Diurnal variations in the cortical visual and auditory evoked potentials

Visual and auditory evoked potentials (VEP and AEP) have been recorded for six subjects during the day (11.00-12.00 h) and during the night (23.00-24.00 h). For each subject the following parameters were estimated separately: the confidence limit of the averaged EP, the mean value and the standard error of the latencies of the EP components, EP energy and the product-manner correlation coefficient to evaluate waveform similarity of two evoked potentials. In order to estimate the diurnal variations of VEP and AEP, a comparison is made for each subject separately of the amplitudes and latencies, and their dispersions at N100, P200 for VEP and N100, P200 and P300 for AEP during the day and during the night. During the night hours the amplitudes of the different components and the EP energy are found to increase. The variativity of VEP and AEP increases during the night, while the degree of EP similarity and especially of their late components, decreases. The symmetry of the interhemispheric distribution of VEP, estimated by the peak amplitudes and the energy during the night hours of investigation, is disrupted.