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.