Variability of the human visual evoked potential: normative data.

Abstract 1. 1. Visual evoked responses of twenty normal subjects (50 from each of them) were recorded, graphically digitalized and studied by means of digital computer techniques, mainly for their variability. 2. 2. The amplitude variability of the individual average responses—averaged from the 50 single responses of each subject—is due mainly to the background activity, the variability of the evoked potential being mostly negligible. In about one half of the subjects there is a significant decrease of variability about 80 msec after the stimulus, due to blocking of background activity. Other significant short oscillations in variability of less clear origin are described and discussed. 3. 3. The average group response (across the whole group of subjects) supportes the existence of a standard, though inter-individually very variable, human visual evoked potential. The dispersion of the amplitudes of the individual average responses about the average group responses rises with the wave latency. 4. 4. With the method used it is possible to compute the probability with which a particular wave of the average response is due to the evoked potential or to noise (background activity). Examples are given mainly in connection with the problem of reliability of low voltage, short latency waves of the EEG response. 5. 5. The latency variability is greater with long than with short latency waves. There is also a slight increase in variability in the initial short latency waves (I and II) but this is probably due to the background constituent as these evoked potential waves are of low voltage.