Brightness contrast and evoked potentials.

The amplitude of the evoked potential (EP) generated by an oscillating checkerboard pattern remains constant, even though the brightness contrast of the checks may be significantly altered by changing fixation distance without altering retinal subtense. On the other hand, when the pattern is slightly blurred, the brightness contrast of small checks is changed only slightly, whereas the EP undergoes marked attenuation. Such a double dissociation between brightness effects and the EP shows that steady-state EP amplitude may be a very poor measure of brightness contrast, at least when the sensations are considerably above threshold. For suprathreshold stimuli, check size may also affect brightness contrast and the steady-state EP amplitude in different ways. Moderate blurring may increase the amplitudes of large-check EP’s, while attenuating small-check EP’s. Interactions between blurring and pattern size could thus lead to large systematic errors in EP refraction procedures.

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