The spatial properties of the human electroretinogram

The Relationship Between the Photopic Negative Response and Retinal Ganglion Cell Topography

Purpose

To assess the topographic relationship between the photopic negative response (PhNR) and retinal ganglion cell distribution in healthy individuals.

Method

Data was recorded from 16 healthy participants. The amplitude of PhNRs obtained in response to focal long duration (250 ms) and brief flash (5 ms), red (660 nm) on blue (469 nm) stimuli of increasing size (5° - full field) were measured. The number of retinal ganglion cell receptive fields (RGCf) in each stimulus area was established from the literature and regression analysis used to determine the relationships between: PhNR amplitude and number of RGCfs stimulated, PhNR density and the RGCf density and response per RGCf as a function of eccentricity.

Results

The overall amplitude of the PhNR increased with stimulus size and the response density declined from ∼0.1 μV/deg in the macular region to ∼0.003 μV/deg approximately 45° from the fovea. Contrary to expectations, the relationship between the PhNR and number of RGCf was nonlinear, the response from more eccentric neurons being about three times greater than those in the macular region.

Conclusions

Although the amplitude of the PhNR broadly maps on to the topographic distribution of RGCf the increase in PhNR amplitude with increasing eccentricity is only partly explained by RGCf numbers. Increases in the PhNR amplitude may be due to topographic variations in the contributions from other non-RGC neurons, as well as eccentricity-related morphologic and physiologic differences in RGCs.

Retinocortical Gain in the Foveal Pathway: The Effect of Spatial Frequency and Stimulus Size

The amplitude and the phase of the simultaneously recorded steady-state pattern electroretinogram (PERG) and visual evoked potential (VEP) were evaluated in humans as a function of the vertical diameter (D) of unidimensional Gabor stimuli. In the other dimension, parallel to the horizontal gratings, the patterns all had constant diameter (see Methods and Materials). Spatial frequencies (SFs) of 1 cycle per degree (cpd) and 5.3 cpd were counterphase modulated at a rate of 6.8 Hz. After off-line artifact rejection, the response was subject to Fast Fourier Transformation (FFT). Amplitude and phase of the first and second harmonics of both ERG and VEP were displayed for each SF and stimulus diameter.

Both ERG and VEP amplitude were found to increase as a function of D. Using trend analysis we found that ERG amplitude increased linearly as a function of D. VEP amplitude was found to be both linear and cubic, as a function of D. We calculated the ratio of VEP amplitude and ERG amplitude at each D and termed it retinocortical gain (G). G normalized to stimulus area was high for small D and decreased with D independently of SF. Unity gain occurred at stimulus sizes of 6° to 7°.

ERG phase was found to be more negative at 5.3 cpd than at 1 cpd. Although no significant difference was found between VEP phases at the two SFs tested, neither ERG nor VEP phase changed as a function of D.

The results suggest that retinocortical gain is highest for the foveally centered low spatial frequency small patch Gabors. The results provide support for the notion of the “foveal window” in human vision.

The electroretinogram evoked by the excitation of human foveal cones

1. A 2 degrees test stimulus foveally fixed and viewed against a blue background (40 degrees in extent and producing 2.0 x 10(4) scotopic td of retinal illuminance) evokes a small voltage which can be recorded from the human eye with a conventional contact lens electrode if the test stimulus is flashed at a rate of 15 c/s, and the responses to at least several hundred flashes are averaged.2. The action spectrum of the response obtained in this way agrees reasonably well with the observer's psychophysical foveal luminosity curve.3. For the peripheral retina, the action spectrum is similar to that of the fovea when allowance is made for differences in screening macular pigment.4. Such responses diminish when the test stimulus is focused on to the peripheral retina and disappear when the test light is focused on the blind spot.5. Therefore, the response to the test light fixated centrally is the result of the excitation only of cones mainly, if not exclusively, in the fovea.6. When the intensity of the background is reduced by a factor of 10, the action spectrum shows evidence of the effect of excitation of rods in the blue part of the spectrum and of cones in the red. These red and blue responses add linearly when combined together, provided they are adjusted to coincide in phase.

Rapid pupil-based assessment of glaucomatous damage

PURPOSE

To investigate the ability of a technique employing pupillometry and functionally-shaped stimuli to assess loss of visual function due to glaucomatous optic neuropathy.

METHODS

Pairs of large stimuli, mirror images about the horizontal meridian, were displayed alternately in the upper and lower visual field. Pupil diameter was recorded and analyzed in terms of the "contrast balance" (relative sensitivity to the upper and lower stimuli), and the pupil constriction amplitude to upper and lower stimuli separately. A group of 40 patients with glaucoma was tested twice in a first session, and twice more in a second session, 1 to 3 weeks later. A group of 40 normal subjects was tested with the same protocol.

RESULTS

Results for the normal subjects indicated functional symmetry in upper/lower retina, on average. Contrast balance results for the patients with glaucoma differed from normal: half the normal subjects had contrast balance within 0.06 log unit of equality and 80% had contrast balance within 0.1 log unit. Half the patients had contrast balances more than 0.1 log unit from equality. Patient contrast balances were moderately correlated with predictions from perimetric data (r = 0.37, p < 0.00001). Contrast balances correctly classified visual field damage in 28 patients (70%), and response amplitudes correctly classified 24 patients (60%). When contrast balance and response amplitude were combined, receiver operating characteristic area for discriminating glaucoma from normal was 0.83.

CONCLUSIONS

Pupillary evaluation of retinal asymmetry provides a rapid method for detecting and classifying visual field defects. In this patient population, classification agreed with perimetry in 70% of eyes.

