What are the technical aspects of visual evoked potentials (VEPs) for clinical use?

Updated: Oct 25, 2019
  • Author: Andrew B Evans, MD; Chief Editor: Selim R Benbadis, MD  more...
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A checkerboard pattern (or, less often, a flash) is used as stimulation. Responses are collected over Oz, O1, and O2 and with hemifield studies at T5 and T6 with the standard electroencephalographic (EEG) electrode placement. Monocular stimulation is used to avoid masking of a unilateral conduction abnormality. Sedation should not be used, and note should be taken of medications that the patient is taking regularly.

Testing circumstances should be standardized, including a seating distance of 70-100 cm from the monitor screen, which gives a check size of approximately 30 seconds of visual angle. The vision should be corrected to the extent possible in case of a visual problem. Pupil size and any abnormality should be noted. The P100 waveform is at its maximum in the midoccipital area. Stimulus rates of 1-2 Hz are recommended, and the filter setting should be in 1- to 200-Hz bandwidth (the outside limits are 0.2 and 300 Hz).

The recommended recording time window (ie, the sweep length) is 250 msec; 50-200 responses are to be averaged. A minimum of 2 trials should be given. The responses are averaged, and the P100 positive polarity waveform that appears in the posterior head region is analyzed. The mean latency is about 100 msec. Normative data should be assembled on a laboratory-by-laboratory basis.

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