On the validity of interblock averaging of P300 in clinical settings

The reduction of long-latency auditory ERPs amplitude, including P300, to r epeated stimuli has been well documented in the literature on habituation. The effect of block repetition on auditory ERPs recorded for clinical purpo ses, where interblock intervals are commonly longer than those employed in habituation studies, was studied in a sample of 38 adults submitted to two blocks of a counting oddball paradigm. Four different experimental conditio ns were considered, differing in target probability, delivery or not of a p revious passive oddball tone sequence, and the performance or not of other oddball tasks requiring more complex discriminative responses between the t wo blocks. Results showed that: (1) N1 amplitude to the frequent non-target stimuli decreased in the second block under all the conditions; (2) when t he two blocks were consecutive (separated by 2-3 min), P300 amplitudes were unaffected by block repetition, this whatever the probability of the targe t (25% vs. 10%) and whether or not a passive oddball sequence preceded the two active blocks; (3) P300 amplitude was only affected by stimulus repetit ion in those subjects who performed more complex cognitive tasks between th e first and second blocks and; (4) latency values were unaffected by repeti tion. It is hypothesised that the N1 amplitude decline may be caused by a d ecrease in alertness or arousal level produced by stimuli repetition. Reduc tion in P3 amplitude only appeared when more difficult tasks had to be done between the two oddball blocks and may reflect a decrease in the amount of attentional resources allocated to the second block, due either to fatigue or over training. The practice of using a grand average of several repetit ions of the oddball paradigm, as recommended for the clinical use of long-l atency ERPs, seems to be adequate provided that long interblock intervals a re used and that the subject is not engaged in tasks requiring a high menta l workload between the trial blocks. (C) 1999 Elsevier Science B.V. All rig hts reserved.