Retinal function following transpupillary thermotherapy for occult choroidal neovascularization in age-related macular degeneration: a short-term study by focal electroretinography

PURPOSE

To assess short-term changes in macular function after transpupillary thermotherapy (TTT) in patients with occult subfoveal choroidal neovascularization (CNV) secondary to age-related macular degeneration (AMD), using focal electroretinography (FERG).

METHODS

Twenty-five patients with occult subfoveal CNV due to AMD were treated with TTT delivered using an infrared (810 nm) diode laser (spot size 3.0 mm, laser power 400-600 mW, duration 60 seconds). All patients were clinically evaluated before, 1 and 6 weeks after treatment. Snellen visual acuity (VA) was measured at each visit. Fluorescein angiography (FA) was performed at baseline and 6 weeks after TTT. Focal ERGs were recorded in all patients immediately before and 1 week after TTT in response to an 18-degree diameter, 41 Hz flickering spot (630 nm) centred on the fovea, presented on a steady background in Maxwellian view. A subgroup of 12 patients was also re-tested by FERG at 6-weeks post-TTT.

RESULTS

No significant changes in mean FERG amplitude and phase were observed across the different recording sessions before and after TTT. One week after TTT, four patients had significant (> 2 SD from baseline variability) increases in FERG amplitude and/or phase advances, one had a decrease in amplitude and four had phase delays, compared to baseline. The remaining 15 patients had stable FERGs. Six weeks after TTT, four patients had significant increases in FERG amplitude and/or phase advances, four had decreases in amplitude and/or phase delays, and four had stable FERGs, compared to baseline. Improvement in FERG parameters after TTT was always associated with an improvement in VA and a decrease in exudation. Patients with post-TTT FERG deterioration had stable or deteriorated clinical pictures. At either 1 or 6 weeks post-TTT, the FERG amplitude increase was inversely correlated (p < 0.05) with the baseline FERG amplitude and VA.

CONCLUSIONS

Three major conclusions can be drawn: in a short-term follow-up, TTT was not found to be associated with significant changes in macular function; FERG improvement was associated with VA improvement, and the increase in FERG amplitude was greatest in patients with the worst baseline acuity.

Transient and steady state focal and pattern electroretinogram nerve section losses in cats with unilateral optic

This study shows the ketamine/xylazine anaesthetised cat is a useful model for the effect of unilateral optic nerve section on pattern electroretinograms (PERGs), especially if stimuli extending to previously untested low spatial frequencies and preferably down to the focal ERG (FERG) are included. The transient reversal rate, seldom used in animals,has advantages over steady state recording. Transient PERGs had signs of true spatial tuning, a higher amplitude and signal noise ratio and showed the effect of optic atrophy at low spatial frequencies more rapidly.

Diagnosis and classification of macular degenerations: an approach based on retinal function testing

The results from literature concerning some aspects of retinal function in macular degenerations (MDs) were reviewed in order to evaluate whether (a) specific patterns of retinal dysfunction may be linked to different clinical phenotypes, and (b) distinct functional profiles may help in orienting molecular diagnosis of diseases. Examined clinical phenotypes included: Stargardt disease/fundus flavimaculatus (St/FF), age-related maculopathy (ARM) and macular degeneration (AMD), pattern dystrophies (PD), Best vitelliform dystrophy (BVD), Sorsby's fundus dystrophy (SFD), autosomal cone-rod dystrophies (CRD). The following functional tests were evaluated: (1) electroretinogram (ERG) (scotopic and photopic according to ISCEV standards, rod and cone photoresponses, rod and cone b-wave intensity-response function, focal ERGs); (2) dark adaptometry (pre-bleach sensitivity and post-bleach recovery kinetics); (3) fundus reflectometry (pigment density and regeneration kinetics). Specific patterns of retinal dysfunction were identified for St/FF, ARM/AMD, SFD and BVD, whereas partially overlapping profiles were found for PD and CRD. Specific functional patterns were associated with different peripherin/RDS gene mutations, as well as with CRX mutations. Combined analysis of different retinal function tests may help to identify different phenotypes of MD, and to orient molecular diagnosis for selected genotypes.

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.

Impaired VEP after photostress response in multiple sclerosis patients previously affected by optic neuritis

The aim of our work was to evaluate if an optic nerve involvement (multiple sclerosis patients previously affected by optic neuritis) may induce any change in visual evoked potential (VEP) after photostress response. VEP in basal conditions and after photostress were assessed in 10 patients with defined multiple sclerosis without a history of optic neuritis (MSWO); in 14 patients with defined multiple sclerosis previously affected by optic neuritis but with complete recovery of the visual acuity (MSON) and in 14 age-matched controls. In order to complete the investigation of the retinal function, Transient Pattern electroretinogram (PERG) and steady-state focal-ERG (counterphased gratings presented at 8 Hz in the macular region) were performed in MSON patients only. In MSWO eyes VEP parameters in basal condition and after photostress did not undergo significant changes compared to controls (ANOVA; P > 0.05). In MSON eyes we observed basal VEP with delayed P100 peak latency and reduced N75-P100 amplitude when compared with the control ones (P < 0.01). In MSON eyes the parameters of VEP after photostress underwent large changes and longer recovery time (RT) than in control and MSWO eyes (P < 0.01). In addition; in MSON eyes we found increased transient PERG P50 latency (P < 0.01) and reduced P50-N95 amplitude (P < 0.01); Focal-ERG (that displays a major component at 16 Hz; 2nd harmonic:2P) with reduced 2P amplitudes and delayed 2P phases (P < 0.01). Our results indicate that patients previously affected by optic neuritis present an abnormal VEP after photostress response and this may be ascribed predominantly to an involvement of the inner retinal layers as indicated by the concomitant impairment of PERG and focal-ERG responses.

