The positive potential of the ERG shows a decreasing course during light adaptation when the difference between the intensities of the test and adapting light is large. This is contrary to psychophysical measurements. In the human ERG, a test light of large area and high intensity is generally required to elicit adequate potentials, while psychophysical measurements of light adaptation use relatively weak test flashes to measure sensitivity by means of the difference threshold. If the intensity of the test light is reduced, or that of the adapting light is increased, the decreasing course of the ERG becomes less apparent, and when a red test light is used, the curve even shows a slight increase. When a red test light now is used on a weak adapting field, the ERG reveals both an x wave and a b wave. The curve of the x wave shows a slight increase, while the b wave shows a decreasing course. An analysis has been made of the differences among light adaptation curves of the human ERG measured under the various conditions, as compared with psychophysical measurement. It is suggested that the differences reflect the relative contributions of the photopic and scotopic mechanisms of the eye.
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