Retinal and cortical activity in human subjects during color flicker fusion

Pattern electroretinograms (PERG) and cortical visually evoked potentials (VEP) were simultaneously recorded from 7 visually normal and 1 protanopic subjects. Stimuli were color checkerboards (0.5 degrees check size), phase-reversing at 17 Hz (i.e. 34 reversals/sec). Using a stepwise sweep procedure, the luminance of the red (lambda peak = 550 nm) and green (lambda peak = 630 nm) checks varied in 11 steps in opposite directions from 0 to 30 cd/m2, embracing the subjective equiluminance point. For normal subjects at subjective equiluminance, the VEP amplitude dropped sharply down to 13 +/- 2% of the value at pure luminance contrast. The PERG, however, was only reduced to 56 +/- 10% at this point, an attenuation 4 times less than that of the VEP. In contrast to normal subjects, in the protanopic subject the PERG was sharply reduced at equiluminance, parallel to the VEP. This would be expected when L-cones are missing. Assuming that the PERG reflects the activity of the retinal ganglion cells, our findings suggest that human retinal ganglion cells respond well under the condition of equiluminant flicker fusion, which is in agreement with recent single-cell studies in the monkey. Consequently, the temporal low-pass filter, which mediates color-flicker fusion, would seem to lie central to the retinal ganglion cells.