The effect of stimulus size on human cortical potentials evoked by chromatic patterns

The effect of stimulus size on the pattern onset-offset visual evoked potential elicited with stimuli of two different wavelengths is studied under intensive yellow adaptation: (1) The onset response obtained with a 460 nm pattern is of negative polarity (N1) and saturates in amplitude with a stimulus radius of 7 deg. The onset response obtained with a 550 nm pattern is of positive polarity and continues to increase up to the maximum size (32.2 deg). (2) The peak time of N1 (460 nm) decreases with increasing stimulus size, that of P1 (550 nm) remains constant. These results are discussed as reflecting either varying retinal and brain anatomy, or cone activity, color-opponent activity, or luminance contrast activity.

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.

The focal cone electroretinogram

The focal cone electroretinogram (ERG) in monkey retina has been examined with a 3 deg pulse of laser light (544 and 633 nm) centered on a 25 deg steady white rod saturating field. The stimuli were viewed simultaneously through a slit lamp and corneal contact lens. Cone ERGs were studied at different eccentricities from the fovea and compared with full-field corneal and intraretinal ERGs. The cone ERG is maximum at the fovea. There are two components to the on- (b-wave) and off- (d-wave) response, one slower, more long wavelength sensitive and more foveally oriented than the faster response. This makes the foveal cone ERG slower and more longer wavelength sensitive than the perifoveal ERG. This difference disappears at high rates (> 20 Hz) of stimulation. The foveal cone ERG is larger and slower than that of more peripheral retina. The slowness appears to be due to a subcomponent of the response which is especially prominent in the fovea and has a slightly greater long wavelength sensitivity than the more peripherally generated ERG. It may depend on a unique difference in L-M cone bipolar systems or in L-M cone interactions that are more prominent near the fovea.

Flash and pattern electroretinogram changes with optic atrophy and glaucoma

We investigated recent reports that, contrary to common belief, glaucoma can affect flash as well as pattern electroretinograms. An extensive flash and pattern electroretinogram test protocol was used in a large sample of glaucoma patients and age-matched controls who were either visually normal or had other optic nerve diseases. All electroretinogram parameters were reduced and delayed in normal people > 55 years of age. The effect did not increase in later decades. In patients aged < or = 55 years, flash electroretinograms showed mild reductions and delays from optic atrophy alone. Glaucomatous ERG changes were larger and increased with disease severity. Pattern electroretinograms and oscillatory potentials were almost equally reduced in optic atrophy and all degrees of glaucoma. Mildly affected patients > 55 years of age had similar electroretinogram change to age-matched normals in most conditions. Advanced glaucoma patients showed similar differences from normal irrespective of age. This suggests that direct diagnostic application of these results to older patients will be difficult, that the ERG changes in glaucoma cannot be attributed simply to optic atrophy and that additional widespread outer retinal damage occurs in glaucoma.

The VEP thresholds for full-field stimuli in dark-adapted infants

Dark-adapted adults' electroretinographic b-wave thresholds are approximately 2 log units below a-wave thresholds and approximately 3 log units above the perceptual threshold, and their perceptual and the visually evoked cortical potential (VEP) thresholds are similar. Dark-adapted infants' scotopic a- and b-wave thresholds for full-field stimuli are both about 0.5 log units above those of adults, but their scotopic VEP thresholds for such stimuli have not been studied. We obtained scotopic VEP thresholds for brief, full-field stimuli from dark-adapted, infants and adults to consider the relationships of the cortical responses to the responses of more distal structures, namely the rod photoreceptors (scotopic a-wave) and ON-bipolars (scotopic b-wave). The median VEP threshold of infants is 0.5 log unit above that of adults. Thus, the relationships of a- and b-wave and VEP thresholds in infants are similar to those in adults. These results are consistent with rod cell immaturities being the primary determinant of the difference between infants' and adults' thresholds.

The first and second harmonics of the macular flicker electroretinogram: differential effects of retinal diseases

We evaluated the effects of retinal diseases on the macular electroretinogram first and second harmonic components, which are dominated by outer and inner retinal activity, respectively. Macular electroretinograms in response to a uniform field (9 degrees x 9 degrees) flickering sinusoidally at either 32 or 8 Hz (peak frequencies of the first and second harmonics, respectively) were recorded in 14 patients with maculopathies involving photoreceptors (e.g., age-related macular degeneration), in 16 patients with postreceptoral macular diseases (e.g., branch occlusion of central retinal artery), and in 38 normal controls. Amplitude and phase of the first and second harmonic response components were evaluated by Fourier analysis. When compared to controls, patients with photoreceptor diseases had reduction in both first and second harmonic mean amplitudes and second harmonic phase delay; patients with postreceptoral diseases had normal first harmonic components but reduced and delayed second harmonic components. A discriminant analysis, by using first and second harmonic values, correctly classified 13 of 14 patients with photoreceptor diseases and 14 of 16 patients with postreceptoral disorders. These results indicate that combined evaluation of the macular electroretinogram first and second harmonic components is a useful test for identifying the site(s) of retinal dysfunction in patients with macular diseases.

