Objective determination of dark adaptation: accuracy of the evoked potential method.

The psychophysical threshold and the appearance of the visually evoked potential (VEP) were compared in the dark adapted state. Nineteen normal subjects and 11 persons with defective dark adaptation were studied. The VEP threshold was an average of 0.2 log units (standard deviation 0.2) above the subjective threshold in the normal group, and 0.4 log units above it in the night-blind subjects. In 7 normal subjects, the alpha-activity of the electroencephalogram interfered with the VEP recording and rendered the evaluation more difficult. The results indicate that the VEP can be used as an accurate method of objectively measuring dark adaptation.