERG, PERG, and b-wave and high doses (above 150 nmol) reduced them. Light microscopic examination of retinas showed 25 nmol KA only damaged dendrites of ganglion cells. NMDA damage was slight with < 200 nmol. These data show that the cat PERG has a proximal component which is very sensitive to low doses of KA; the PERG and FERG are very similar; the STR and PERG are generated by different structures and that the OPs and the FERG and PERG are all generated close to the ganglion cell layer, proximal to the STR.

Retinal and cortical activity in human subjects during color flicker fusion

Pattern electroretinograms (PERG) and cortical visually evoked potentials (VEP) were simultaneously recorded from 7 visually normal and 1 protanopic subjects. Stimuli were color checkerboards (0.5 degrees check size), phase-reversing at 17 Hz (i.e. 34 reversals/sec). Using a stepwise sweep procedure, the luminance of the red (lambda peak = 550 nm) and green (lambda peak = 630 nm) checks varied in 11 steps in opposite directions from 0 to 30 cd/m2, embracing the subjective equiluminance point. For normal subjects at subjective equiluminance, the VEP amplitude dropped sharply down to 13 +/- 2% of the value at pure luminance contrast. The PERG, however, was only reduced to 56 +/- 10% at this point, an attenuation 4 times less than that of the VEP. In contrast to normal subjects, in the protanopic subject the PERG was sharply reduced at equiluminance, parallel to the VEP. This would be expected when L-cones are missing. Assuming that the PERG reflects the activity of the retinal ganglion cells, our findings suggest that human retinal ganglion cells respond well under the condition of equiluminant flicker fusion, which is in agreement with recent single-cell studies in the monkey. Consequently, the temporal low-pass filter, which mediates color-flicker fusion, would seem to lie central to the retinal ganglion cells.

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).

Steady-state visual evoked cortical potentials from stimulation of visual field quadrants. Optimizing pattern variables for the size of the field to be investigated

The effects of check size and stimulus size were investigated to optimize the steady-state visual evoked cortical potentials from pattern-reversal stimulation of the visual field quadrants. Check sizes of 15', 30', 60', 90', 120' and 180' were investigated at a pattern reversal rate of 11.6 per second for field sizes varying from 2 degrees x 2 degrees to 24 degrees x 24 degrees. The visual evoked cortical potentials were recorded from mid occipital, right occipital and left occipital positions. In the inferonasal quadrant, the largest amplitudes were obtained with 30' and 60' check sizes; however, for these check sizes, the visual evoked cortical potential yielded limited additional information for field sizes greater than 4 degrees x 4 degrees and 6 degrees x 6 degrees, respectively. When a field size of 12 degrees x 12 degrees was investigated, a 90' check size was optimal. The results indicated that, with the above recording positions and check sizes of 15' to 120', there is an optimal number of pattern elements, 40 to 100, for stimulation of the inferonasal quadrant. This should be taken into account when a check size is selected to investigate a field quadrant of a particular size. Digital signal processing techniques were applied to analyze the visual evoked cortical potential, and the system shows promise for objective examination of the visual field.

Lateral interaction component and local luminance nonlinearities in the human pattern reversal ERG

Two different mechanisms are presumed to contribute to pattern electroretinograms: non linearities of the local luminance response and nonlinear effects of lateral interactions. Previous attempts to discriminate the two components relied on the theoretical MTF of the optics. In this study, techniques of nonlinear systems analysis are used to extract the two components from the response to pattern reversal of different check sizes. The decomposition is based on the structure of the second order pattern reversal kernels alone. Detailed information about the two mechanisms can be gleaned from the kernel structure. The component properties are compared and discussed.

Electroretinograms to checkerboard pattern reversal in cats: physiological characteristics and effect of retrograde degeneration of ganglion cells

Pattern electroretinograms (P-ERGs) evoked by alternating checks were studied in cats. Spatial frequency functions for transient and steady-state P-ERG waves showed a bimodal distribution with a preferred frequency at 0.6-0.75 c/deg and a second amplitude increase at frequencies lower than 0.5 c/deg. Decreasing the pattern luminance by 0.5 log units produced a shift of the spatial tuning curve toward lower spatial frequencies. No temporal tuning was noted in the temporal frequency functions. The bimodal distribution suggests that at spatial frequencies higher than 0.5 c/deg, the tuning reflects a 'contrast response' originating in cells with center surround organization. At spatial frequencies below 0.5 c/deg, the 'luminance response' becomes predominant and is generated in cells sensitive to mean luminance changes. Transient and steady-state P-ERGs to medium and high spatial frequencies were abolished by section of the optic nerve, while low spatial frequency stimuli at or below 0.3 c/deg continued to evoke P-ERGs at 1, 5 and 10 months after surgery. Quantitative whole mount retina microscopic examination confirmed the retrograde degeneration of the ganglion cells. It is concluded that both transient and steady-state P-ERGs to small and medium spatial frequencies checks are predominantly related to ganglion cell activity, while P-ERGs to low spatial frequencies reflect preganglionic cell activity.

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.

Isolation of scotopic human electroretinograms using color adaptation and pattern reversal stimuli

The human electroretinogram was recorded in response to alternations of a 500 nm checkerboard stimulus pattern presented against adaptation backgrounds of different wavelengths. A modification of Stiles' color adaptation paradigm was adapted to electroretinography. The results provided electrophysiological t.v.i. curves (threshold versus intensity curves) and permitted a determination of spectral sensitivity. The t.v.i. curves matched those seen in psychophysics. The spectral curve showed a close agreement with the scotopic CIE function.