Early selective neuroretinal disorder in prepubertal type 1 (insulin-dependent) diabetic children without microvascular abnormalities

The duration of diabetes before puberty is not considered relevant to the future development of complications. To evaluate the effects of diabetes on the neural retina, we analysed macular function by steady-state focal electroretinography in 20 prepubescent diabetic children without vascular retinopathy and in 39 sex- and age-matched normal children. The mean (+/- SD) response related to retinal cellular elements between the photoreceptors and ganglion cells was significantly lower in diabetic children than in the control group (0.38 +/- 0.12 vs. 0.51 +/- 0.13 microV; unpaired t-test = 3; P = 0.005). Similarly, ganglion cell function showed a significant impairment in diabetic children with respect to the control group (0.4 +/- 0.13 vs. 0.53 +/- 0.09 microV; unpaired t-test = 5.4; P = 0.0001), whereas the photoreceptors appeared unaffected. Metabolic control and disease duration were not correlated with functional deficits. Our results suggest that before puberty, early diabetes may have a selective effect on the neural retina prior to the appearance of microvascular changes. A focal electroretinogram could identify diabetic children with neurosensory disorders who may have a higher risk of developing microvascular retinopathy.

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.

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.

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

The photopic, focal electroretinogram elicited by sinusoidal modulation (8 Hz) of the luminance of a uniform field (flicker electroretinogram) presented in the macular region contains two main components: the first harmonic at the stimulus frequency (1F) and the second harmonic at twice the stimulus frequency (2F). Physiologic and clinical studies have demonstrated that the 1F originates in the outer retina, whereas the 2F has multiple postreceptoral sources, with a contribution of the innermost retina. The 2F, unlike 1F, is selectively altered in amplitude and phase in the early stages of optic neuritis and glaucoma. Both 1F and 2F are altered in maculopathies. An additional property of the focal electroretinogram is that the 2F, unlike 1F, has a very limited summation area (i.e., the function relating 2F amplitude with stimulus area is saturated for central stimulus sizes of the order of the macular region). This overrepresentation of the macular activity suggests that the 2F component of the flicker electroretinogram may provide a specific macular test even for full-field stimuli presented through opacities of the optical media.

The field topography of ERG components in man--I. The photopic luminance response

A technique of multi-input systems analysis is used to explore the field topography of ERG responses to local luminance modulation. Variations in amplitude and wave form are studied within the central 23 degrees. Outside the fovea, the amplitude appears to follow a simple power law rx as a function of eccentricity r where x is approximately -2/3. The largest inter-subject variability is found in the fovea. Some nasal-temporal asymmetry is observed in all subjects with higher response densities in the temporal field outside the blind spot. The topography of the luminance response shares all these properties with the density of retinal cones.

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).

Macular dysfunction in multiple sclerosis revealed by steady-state flicker and pattern ERGs

Recent evidence indicates that the 2nd harmonics of steady-state (8 Hz) electroretinograms to either sinusoidal flicker (FERG) or to counterphased gratings (PERG) presented in the macular region (9 degrees) represent different subsets of generators in the inner retina. We evaluated the steady-state macular FERG and PERG 2nd harmonics (2F and 2P, respectively) in 19 normal subjects (19 eyes) and in 23 multiple sclerosis patients (44 eyes; 25 eyes with a history of clinical optic neuritis, and 19 eyes with no history of optic neuritis, subclinical eyes). The mean 2F and 2P amplitudes were significantly reduced, as compared to controls, in both subclinical and optic neuritis eyes. The 2P phase was significantly delayed, as compared to controls, in subclinical eyes, whereas 2F phase was delayed in eyes with optic neuritis. 2F was outside the 95% confidence limits (in amplitude or phase) in 11/19 subclinical eyes and in 25/25 optic neuritis eyes. 2P was outside the normal range in 12/19 subclinical eyes and in 24/25 optic neuritis eyes. These results show that FERG and PERG 2nd harmonics are significantly altered in multiple sclerosis eyes with or without a clinical history of optic neuritis. This finding suggests a dysfunction of inner macular layers which may result from direct retinal involvement or retrograde degeneration.

Macular electroretinograms to flicker and pattern stimulation in lamellar macular holes

Steady-state macular (9 degrees x 9 degrees) electroretinograms in response to either sinusoidal flicker (focal electroretinogram) or counterphased sinusoidal gratings (pattern electroretinogram) were recorded in 14 patients with inner lamellar macular holes, in 4 patients with full-thickness macular holes and in 14 age-matched controls. Fourier analysis of focal and pattern electroretinograms yielded three main components: a first and a second harmonic to flicker, and a second harmonic to pattern. Recent evidence indicates that the first harmonic to flicker is of receptoral origin, whereas the flicker and pattern second harmonics represent, at least in part, the activity of different generators in the inner retina. When compared to controls, patients with inner lamellar holes showed significant amplitude reduction and phase delay for both flicker and pattern second harmonics, but not for the flicker first harmonic. Patients with full-thickness holes showed significant amplitude reduction also for the flicker first harmonic. These results indicate a prevalent functional involvement of the inner retina in lamellar macular holes, which can be clinically detected by evaluating focal and pattern electroretinogram second harmonics.

Analysis of the electroretinogram in toxoplasma retinochorioiditis

The decision to use therapy in toxoplasma retinochorioiditis depends on the location of the active lesion and the presence of vitreous activity. In eyes with dense vitreous clouding it can be difficult to see whether the macular region is involved or not. In theory the localisation of a lesion can be estimated on the basis of the flash ERG. The standard flash electroretinogram was recorded in 23 patients with inactive toxoplasma retinochorioiditis lesions in the retina. In 17 cases a lesion was present within the central 12 degrees of the visual field, 8 of these had a reduced photopic ERG. In 15 patients lesions were found outside the central 12 degrees, in 8 of whom the scotopic ERG was reduced. We conclude that the ERG can be of use in indicating the scar location in patients with dense vitreous clouding.

