Inter-participant variabilities and sample sizes in P300 and P600.

Reliably and efficiently detecting physiological differences between conditions of interest is of importance in psychophysiology. In particular, when it comes to the observation of relatively small differences, such as a P600 effect, a language-related brain potential elicited by ungrammatical sentences compared to grammatical sentences, inter-participant variability is a critical factor since a larger inter-participant variability decreases statistical significance, and therefore increases the necessary sample size. The present study investigated how stable individual P600s are, at which sample sizes the P600 becomes stable, and how many participants are necessary to observe a P600 effect. P600s were recorded from 48 participants, as well as P300 (P3b) from 40 participants for comparison. Unlike the P3b effect, which had an approximately 10 μV difference between the target and standard stimuli, P600 increased in amplitude by only 1.4-1.7 μV at Pz during the processing of ungrammatical sentences relative to the grammatical counterparts. The sample size analysis suggests that 20 to 30 participants are needed to detect a P600 effect at Pz, and the distribution of variances does not change significantly with a larger sample size.