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Visual evoked potential (EP) recorders designed for detecting, amplifying, and recording the changes in the electric potential between the retina and the cornea in response to specialized external visual stimuli (e.g., alternation of light and dark conditions). The results may be displayed as a graphic of the measured voltages versus time (an electrooculogram). These recorders typically consist of detachable electrodes usually placed on the nasal and lateral canthi of each eye; filters (to suppress interference); memory (e.g., electronic or optical/magnetic disc); and a display. Electrooculographs usually include capabilities to induce light adaptation and to provide additional stimuli at an appropriate repetitive level to elicit the bioelectric response needed for recording. Electrooculographs measurements (typically in microvolt) are typically expressed as the ratio of the maximum value obtained during the light period to the minimum value at the dark period (known as Arden ratio). Decreases in this ratio are associated with retinal pigment disorders. Some recorders with additional capabilities to perform electroretinography, other specialized forms of visual EP response, and/or for recording the velocity of the short, fast movements of the eyes made during reading and sleep (i.e., saccadic velocity) are also available.
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