Multifocal electroretinograms combined with periodic flashes: direct responses and induced components

To investigate the retinal responses generated by flash interactions in a recently introduced "global (full-screen) flash" stimulus paradigm to record the multifocal electroretinogram (mfERG).

Five normal individuals were studied with stimulation combining multifocal (pseudorandom) flashes with interleaved global (periodic, full-screen) flashes. The intensities of the two flashes were independently varied. Two distinct first-order response components were obtained: the mean response to the focal flashes (referred to as the direct response, DR) and the effect of the focal flash on the responses evoked by the global flash (the induced component, IC).

Increasing the global flash luminance reduced DR amplitude and shortened DR implicit time. IC amplitude peaked with a moderate global flash (1.33-2.67 cd.s/m2). With a global flash of the right intensity, a weak focal flash could evoke a considerable IC even when the DR was barely detectable. Moderate global flashes maximized the IC, and its intersubject variability was reduced at lower focal flash luminances. IC topography had a large naso-temporal asymmetry.

While the DR is the average response generated by the local flashes regardless of the context of preceding and following flashes, the IC represents the difference in the global flash response in the presence and absence of the preceding focal flash. As the focal flashes were always preceded by the periodically occurring global flashes, the DR reflects the resulting adapted or desensitized state of the retinal patch. The pure, nonlinear IC is thought to reflect predominantly inner retinal function.