Isolation of focal rod electroretinograms from the dark-adapted human eye.

PURPOSE To isolate focal rod electroretinograms (ERGs) from the dark-adapted human eye. METHODS In two normal volunteers, dark-adapted focal rod ERGs were recorded from the peripheral retina in response to 30 degree diameter blue flashes of varying retinal illuminance and from different retinal regions in response to 10 degree diameter bright blue flashes. Dark-adapted focal rod ERGs also were recorded from a patient with the multiple evanescent white dot syndrome (MEWDS) and an enlarged blind spot in response to 30 degree diameter blue flashes presented within and outside the scotoma. The slower and larger stray light rod component elicited by these flashes was removed by subtracting the matching rod response to a dimmer, full-field flash, or was ignored when it did not overlap the faster and smaller focal rod component. RESULTS The focal rod ERG had a waveform and sensitivity similar to those of the full-field rod ERG, was approximately proportional in amplitude to the density of rods directly illuminated, and was nondetectable within the retinal area corresponding to the enlarged blind spot of the patient with MEWDS. CONCLUSIONS Focal rod ERG a- and b-waves, in response to stimuli as small as 10 degrees, can be recorded from different regions of the dark-adapted human retina to evaluate localized rod function.