The pattern electroretinogram (PERG) is the response of central retina and is usually measured using a reversing black and white checkerboard. It is important that there is no luminance change during pattern reversal. It allows both a measure of central retinal function and, in relation to its origins, an evaluation of retinal ganglion cell function. It is thus of great value in the electrophysiological differentiation between macular dysfunction and generalised retinal dysfunction or optic nerve dysfunction in the older child with visual acuity loss (see  for a comprehensive review). It is important to preserve the optics of the eye for PERG recording, which requires non-contact lens electrodes in contact with the cornea or bulbar conjunctiva to preserve the optics of the eye, and no mydriasis. The gold foil, the DTL and the H-K loop electrode are all suitable. Ipsilateral outer canthus reference electrodes are mandatory; there is contamination from the cortically generated VEP if forehead or ear "reference" electrodes are used . In young children incapable of tolerating corneal electrodes, surface electrodes may be used . The signal:noise ratio is inevitably lower compared with corneal recordings and responses are particularly sensitive to alterations in fixation. Continuous monitoring and "interrupted averaging" [40, 72] during lapses in fixation or to allow blinking is essential in order to optimise the quality of recordings. Surface PERGs may be elicited using large stimulus fields; the use of such stimuli may be beneficial in terms of signal:noise ratio and by encouraging better fixation, but require cautious interpretation and comparison with corresponding normal values. In general, the presence of any PERG excludes severe macular dysfunction.
There are two main components in the so-called transient PERG: P50 at approximately 50 ms and a larger N95at 95 ms  (Fig. 9.1). Assessment concentrates on the amplitude of P50, measured from the trough of the small early negative N35 component; the latency of P50 measured to peak; and the amplitude of N95, measured to trough from the peak of P50. The N95 is a contrast-related component generated in the retinal ganglion cells. Approximately 70% of P50 appears to be ganglion cell-related, but the remainder is not related to spiking cell function and may be generated more distally in the retina . The exact origins have yet to be ascertained. Although the PERG is generated in inner retina, the P50 component is "driven" by the macular photoreceptors and thus reflects macular function.
For optimal recording of the PERG, an analysis time of 150 ms or greater is usually used. It is a low-amplitude signal and computerised averaging is essential. The necessary stringent technical controls are important and are fully discussed elsewhere . Binocular stimulation and recording is usually preferred so the better eye can maintain fixation and accommodation, but if there is a history of squint it is necessary to use monocular recording. P50 is sensitive to optical blur, and accurate refraction is important. PERG amplitude is related almost linearly to stimulus contrast at low stimulus frequencies. ISCEV recommends a high-contrast black and white reversing checkerboard with approximately 50-min checks in a 10- to 16-degree field.
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