Diagnosing functional visual deficits with the P300 component of the visual evoked potential

The visual evoked potential (VEP) is routinely used to assess visual function, though it occasionally does not reflect a patient's conscious experience. Reports of normal flash or pattern VEPs obtained from blind persons are extreme examples of this problem. The difficulty in interpreting VEPs in light of such findings can be partly overcome by obtaining a cognitive component of the evoked potential, P300. We obtained traditional visual acuity measurements, pattern-reversal VEPs, and VEPs containing P300s from three patients with clinically diagnosed functional visual deficits. The P300s were obtained in response to stimuli that the patients claimed they could not see, supporting the clinical conclusions that malingering or hysteria was involved. The P300 component can be helpful in assessing the subjective visual experience of patients suspected of having functional visual loss.