Analyses of single-trial N1 amplitude and latency variability and their influence on the average evoked potential

Sixteen adult subjects listened to tone pips superimposed on Bach concertos and either attended to the tones or to the music. Auditory evoked potentials (EPs) were recorded to the tones in both conditions from C3 and C4 each referenced to linked ears. Although the expected Condition X Hemisphere interaction was not found, a main effect for Condition showed that N1 amplitude in the average EPs (AEPs) was greater in the Tones condition. P2 showed no significant main effects or interactions. The AEP was used as a template to identify N1 in each single-trial wave form. The difference in AEP amplitude between conditions was subsequently found to be due primarily to enhanced N1 amplitude in the single-trial wave forms, and not to a decrease in latency variability. This template-matching procedure appears to be a valid method for identifying N1 in single-trial wave forms based on the following findings: (1) multiple regression analyses demonstrated that 90-99% of the variance of the amplitude of N1 in the average EP could be accounted for by a weighted linear combination of single-trial amplitude and latency variability; (2) the values of the correlation coefficients between the AEP template and the single-trial EPs were generally very high, averaging approximately +0.91; (3) the distributions of the latency of N1 in the single-trial EPs were not flat as would be expected if random noise were being identified as N1.