The second harmonic of the electroretinogram to sinusoidal flicker: spatiotemporal properties and clinical application

Asymmetric Functional Impairment of ON and OFF Retinal Pathways in Glaucoma

Purpose

To investigate ON-pathway versus OFF-pathway dysfunction in glaucoma using handheld electroretinography (ERG) with a temporally modulated sinusoidal flicker stimulus.

Design

Cross-sectional study.

Participants

Fifty-nine participants accounting for 104 eyes, comprised of 19 control eyes, 26 glaucoma suspect eyes, and 59 glaucoma eyes.

Methods

Participants underwent portable ERG testing, which included the photopic flash, photopic flicker, photopic negative response stimulus, ON-OFF stimulus, and a custom-written sinusoidal flicker stimulus that was modulated from 50 to 0.3 Hz.

Main Outcome and Measures

The ERG response amplitudes were measured by the handheld ERG. For the custom-written sinusoidal flicker stimulus, we derived and compared the log10 first harmonic frequency response amplitudes. Patient discomfort and fatigue after ERG testing were rated on a scale from 1 to 5.

Results

Baseline demographics were not significantly different between groups, except for ocular characteristics. Analysis was performed adjusting for participant age, sex, race, and dilation status, and the sinusoidal frequency responses were stratified at 10 Hz because higher frequencies are associated with the OFF-pathway, whereas lower frequencies are associated with the ON-pathway. After stratification, glaucoma eyes showed an adjusted decrease of 32.1% at frequencies of more than 10 Hz (95% confidence interval [CI], -51.8% to -4.1%; P = 0.03). For 10 Hz stimulus frequencies or less, an adjusted 11.5% reduction was found (95% CI, -39.5% to 29.1%; P = 0.50). Glaucoma suspect eyes did show a decreased response, but this was not significant at either frequency range. When comparing handheld ERG with traditional visual field assessments, participants found the handheld ERG to result in much less discomfort and fatigue.

Conclusions

Our finding that glaucoma participants showed greater decreases in ERG response at higher frequencies supports the hypothesis that the OFF-pathway may be more vulnerable in human glaucoma. Using a handheld ERG device with a sinusoidal flicker stimulus may provide an objective assessment of visual function in glaucoma.

Miniaturisation reduces contrast sensitivity and spatial resolving power in ants

Vision is crucial for animals to find prey, locate conspecifics, and to navigate within cluttered landscapes. Animals need to discriminate objects against a visually noisy background. However, the ability to detect spatial information is limited by eye size. In insects, as individuals become smaller, the space available for the eyes reduces, which affects the number of ommatidia, the size of the lens and the downstream information processing capabilities. The evolution of small body size in a lineage, known as miniaturisation, is common in insects. Here, using pattern electroretinography with vertical sinusoidal gratings as stimuli, we studied how miniaturisation affects spatial resolving power and contrast sensitivity in four diurnal ants that live in a similar environment but varied in their body and eye size. We found that ants with fewer and smaller ommatidial facets had lower spatial resolving power and contrast sensitivity. The spatial resolving power was maximum in the largest ant Myrmecia tarsata at 0.60 cycles per degree (cpd) compared to the ant with smallest eyes Rhytidoponera inornata that had 0.48 cpd. Maximum contrast sensitivity (minimum contrast threshold) in M. tarsata (2627 facets) was 15.51 (6.4% contrast detection threshold) at 0.1 cpd, while the smallest ant R. inornata (227 facets) had a maximum contrast sensitivity of 1.34 (74.1% contrast detection threshold) at 0.05 cpd. This is the first study to physiologically investigate contrast sensitivity in the context of insect allometry. Miniaturisation thus dramatically decreases maximum contrast sensitivity and also reduces spatial resolution, which could have implications for visually guided behaviours.

Electrophysiological Assessment of Visual Pathways in Glaucoma

Purpose.

To assess nerve conduction in visual pathways in patients with open-angle glaucoma.

Methods.

Pattern-electroretinograms (PERG) and visual-evoked potentials (VEP) were simultaneously recorded in 16 patients with open-angle glaucoma (POAG) and 15 age-matched controls. The visual stimuli were checker-board patterns (the check edges subtend 15’; the contrast was 70% and reversed at the rate of 2 reversals/s).

Results.

POAG patients showed significantly higher PERG and VEP latencies (ANOVA: P<0.01) and significantly lower amplitudes than controls; the retinocortical time (RCT: difference between VEP P100 latency and PERG P50 latency) was longer (P<0.01) in POAG than controls and the longer RCT was correlated with the reduced PERG amplitude (r:0.798, P<0.01).

Conclusions.

This suggests that POAG patients have an involvement of the innermost retinal layers and impaired nerve conduction in their visual pathways.

