Differences between pattern-evoked electroretinograms obtained by a scanning laser ophthalmoscope and by a mechanical mirror system.

Pattern electroretinograms were studied in response to stripe patterns of different spatial frequencies and intensities in the pattern-onset and reversal mode by means of a scanning laser ophthalmoscope and a mechanical mirror system (maxwellian view). The stimulus conditions in both procedures were as close as possible: 31 degrees square field, 633-nm wavelength and the same equipment for recording. The remaining differences between the two methods were the line and pixel structure in the scanning laser ophthalmoscope and the buildup speed (40 ms in the scanning laser ophthalmoscope and less than 1 ms in the mirror system). This difference was reflected in the response peak times of the pattern electroretinogram, which were up to 10 ms in the scanning laser ophthalmoscope measurement. The pattern-related onset response was smaller with the scanning laser ophthalmoscope, indicating a strong sensitivity to the slow build-up speed, while the offset and reversal responses showed no differences in amplitudes. All differences were found only with the use of high luminances. The results suggest that responses evoked with patterns displayed on a television screen could be faster and larger if stimulators faster than the conventional television standard were used. Because of the smallness of onset amplitudes, the pattern-reversal mode might be better for stimulation in a clinical study when the scanning laser ophthalmoscope is used.