Fig. 1. The typical recording arrangement for visual evoked potentials. The patient is seated at an appropriate distance from the visual stimulus wearing the appropriate refraction (corrected for the test distance). The appropriate recording (active) electrode is attached to the posterior scalp and connected to the positive input of the differential amplifier. A similar reference electrode is attached to a visually neutral site on the head (e.g., the earlobe) and connected to the negative input of the differential amplifier. A ground (GND) electrode is attached to the forehead or arm. A stimulus generator is used to select the desired stimulus type (flash, pattern reversal, or pattern onset-offset), temporal frequency, pattern type, (grating or checks) and size of the pattern elements. The stimulus generator also sends a signal to the signal averaging computer that is synchronous with the stimulus presentation and triggers each averaging epoch. The signal averaging computer usually controls the duration of the averaging epoch, the number of averages collected, the signal conditioning (e.g., filtering), and signal analysis.