Selective reduction of oscillatory potentials and pattern electroretinograms after retinal ganglion cell damage by disease in humans or by kainic acid toxicity in cats

We recorded pattern electroretinograms, scotopic threshold responses, oscillatory potentials and ganzfeld flash electroretinograms in patients with glaucoma or other optic nerve diseases and in cats with inner retinal damage caused by intravitreal injections of kainic acid. In both studies, the scotopic b-wave and the scotopic threshold responses were normal but the oscillatory potentials and pattern electroretinograms were not. The photopic b-wave was also often reduced in patients with scotopic oscillatory potential reduction, and the reduction was proportionate to the oscillatory potential change. Oscillatory potentials were as frequently reduced as pattern electroretinograms in both patient groups, and in the few cases where only one response was reduced, there was no bias toward either measure. In cats, the effects of intravitreal injection of various doses of kainic acid on the retina were evaluated electrophysiologically, and structural damage was assessed histologically. After 25 nmol of kainic acid, the pattern electroretinograms and oscillatory potentials were reduced but neither the b-waves nor the scotopic threshold responses, were affected. Histologic studies of retinas after this dose showed swollen dendrites that were restricted to the outer part (off-sublamina) of the inner plexiform layer. Serial semithin sections indicated that most, if not all, of the swelling was confined to dendrites of large ganglion cells. Our results indicate that the size and sensitivity of the oscillatory potential response may have a role in the diagnosis and management of early glaucoma and optic nerve disease, and that the photopic electroretinogram may give similar information.

Local ERG for clinical examination of eye diseases

We used a suction electrode with a frosted front window to record a full-field (general) ERG, and a suction electrode with a short-focus lens and a red-green light diode to record a local ERG. As the suction electrode moved with the eye, the stimulus struck the macula in all positions of gaze. Our data showed that only mildly intense stimuli with an angular size of 10 to 15 degrees induced a retinal response, with a macular contribution of 70%. Patients with various dystrophic changes of the retina, such as Stargardt's dystrophy, vitelliform dystrophy of the Best type, X-linked juvenile retinoschisis, and age-related macular dystrophy of different stages, were examined. In patients with retinitis pigmentosa, the cones were involved in the pathologic process to varying degrees; the behavior of the local ERGs helped determine the final visual acuity. We also observed a reduction in the local ERG in patients with Stargardt's dystrophy that disappeared as the disease progressed. In age-related macular dystrophy, a disturbance of the electrogenesis mainly occurred in the central retina in advanced stages of the disease.

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.

The human focal electroretinogram as a function of stimulus area

Focal electroretinograms to on-off luminance modulation (2.7 Hz) of uniform fields with a different area (144-2.25 deg2) centered on the fovea were recorded in two young normal subjects. The electroretinogram responses typically consisted of an a-wave, a b-wave, a PIII component and a d-wave. The concentric reduction of the test field size caused progressive amplitude decrement of all components, while their response density increased. The spatial summation properties of the b-wave differed from those of the a-wave, PIII component and d-wave so that the relative contribution of the b-wave to the electroretinogram waveform depended on the stimulus area. For a critically small field size, the electroretinogram was dominated by the PIII and d-wave components. Only these two electroretinogram components displayed a density profile that could be compared with that of cone distribution.

The effect of various anaesthetics on the spatial tuning of two major wave peaks in the transient pattern electroretinogram of the cat: evidence for pattern and luminance components

The main PERG component of the transient contrast reversal pattern electroretinogram (PERG) in cats was a negative wave (3.5 microV average, SD 1.7 microV) peaking at about 130 msec (N130) with a spatial resolution above 5.5 c/deg, close to behavioural estimates. The early positivity (P35) was more variable, smaller and had lower spatial resolution. Different anaesthetic protocols affected both the waveform and the amplitude by spatial frequency functions. Responses of urethane anaesthetised cats were like those reported previously for decerebrate cats or cats paralysed and ventilated with N2O/O2/CO2 (75%/24%/1%). P35 was evoked only by coarse stimuli and N130 amplitude decreased linearly as spatial frequency increased. When the luminance response amplitude, predicted from the optical transfer function of the eye, was subtracted, spatial tuning appeared. An anaesthetic mixture of ketamine hydrochloride and xylazine depressed both P35 and N130 at low spatial frequencies while enhancing them at high frequencies. In paralysed animals ventilated with N2O/O2 (67%/33%) P35 was larger and recordable to 1.6 c/deg. Peak times were reduced and the inter-peak time halved. Other anaesthetics depressed the ERGs. These effects suggest that cats are a good model for studying N130 in isolation or its interaction with P35 and that both PERG peaks include luminance and pattern components.

The clinical utility of the foveal electroretinogram: a review

The historical development of the foveal electroretinogram is reviewed. The factors required to obtain a local cone electroretinogram with a small focal source of illumination against a lighted background are discussed. From these considerations and various studies of macular degeneration, the optimal size of the foveal source is found to be 5 degrees diameter or less. Applications of the foveal electroretinogram in various diseases are summarized. The foveal electroretinogram is abnormal with reduced acuity in age-related macular degeneration, juvenile macular degeneration (Stargardt's disease), retinitis pigmentosa, and diabetic retinopathy. Normal foveal electroretinograms are found in optic atrophy, amblyopia, and optic neuritis. Foveal electroretinograms are normal or abnormal in Best's disease, central serous retinopathy, and several other diseases discussed.

