Pupil size and the pattern evoked visual response.

The effect of artificially varying the pupil size upon the pattern evoked visual response was examined in six healthy subjects and six patients with multiple sclerosis who had abnormal visual evoked responses. Slight depression of amplitude was seen whether the pupil was small or large, but the most obvious effect was a reduction of latency with a large pupil and an increase of latency with a small pupil. It is suggested these phenomena relate to the amount of light entering the eye and not any change of acuity.