Factors contributing to amplitude variability of the steady-state visual evoked response.

Ten steady-state visual evoked responses (VER's) were recorded from each of 47 normal, adult subjects. For each subject, the mean and SD for the 10 VER amplitudes were calculated and used to determine amplitude variability. Although some subjects produced extremely reliable VER's, data from the majority showed a considerable degree of variability. A number of factors including trend (the tendency of the VER amplitudes to increase or decrease with repeated measurements), noise, attention, binocularity, accommodation, eye movements, artifacts, and electrode placement were evaluated to determine their relative contributions to this variability. Noise and trend factors produced a large proportion of the variability (75%) whereas the other factors were found to be relatively insignificant.