Simultaneous foveal and parafoveal electroretinograms in hereditary degeneration of the central retina

Simultaneous foveal and parafoveal electroretinograms (ERG) in response to two identical checks (6 degrees per side) alternating at constant mean luminance were recorded in 26 patients (52 eyes) affected by central hereditary chorioretinal diseases and in 14 age-matched normal subjects (14 eyes). Patients were divided into four groups according to clinical diagnoses: 1. Stargardt's disease; 2. cone dystrophy; 3. vitelliform degeneration; 4. pattern dystrophy. The amplitude and latency of the foveal ERG and the amplitude ratio between foveal and parafoveal ERG (F:PF ratio) were measured. The mean foveal ERG amplitude was significantly lower than the control mean in all patient groups. The foveal ERG latency showed a trend to a increase in all pathological groups. However, this difference was not statistically significant. The mean value of F:PF ratio was significantly reduced as compared with the control mean in Stargardt's disease and cone dystrophy only. In 46 of 52 affected eyes (88.5%) at least one of the electrophysiological parameters was abnormal. Our results suggest that the simultaneous foveal and parafoveal ERG recording may be a sensitive technique in hereditary degenerations of the central retina. This method may also contribute to a better understanding of cone degeneration pathophysiology.

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.

Computation of the luminance and pattern components of the bar pattern electroretinogram

Pattern onset electroretinograms (PERGs) were recorded from four normal subjects. Square-wave gratings of 75% contrast were presented in three approximately contiguous, concentric zones of outer angular radius, 5.1 degrees, 12.6 degrees, and 23.6 degrees. The zones were calculated to give equal numbers of ganglion cell receptive fields. The recorded PERGs were considered to include luminance and pattern components which have low and bandpass spatial tuning functions respectively. These components combine in the PERG to produce a broad spatial tuning characteristic. The amplitude of PERGs in response to low spatial frequency stimuli is widely reported to be linearly related to contrast. The retinal illuminance response at every spatial frequency was computed from the eye's modulation transfer function. This function characterizes the reduction in contrast that occurs because of optical degradation. The computed retinal illuminance response was subtracted from the PERG waveform and a pattern-specific response was revealed. The latter had a highly tuned bandpass function which peaked at higher spatial frequencies than the PERG at corresponding peripheral angles.

Pharmacological dissociation of the b-wave and pattern electroretinogram

Electroretinographic responses to modulation of either luminance (focal ERG) or spatial contrast (pattern ERG) were recorded from the pigeon eye before and after intravitreal injection of glutamate analogues DL alpha amino adipic acid (DL alpha AA) and 2-amino-4 phosphonobutyric acid (APB). Both toxins reversibly abolished the b-wave. The pattern ERG was still present, however, when the b-wave had been abolished by the toxins. This result demonstrates that the b-wave and the pattern ERG can be pharmacologically dissociated and suggests the possibility that in pigeons the b-wave and pattern ERG reflect the activity of different generators.

Simultaneous macular and paramacular ERGs recorded by standard techniques

A technique for the simultaneous recording of two small-field electroretinograms (ERGs), macular and paramacular, employing standard apparatuses for stimulation and analysis is described. The stimuli consisted of two adjacent checks (6 deg/side) obtained by masking the display of a commercially available TV pattern stimulator. The checks were square-wave-modulated in counterphase at 3.12 Hz. The subjects fixated the center of one of two checks. In one stimulus cycle, two ERGs could be distinguished: one from the macular area and the other from the adjacent area. The macular ERG is about twice as large as the paramacular. ERGs recorded by this technique can be considered focal since (i) they show a sharp fall-off in amplitude when the stimulus is displaced from the fovea, and (ii) they are no longer recordable when the stimulus is centered on the optic disk or on a large macular scar.

Normal strobe electroretinograms without pattern electroretinograms in albino rats

Electroretinograms (ERGs) were obtained from pigmented and albino rats to step luminance changes of an unpatterned TV screen. Surround luminance was increased until the ERG became small and focal. In pigmented rats the ERG at on was positive, earlier, and about twice the amplitude of the negative ERG at off. All pigmented rats had pattern ERGs-0.5 cycles/deg in dark agouti rats and an octave less in hooded rats. Implicit peak times were similar to that of the sum of on plus off focal ERGs from the same animals (85 ms). In albino rats off responses were more like on. The resultant sum was consequently small. Both peak times were similar and did not move earlier than 120 ms as surround luminance increased. Pattern ERGs could not be recorded from albinos at any spatial frequency or surround luminance. These pigmented rat ERGs seem to have two major components. One follows luminance linearly; the other is a fully rectified nonlinearity with about one-third the amplitude. The albino rat retina apparently lacks the latter component. These deficiencies may occur in albinos of other species and be associated with their visual system abnormalities.

The differential effect of optic nerve disease on pattern and focal electroretinograms

We studied nine cases of retrobulbar neuritis with confirmed multiple sclerosis and six cases of optic atrophy from other causes. Pattern and focal electroretinograms (macular ERGs) were recorded with high (400 cd/m2) and low (40 cd/m2) intensity stimuli. Contrast sensitivity was also measured with a simple printed test. Luminance was not markedly important. High spatial frequency contrast sensitivity was significantly correlated with pattern ERG amplitude. Pattern and focal ERG amplitude ratio was usually reduced, but the effect was not correlated with contrast sensitivity or large enough to be useful clinically. In optic atrophy the pattern ERG (PERG) was clearly more severely reduced than the focal ERG (FERG). In retrobulbar neuritis both ERGs were equally and more severely reduced even though the visual losses were less. In unilateral cases the PERG increased then decreased after the initial attack, as previously described (Arden et al., 1982). The results suggest that retinal layers beyond the ganglion cells may be affected in retrobulbar neuritis, but proximally generated, pattern-specific ERG components are selectively lost in optic atrophy.

