Visual evoked responses to pattern-reversal: the signal-to-noise ratio.

Visual evoked responses (VERs) to checkboard-reversal photic stimuli were recorded in 30 healthy experimental subjects with a mean age of 32 years. In 20 cases the stimuli were presented monocularly and in 10 cases binocularly. The estimated mean signal-to-noise ratio (SNR) of the individual VERs was found to be 0.245 +/- 0.092 for monocular stimulation and 0.444 +/- 0.23 for binocular stimulation. The power-based SNR of the averaged VERs was 24.5 and 44.4 respectively. The SNR was raised the most effectively by a new selective averaging variant applied after latency correction of the individual VERs; in the case of monocular stimulation the SNR rose to a mean 58.94 and in binocular stimulation to a mean 87.42. The mean proportion of discarded single VERs was 24%.