A new interpretation of components in the ERG signals to sine wave luminance stimuli at different temporal frequencies and contrasts

Full-field electroretinograms were recorded from five normal human subjects using white light (mean luminance: 250 cd/m2) sine wave stimuli at different frequencies and contrasts. In agreement with previous studies, we found that the amplitude of the fundamental component displayed a dip at about 12 Hz, coinciding with a maximum in the second harmonic component, indicating frequency doubling of the responses. By including measurements at different contrasts, we were able to recognize two (sine-like and transient) response components. We found that the waveform of the transient response was relatively frequency independent. An algorithm to separate the two components was developed. The interaction between these two components can explain the frequency-doubled responses around 12 Hz. The sine-like component is more linear and prominent in the low-frequency region, whereas the transient seems to be more nonlinear and prominent in the high-frequency region.

Intravitreal administration of the anti-TNF monoclonal antibody infliximab in the rabbit

BACKGROUND

Tumor necrosis factor (TNF) is known to play an important role in various immune-mediated ocular diseases; intravenous administration of the anti-TNF monoclonal antibody infliximab has proved beneficial in such cases. Since intravitreal injection (when available) is a substitute for systemic administration of various drugs targeting the eye, we aimed to evaluate the safety of intravitreal injection of infliximab in the rabbit eye.

METHODS

Seven groups of New Zealand white rabbits (four animals in each group) received a single unilateral intravitreal injection (0.1 ml) of increasing doses of infliximab (namely 1, 2, 5, 8, 10 or 20 mg infliximab [Remicade]) or a sham injection respectively. Slit-lamp biomicroscopy, fundoscopy and electrophysiology recordings, i.e. scotopic, photopic and flicker responses, were performed at baseline and after 1, 5, 10, 15, 30 and 45 days. Infliximab-injected eyes were compared with sham-injected and with uninjected fellow eyes (n = 28). Animals were euthanized on day 45 for histopathological examination of the retinas.

RESULTS

Clinical examination and electrophysiological testing were consistently unremarkable after either sham or 1 mg or 2 mg infliximab injections. In contrast, electrophysiological recordings were significantly reduced in a dose-dependent manner from day 1 through day 45, after 5, 8, 10 and 20 mg infliximab injections. Flicker responses were the most sensitive in detecting the lower toxic dose of 5 mg. Histopathological findings were similar in uninjected and sham-injected eyes, as well as after 1 mg or 2 mg infliximab injections. Consistent with the functional abnormalities, retinal deformities and diffuse edema were observed after injection of 5 mg or higher doses of infliximab.

CONCLUSIONS

Intravitreal infliximab may be safely administered up to a dose of 2 mg in the rabbit eye. Such doses can be used in the design of future clinical trials assessing the effects of infliximab for selected patients with immune-mediated ocular conditions.

A fast visual evoked potential method for functional assessment and follow-up of childhood optic gliomas

OBJECTIVE

To evaluate a fast technique of visual evoked potentials (VEPs) recording, in response to steady-state luminance stimuli (SS-LVEPs), for functional assessment and follow-up of childhood optic gliomas (OGs).

METHODS

Eighteen OG patients (age range: 3.5-18 years), with different degrees of optic pathway damage severity, were examined. Sixteen age-matched normal subjects served as controls. Ten of the 18 OG patients were re-tested 1-3 months after the first examination. SS-LVEPs were elicited by a sinusoidally-modulated flickering (8 Hz) uniform field, generated by a light emitting diode (LED)-array and presented monocularly in a mini-ganzfeld. Amplitude and phase of the Fourier-analyzed response fundamental (1F) and second harmonic (2F) were measured. The full VEP protocol had a median duration of 6 min (range: 4-12).

RESULTS

When compared to normal control values, median 1F and 2F SS-LVEP amplitudes of OG patients were reduced (P<0.01), with a borderline increase in 2F phase lag (P<0.05). In 11 OG patients with asymmetric optic pathway damage in between-eye comparisons, median 1F amplitude losses were greater (P<0.01) in fellow eyes with more severe damage. No significant interocular difference was observed in control subjects. Median test-retest changes of 1F and 2F component were <20% and 30 degrees for amplitude and phase, respectively. In individual OG patients, 1F and 2F amplitudes were positively correlated (P<0.01) with visual acuity. 1F amplitude losses were correlated (P=0.01) with the severity of optic disc atrophy. Considering both 1F and 2F abnormalities, diagnostic sensitivity of SS-LVEP in detecting OG-induced optic pathways damage was 83.3%.

CONCLUSIONS

The present findings support the use of this technique, as an alternative to pattern VEPs, for functional assessment and follow-up of OG in uncooperative children.

