Effect of defocusing and of distracted attention upon recordings of the visual evoked potential.

Pattern reversal visual stimuli are used to evoke potentials (VEPs) for assessment of visual acuity and for localizing defects along the visual pathways. Our goal was to assess the importance of attention and defocusing to the recordings of pattern VEP. Forty-one volunteers with normal (6/6) corrected visual acuity participated in this study. Twenty-one were asked to defocus intentionally the visual stimulus (located 200 cm away) by fixating at a target 25 or 50 cm from the eye. Twenty other subjects performed auditory tasks to distract their attention from the visual stimulus. Pattern VEPs were elicited by different check sizes. The amplitude and time-to-peak of the P100 wave were measured. Intentional defocusing caused amplitude reduction and prolongation of the time-to-peak in young subjects (20-34 years old). With the smallest checks used (7.5') we could not record a reliable response from 43% of the young subjects (6 out of 14). In older patients (35-61 years old), intentional defocusing induced negligible effects on pattern VEPs regardless of check size. There were no effects of auditory distraction upon the pattern VEPs. Our data suggest that intentional defocusing can produce false positive results (reduced VEP with prolonged time-to-peak) only when small checks are used in young subjects. Divided attention has negligible effect on the recordings of pattern VEPs. With proper controls, the pattern VEP test can be used for objective assessment of visual function.