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

PURPOSE. The interindividual variability of the visual evoked potential (VE P) has been recognized as a problem for interpretation of clinical results. This study examines whether VEP variability can be reduced by scaling resp onses according to underlying electroencephalogram (EEG) activity. METHODS. A multifocal objective perimeter provided different random check p atterns to each of 58 points extending out to 32 degrees nasally. A multich annel VEP was recorded (bipolar occipital cross electrodes, 7 min/eye). One hundred normal subjects (age 58.9 +/- 10.7 years) were tested. The amplitu de and inter-eye asymmetry coefficient for each point of the field was calc ulated. VEP signals were then normalized according to underlying EEG activi ty recorded using Fourier transform to quantify EEG levels. High alpha -rhy thm 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% (coeffi cient of variability [CV]). There was a strong correlation between EEG acti vity 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 CV s dropped to 24.6% +/- 3.1% (P < 0.0001) and 24.0% +/- 3.2% (P < 0.0001), r espectively. The gender difference was effectively removed. CONCLUSIONS. Using underlying EEG amplitudes to normalize an individual's V EP substantially reduces intersubject variability, including differences be tween men and women. This renders the use of a normal database more reliabl e when applying the multifocal VEP in the clinical detection of visual fiel d changes.