Recording the pattern electroretinogram: a cautionary note.

It is possible to record a pattern electroretinogram (PERG) of near normal amplitude in a situation when the eye containing the active electrode is occluded. Because PERG recording requires high amplification and sensitive signal retrieval techniques, the electrode in the occluded eye records a distant potential from the unoccluded eye. Referencing the active electrode to an ipsilateral ear diminishes, but does not eliminate the referred PERG potential. Such unlooked for interaction may provide misleading data in situations where binocular viewing is used because of poor vision in one eye; therefore, occlusion of the eye not being tested should be undertaken whenever possible.