Proximal retinal contribution to the intraretinal 8-Hz pattern ERG of cat

We investigated the retinal depth of origin of the pattern ERG (PERG) in cat by recording extracellular responses to contrast reversal grating stimuli and performing a current source-density analysis of the depth profiles. The local ERG (LERG) was recorded within the retina; the waveform varied with grating spatial phase position of the microelectrode. The LERG at any arbitrary single spatial phase was not equivalent to the corneal PERG. To obtain an "intraretinal PERG" equivalent to the corneal PERG, we summed LERGs from many spatial phases over a complete grating cycle. This intraretinal PERG was analogous to the corneal PERG in having only even harmonic components and also in being independent of the grating spatial phase. The absence of the fundamental and odd harmonics appeared to result from the summation of responses across grating spatial phase rather than from an intrinsic generator mechanism. The spatial contrast sensitivity function for the intraretinal PERG at 8 Hz was tuned, with peak sensitivity between 0.5 and 0.75 cycles per degree in the area centralis. The 8-Hz PERG response was maximum in the proximal retina at approximately 23% depth from the internal limiting membrane (ILM). This depth lies within the inner plexiform layer and is proximal to the depth at which the b-wave was maximum (49-61% depth). PERG and flash-elicited ERG responses exhibited different amplitude profiles as a function of retinal depth. Analysis of the 8-Hz PERG depth profiles identified generators in the proximal retina between the ILM and the inner plexiform-inner nuclear boundary. The analysis suggested that at least two generators were present. The proportional contribution to the corneal PERG from each of the two or more proximal generators was not determined. Contributions to the corneal PERG from more distal generators, however, were not specifically excluded. The intraretinal recordings suggest that under some conditions contrast reversal gratings at 8 Hz can elicit current generators in the proximal retina of cat. These generators appear to respond to local retinal illumination, and the intraretinal PERG results from nonlinear elements of at least two generators. Spatial tuning of the intraretinal PERG may reflect properties of the underlying LERG components, including the M-wave.