PATTERN ELECTRORETINOGRAM AND VISUAL-EVOKED POTENTIAL AMPLITUDES ARE INFLUENCED BY DIFFERENT STIMULUS FIELD SIZES AND SCOTOMAS

The pattern electroretinogram and the visual evoked potential were rec orded simultaneously with various stimulus fields and artificial scoto mata of increasing sizes. In contrast to an earlier study, a smaller c heck size (20') and two stimulus field sizes (20-degrees x 20-degrees and 10-degrees x 10-degrees) for the scotomata were used. With a conce ntric decreasing stimulus field, a reduction of both the pattern elect roretinogram and visual evoked potential was found. Both showed a simu ltaneous reduction of amplitudes, but, compared with the amplitude in the full field, the reduction was more extensive for the pattern elect roretinogram at each test field size. This implies a greater contribut ion to the pattern electroretinogram from more eccentric retinal parts . An artificial central scotoma of increasing size in the 20-degrees x 20-degrees field had less influence on the pattern electroretinogram than on the visual evoked potential. The percentage amplitude loss of the visual evoked potential was more pronounced. The visual evoked pot ential was eventually abolished by a scotoma size from 10-degrees x 10 -degrees upward, while the pattern electroretinogram was still registr able. When scotomata of similar size were introduced in a smaller (10- degrees x 10-degrees) field, percentage pattern electroretinogram and visual evoked potential amplitude losses were less separated than in a larger (20-degrees x 20-degrees) test field.