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The contrast of a retinal image is less than that of the external stimulus owing to a process of optical degradation. Theoretical studies have shows that this affects the pattern and illuminance detectors of the retina differently and provides a new insight into the nature of contrast stimulation and the mechanisms responsible for the pattern electroretinogram. Consensus data on the optical transfer function of the eye are applied to the Fourier transform of the pattern stimulus and the retinal illuminance distribution is determined. The checkerboard image is shown to undergo substantial degradation for those check sizes used in experimental and clinical observations. Current concepts of contrast and modes of stimulation are examined and methods are described for quantifying the effects on stimulation of illuminance and pattern detectors. The findings are applied to experimental data on spatial tuning functions of electroretinograms elicited by checkerboard pattern stimulation. It is concluded that the signal predominantly originates from local illuminance, but that this can account for only part of the response with small check sizes. The remainder must be a highly selective response to spatial frequency. If the predicted degradation is accepted, a similar conclusion must be reached for many previously reported tuning functions of the pattern electroretinogram as well as to the present experiment on eight normal eyes.

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