Electroencephalogram-based scaling of multifocal visual evoked potentials: effect on intersubject amplitude variability.

PURPOSE: The interindividual variability of the visual evoked potential (VEP) has been recognized as a problem for interpretation of clinical results. This study examines whether VEP variability can be reduced by scaling responses according to underlying electroencephalogram (EEG) activity.
METHODS: A multifocal objective perimeter provided different random check patterns to each of 58 points extending out to 32 degrees nasally. A multichannel VEP was recorded (bipolar occipital cross electrodes, 7 min/eye). One hundred normal subjects (age 58.9 +/- 10.7 years) were tested. The amplitude and inter-eye asymmetry coefficient for each point of the field was calculated. VEP signals were then normalized according to underlying EEG activity recorded using Fourier transform to quantify EEG levels. High alpha-rhythm and electrocardiogram contamination were removed before scaling.
RESULTS: High intersubject variability was present in the multifocal VEP, with amplitude in women on average 33% larger than in men. The variability for all left eyes was 42.2% +/- 3.9%, for right eyes 41.7% +/- 4.4% (coefficient of variability [CV]). There was a strong correlation between EEG activity and the amplitude of the VEP (left eye, r = 0.83; P < 0.001; right eye, r = 0.82; P < 0.001). When this was used to normalize VEP results, the CVs dropped to 24.6% +/- 3.1% (P < 0.0001) and 24.0% +/- 3.2% (P < 0.0001), respectively. The gender difference was effectively removed.
CONCLUSIONS: Using underlying EEG amplitudes to normalize an individual's VEP substantially reduces intersubject variability, including differences between men and women. This renders the use of a normal database more reliable when applying the multifocal VEP in the clinical detection of visual field changes.