Monocular stimulation generates a cerebral response that is best recorded over the occipital region. Either a flash or a pattern that occurs without change in background luminance can be used to elicit the response. Responses to approximately 100 stimuli are generally averaged to obtain robust and unambiguous potentials. For pattern stimulation, the seated subject looks at the center of a television screen on which is displayed a checkerboard pattern of white and black squares. The pattern reverses at about 1 Hz so that the white squares become black, and vice versa, without change in total luminance. The size of the checks is usually between 15 and 50 minutes of arc at the subject's eye, and the entire pattern should subtend at least 10 degrees of visual angle. Pattern-reversal stimuli generate robust responses and have a higher yield of abnormalities than flash stimuli. Small checks (15 to 30 minutes) preferentially activate the macular region and are the most widely used stimulus.
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