Averaging, selective averaging and latency-corrected averaging.

Although latency-corrected averaging and selective averaging represent frequency used alternatives to the time-locked averaging, little is known about the changes of the signal-to-noise ratio (SNR) associated with their application. For this reason, visual evoked responses (VERs) to blank and checkerboard patterned flashes of light (5.6 cd/m2) were recorded in 15 subjects using a referential (Oz - A1 + 2) and a bipolar (Oz - Pz) derivation, and interstimulus intervals (ISI) of 1.4s and 2.4s; SNRs were calculated for all combinations. With time-locked averaging (referential derivation) the mean SNR of single VERs was 0.35 to blank stimuli and 0.68 to patterned stimuli. With latency-corrected averaging, the means SNR rose to 0.56 (0.74), and with selective averaging it increased to 0.81 (1.00). A combined procedure of selective and latency-corrected averaging yielded the highest mean SNR values of single VERs: 0.98 (1.21). Increasing ISIs from 1.4 - 2.4s did not change the results; the mean SNR values were always higher in bipolar than in referential derivations. The selective, latency-corrected averaging produced higher SNRs, and the group mean number of detectable single VERs (62% of the single VERs to blank stimuli, and 74% of the single VERs to patterned stimuli) and the latency variability of the single VERs: -0.33 +/- 14.5 ms (blank stimulation) and -0.48 +/- 13.96 ms (patterned stimulation).