Spatial characteristics of the oscillatory potentials of the electroretinogram

The spatial properties of the trans- and intra-retinal oscillatory potentials (OPs), a- and b-waves of the mudpuppy electroretinogram (ERG) were analyzed. A comparison of the intra-retinal potentials was made with the proximal negative response (PNR), an extracellular response, predominantly from amacrine cells. The spatial characteristics of the OPs differed from the a- and b-waves and the PNR. The OPs integrated spatially up to full field illumination. They seem to represent summated neuronal activity over broader areas than that of the PNR and the a- and b-waves. The results indicate that the OPs reflect activity of second order neurons to which visual information converge from the whole retina. These neurons may be the bipolar cells. No evidence that the OPs orginate in the laterally extending amacrines was obtained. The b-wave data further suggested a centre-surround organization for the neurons indirectly involved in the b-wave generation.

Macular electroretinograms and contrast sensitivity as sensitive detectors of early maculopathy

Eighteen patients with early maculopathies of various etiologies were tested with pattern and focal electroretinograms (macular ERGs), with high (400 cd/M2) and moderate (40 cd/M2) stimulus intensities and a four-alternative forced choice (4AFC) contrast sensitivity test in addition to intensive clinical examinations. High spatial frequency contrast sensitivity loss on the 4AFC test was the most striking and consistent feature of all cases. The only eyes not outside normal contrast sensitivity limits were three in which diagnosis was uncertain and the patients had not recognized any problem, including two marginal solar burns. Maculopathy also substantially reduced macular ERG amplitudes. Criterion scores on these tests separated patients from normals more effectively than other noninvasive procedures and only missed one eye detected by contrast sensitivity. Latencies were affected but the delays were of no clinical significance in the individual case. Stimulus intensity was not critical. The results indicate that contrast sensitivity testing and macular ERGs are very reliable indices of central visual dysfunction at a stage when visible macular changes are too subtle for confident diagnosis. Contrast sensitivity has appeal because of its reliability, objectivity, simplicity, and noninvasive nature. It is equally applicable to children and adults. Pattern and focal ERGs can establish that the visual deficit has a retinal origin and can provide the most reliable objective confirmation.

Factors influencing threshold of the fundamental electrical response to sinusoidal excitation of human photoreceptors

The amplitude and phase of the fundamental Fourier component of the electroretinogram (e.r.g.) in response to sinusoidally modulated light were studied in the range 7-50 Hz. Sensitivity was best at the lowest frequency. The threshold-frequency relationship divided into two parts. A weak steady background depressed sensitivity of the low, but increased sensitivity of the high, frequency component. At 8 Hz a small test spot was 0.7 log10 units more effective on the most sensitive part of the retina than on the optic disk. On the fovea, it was 0.1-0.2 log10 units less effective than on the disk. The fovea was 0.7 log10 units more sensitive to 25 Hz than the blind spot. Psychophysical and e.r.g. dark-adaptation curves were similar, but the former was 10(4) times more sensitive than the latter. Four sets of experiments examined the possibility that the Fourier component of the e.r.g. response at the modulation frequency of 8 Hz during the 'rod' phase of the e.r.g. dark-adaptation curve arose from excitation of rods alone. The only hint of a possible cone contribution was a very small but systematic increase in phase delay with increase in background wave number found while measuring the field sensitivity action spectrum. No suggestion was found that the fundamental Fourier component of threshold e.r.g. responses at the modulation frequency of 25 Hz was influenced by photons absorbed in rods.

Macular electroretinograms: their accuracy, specificity and implementation for clinical use

The pattern and the focal electroretinogram (ERG) are both non-invasive, electrophysiological responses recorded from circumscribed retinal areas and are most easily recorded from the macula. This paper describes how our department has incorporated these tests into our clinical protocol, shows how the recording technique and the method of electrode construction may be improved, and describes the normal limits of the macular responses we obtain. The ERG signal-noise ratio we obtained was better than that of the binocular visual evoked potentials (VEPs) recorded simultaneously. Pattern and focal ERGs, using improved methods of recording, show promise of being a valuable addition to the clinical investigation of subtle maculopathies and some forms of optic nerve dysfunction. Three illustrative cases are described. The first demonstrates normal macular ERG responses with abnormal Ganzfeld ERGs due to peripheral retinal damage. The second reveals differential pattern ERG reduction with normal focal ERG in recent optic neuritis. The third case demonstrates reversible simultaneous loss of Ganzfeld ERGs and macular ERGs in vitamin A deficiency.

Pattern ERG of the cat

Recent reports on pattern ERGs reopened the discussion of pattern or luminance origin of the ERG to pattern reversal stimulation. In this report cat ERGs to checkerboard stimuli were recorded. The stimulus parameters were manipulated in order to vary the strength of local luminance and contrast stimulation independently. Special care was taken to eliminate stray light effects. The results clearly show that pattern specific components in the cat ERG, if present at all, are no greater than the variance of the responses from local luminance nonlinearities. Thus a "Pattern ERG" can be recorded by luminance modulation.

Comparison of the focal electroretinogram and the pattern electroretinogram in the pigeon

A comparison has been made, under focal conditions, of the electroretinogram (e.r.g.) and the pattern electroretinogram (p.e.r.g.) of the central yellow field of the pigeon retina, using vitreal and intraretinal recordings. The sum of the on and off e.r.g. is of shorter time-to-peak and of smaller amplitude than the p.e.r.g. elicited by a grating of 1 cycle/degree, as predicted by the spatial tuning of the p.e.r.g. Depth profiles were made to compare with p.e.r.g. and the sum of the on and off e.r.g. The two responses co-vary in terms of amplitude, time-to-peak and form. This suggests that the p.e.r.g. has a conventional electroretinographic location.

