Dissecting the dark-adapted electroretinogram

Although gross recordings of the ganzfeld flash-evoked electroretinogram (E RG) can potentially provide information about the activity of many, if not all, retinal cell types, it is necessary to dissect the ERG into its compon ents to realize this potential fully. Here we describe various procedures t hat have been used in intact mammalian eyes to identify and characterize th e contributions to the dark-adapted ERG of different cells in the retinal r od pathway. These include (1) examination of the very early part of the res ponse to a flash (believed to reflect directly the photocurrent of rods), ( 2) application of high-energy probe flashes to provide information about th e underlying rod photoreceptor response even when this component is obscure d by the responses of other cells, (3) pharmacological suppression of respo nses of amacrine and ganglion cells to identify the contribution of these c ells and to reveal the weaker responses of bipolar cells, (4) use of pharma cological agents that block transmission of signals from rods to more proxi mal neurons to separate responses of rods from those of later neurons, (5) examination of the ERG changes produced by ganglion-cell degeneration or ph armacological block of nerve-spike generation to identify the contribution of spiking neurons, (6) modeling measured amplitude-energy functions and ti mecourse of flash responses and (7) using steady backgrounds to obtain diff erential reductions in sensitivity of different cell types. While some of t hese procedures can be applied to humans, the results described here have a ll been obtained in studies of the ERG of anaesthetized cats, or macaque mo nkeys whose retinas are very similar to those of humans.