Uniocular recording of pattern ERG

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.

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.

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.

Electrophysiological discrimination between retinal and optic nerve disorders

In this study we show how the Pattern ERG (PERG) can be used to distinguish between optic nerve and retinal disease. Records from eyes with RBN and delayed visual evoked responses are compared with those recorded from the normal fellow eyes. In optic nerve disease there is a selective reduction of the later negative component of the PERG. PERGs were also recorded from patients with mild diabetic retinopathy. These were divided into three groups on the basis of the delay in their Visual Evoked Potential (VEP). The amplitude of both positive and negative components of the PERG fall with increasing severity of the disease thus showing that the abnormally long delay in VEP found in some cases is due to retinal disease rather than optic nerve disease.

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.

Pattern electroretinogram: effects of reference electrode position

The pattern electroretinogram has assumed greater clinical and experimental significance because of its inner retinal origins. However, clinical tests may be confounded by an artifact. We tested subjects varying reference electrode position and eye stimulated while employing the Dawson-Trick-Litzkow (DTL) fiber electrode as the active electrode. The presence of a statistically significant artifactual response could not be confirmed. However, the variability of responses elicited with the outer canthus was less and the signal-to-noise ratio greater than with other reference positions.

Check size and defocusing effects on equipotential maps of pattern-evoked electroretinograms

The spatial distribution of the pattern-evoked potentials on the face was studied with a simultaneous 16-channel recording system. The potentials had the first positive component at a peak latency of approximately 55 msec, which was much earlier than the latency of the response picked up from the scalp. The maximal potential value in the maps was found at the region around the stimulated eye. This configuration of the equipotential map was not influenced by defocusing and check size of the pattern, but was altered by changing the contrast. Our results of equipotential maps constructed at around 55 msec after pattern reversal proved that the responses recorded in the facial region originated in the retina and were not a reflection of the scalp potentials.