Electroretinograms evoked in man by local uniform or patterned stimulation

1. We have recorded electroretinograms (e.r.g.s) in normal subjects. Television monitors were used as stimulators. The screens were surrounded by brightly lit white reflecting surfaces to ensure that the responses were developed by defined retinal areas.2. Various types of stimuli were employed. Either (i) a pattern of dark and bright squares was reversed, to evoke a pattern e.r.g. (p.e.r.g.), (ii) the luminance of the uniform screen was abruptly increased and decreased to evoke a focal on-off e.r.g. or (iii) a pattern was made to appear and disappear from a uniform background. In each of these cases, the sequence of changes of luminance at any one point could be made identical. The aim of the experiments was to determine whether the e.r.g. was modified by the spatial organization of the stimulus.3. In other experiments a colour monitor was used so that (i) a red-green flicker, (ii) red-green pattern reversal or (iii) the appearance of a red-green pattern from a yellow background could be used as a stimulus. The responses were caused by the changes in hue, since all the colours were equiluminant.4. With black and white patterns the p.e.r.g. peaks 5 msec later than the focal on-off e.r.g. The largest response is produced by squares of 0.5-1 degrees subtense.5. The ratio of the amplitudes of the p.e.r.g. to the focal on-off response is largest for stimuli confined to the macula and smallest for those projected onto peripheral retina.6. The amplitude of responses to chequerboard reversing patterns increases nearly linearly with contrast up to the maximum contrast available.7. When patterns appear or disappear from a uniform screen, and there is an associated change in the quantity of light entering the eye, recognizable b-waves occur when the average screen luminance increases, independently of whether pattern contrast increases (appearance) or decreases (disappearance).8. When a pattern appears or disappears with no change in luminance, e.r.g.s are evoked at both ;on' and ;off'. The disappearance of the dark parts of the pattern causes the largest logarithmic increase in local retinal illumination. For patterns of square size > 4 degrees the pattern disappearance response is larger than for pattern appearance. As the square size is reduced, the appearance response grows and the disappearance response decreases. The e.r.g.s evoked by the appropriate changes in luminance of a uniform screen are no longer the same as those caused by the appearance and disappearance of the pattern.9. The responses to change of hue are 70% as large as those produced by black and white patterns. The same ratio occurs for pattern and focal on-off e.r.g.s.10. When coloured patterns appear from and disappear to a uniform field, the e.r.g.s. evoked are very similar to those recorded when the appropriate changes of hue occur in a uniform field. This result is quite different to the findings for black and white patterns (see 8 above).11. The results suggest that it is the change in local adaptation caused by the black and white patterns which modifies the e.r.g. and not the presence of contrasting borders.

Pattern electroretinograms and visual-evoked potentials in glaucoma and multiple sclerosis

Steady-state visual-evoked potentials and electroretinograms were simultaneously recorded in four patients with glaucoma and in five patients with multiple sclerosis. The stimuli included a homogenous field and a 2.3 cycles per degree sinusoidal grating that were counter-phase modulated at the rate of 7.5 Hz. We used narrow bandwidth spectral analysis to measure the response amplitudes and signal-to-noise ratios. Transient pattern visual-evoked potentials (1 Hz) were also measured for latency in each eye. We found abnormal pattern electroretinograms, based on the absence of a significant second harmonic component, in three of the four glaucomatous eyes although the homogenous field electroretinograms were normal. In the patients with multiple sclerosis, the pattern electroretinograms were abnormal in two eyes, but the transient visual-evoked potential latency had the highest diagnostic yield (seven of ten eyes).

Vitreal and intraretinal responses to contrast reversing patterns in the pigeon eye

The pattern electroretinogram (PERG) has been recorded vitreally and intraretinally in the pigeon eye. The amplitude of the PERG increases monotonically as pattern contrast is increased, with saturation at high levels. The PERG of the central yellow field has band-pass spatial tuning, with a high frequency cut-off at 8 c/deg. Time-to-peak is shortest at low spatial frequencies. Both PERG and local b-wave are small and positive-going close to the retinal surface, and large and negative-going in the inner nuclear layer. The PERG and b-wave show qualitatively similar depth profiles.

Clinical and experimental evidence that the pattern electroretinogram (PERG) is generated in more proximal retinal layers than the focal electroretinogram (FERG)

A TV monitor was used to evoke either a pattern ERG to a contrast-reversing checkerboard (PERG), or a focal ERG to alternate increases and decreases of luminance of the blank screen within a bright surround (FERG). Both responses are small (approx 2 microV) and fast (approx 50 msec to peak) and are similar in several other properties. However, they differ in timing and respond differently to changes in contrast. Each frame of a TV picture evokes a "raster ERG," even though the screen is blank. The response is focal and specific to a small central strip of the screen. It is simpler to record than the FERG, where the whole screen is flashing. Because the FERG summation area is about 4 deg, small squares (checks) reversing in contrast produce little luminance response. In 5 of 7 cases where the PERG is unilaterally reduced, the FERGs or raster responses were not affected. Thus clinical evidence also suggests that the PERG may be a separate phenomenon to the FERG and produced at a different site. Toxic, traumatic, congenital, and degenerative diseases of the optic nerve reduce the PERG. The comparison is most easily made in unilateral disease. Ten weeks after an optic nerve insult, the PERG becomes reduced in the affected eye as if retrograde degeneration was occurring. In 27 amblyopes of various types, the PERG was reduced in 23 where orthoptic treatment had failed. In 4 patients responding to treatment, PERGs of the amblyopic eye were as large as, or larger than, those of the fellow eye. The loss is greater with smaller checks. Retinal changes do occur after age 4 but so slowly that responses in heavily occluded eyes are not reduced. An additional level in the visual pathway is thus accessible to evoked potential investigation.