Visual electrophysiological responses in persons with type 1 diabetes

Persons with type 1 diabetes show electrophysiological abnormalities of the visual system which are revealed by methods such as flash electroretinogram (FERG), oscillatory potentials (OPs), pattern electroretinogram (PERG), focal electroretinogram (focal ERG), visual evoked potentials (VEP) in basal condition and after photostress. This review reports the changes in electrophysiological responses of the different structures composing the visual system observed in persons with type 1 diabetes before the development of the overt clinical retinopathy. In persons with type 1 diabetes without retinopathy (IDD), the earlier abnormal electrophysiological responses are recorded from the innermost retinal layers and postretinal visual pathways, as suggested by impaired PERGs and delayed retinocortical time (RCT). These are observed in IDD persons with a disease duration shorter than 6 months. Further electrophysiological changes are recorded from the macula (abnormal focal ERG and VEP after photostress) in IDD persons with disease duration greater than 1 year. Additional electrophysiological changes are recorded from the middle and outer retinal layers (impaired FERG and OPs) in IDD persons with a disease duration greater than 10 years. All the electrophysiological tests show a greater degree of abnormal responses in persons with type 1 diabetes when a background retinopathy is present.

Electrophysiological evaluation of the macular cone system: focal electroretinography and visual evoked potentials after photostress

In this article, the methodologies and clinical applications of two electrophysiological tests, the focal electroretinogram (FERG) and the visual evoked potentials (VEPs) after photostress, are described. These techniques provide somewhat complementary results about macular function because they tap the activity of different neural substrates along the pathway of the cone system and allow evaluation of the macular function under steady-state (ie, the FERG) or dynamic (ie, the VEPs after photostress) conditions. The results obtained in patients with different macular pathologies indicate that while the FERG provides direct information about the extent and sites of macular dysfunction, the VEPs after photostress represent an objective, although not specific, index of the dynamic properties of macular performance after exposure to intense light stimulation. The combined use of both techniques appears to be promising for gaining further insights into the diagnosis and pathophysiology of macular diseases.

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.

Neural conduction in the visual pathways in ocular hypertension and glaucoma

BACKGROUND

The aim of our work was to evaluate neural conduction in visual pathways in subjects with ocular hypertension and glaucoma.

METHODS

We assessed simultaneous recordings of pattern electroretinograms (PERG) and visual evoked potentials (VEP) in 16 subjects with ocular hypertension (OHT), in 16 subjects with primary open-angle glaucoma (POAG) and in 15 age-matched controls. The visual stimuli were checkerboard patterns (the check edges subtend 15 min of visual arc; contrast 70%) reversed at the rate of 2 reversals/s.

RESULTS

In OHT and POAG patients we found PERG and VEP latencies significantly longer than in controls. The P50-N95 PERG amplitudes were significantly reduced in OHT and POAG eyes. VEP amplitudes were significantly reduced in POAG eyes, while in OHT they were similar to controls. The retinocortical time (RCT; difference between VEP P100 latency and PERG P50 latency) was longer in POAG patients than in controls; no differences between patients with OHT and controls were observed. Moreover, we observed that in POAG the longer RCT was inversely related to the PERG amplitude.

CONCLUSION

Our results suggest that involvement of the innermost retinal layers in POAG is accompanied by slowed neural conduction in the visual pathways.

Macular flicker electroretinograms in Best vitelliform dystrophy

PURPOSE

The aim of this study was to evaluate the function of the neurosensory retina in Best vitelliform macular dystrophy (BMD) by recording the focal electroretinogram (ERG) fundamental and 2nd harmonic components, which are known to be dominated by receptoral and postreceptoral activity, respectively.

METHODS

FERGs were recorded in response to a uniform field (9 x 9 deg) flickered sinusoidally at either 8 Hz or 32 Hz (peak frequencies for the 2nd and fundamental harmonic, respectively). The fundamental component of the response to the 32-Hz stimulus and the 2nd harmonic of the response to the 8-Hz stimulus were measured in their amplitudes and phases. The fundamental-2nd harmonic amplitude ratio was taken as an index of the relative changes in the FERG components. Eleven patients with BMD and vitelliform stage macular lesions were evaluated. Results were compared with those obtained from 13 patients with Type 2 Stargardt macular dystrophy (STD) according to the Noble and Carr Classification, and 29 normal control subjects. Four BMD and four STD patients were also followed electrophysiologically over a 48 month period.

RESULTS

Compared to controls, BMD patients showed losses of both FERG fundamental and 2nd harmonic amplitudes, and an increase in the fundamental and increase in the fundamental-2nd harmonic ratio. STD patients also showed losses of both fundamental and 2nd harmonic, but the fundamental-2nd harmonic ratio was normal. In BMD patients, but not in those with STD, the fundamental amplitude tended to decrease over the follow-up period.

CONCLUSIONS

The results indicate that BMD involves neurosensory abnormalities early in the disease process. The increased fundamental-2nd harmonic ratio suggests that a postreceptoral dysfunction may be present in addition to that of photoreceptors. This differs from STD, where losses appear to affect primarily the receptoral retina. Receptoral losses in BMD may progress throughout the medium-term follow up.