Effect of the relevance and position of the target stimuli on P300 and reaction time

The relationships among stimulus relevance and the position of target stimuli in a sequence with the P300 component of event related brain potentials and reaction time were investigated. An auditory oddball series was presented to 42 healthy, young, right-handed female participants. In the series, participants were to ignore the standard stimuli and count the deviants from 1 to 6. When reaching the count of 6, they had to restart their count from 1 again. While counting, half of the sample had to press a key after the deviants, numbers 1, 2 and 3, but not after the numbers 4, 5 and 6. The other half of the sample had to press the key when numbers 4, 5 and 6 rather than when numbers 1, 2, 3 appeared. The P300 amplitude of the more relevant (count and press) stimuli was higher than that of the P300 amplitude of the less relevant (count only) stimuli. This is associated with a greater allocation of cognitive resources to the more relevant stimuli. An interaction was also found between the position of the target stimuli in a sequence and the stimulus relevance, which is attributed to the cognitive preparation. Furthermore, the reaction time tended to increase along the sequence of the more relevant stimuli, due perhaps to an increase in physical fatigue. (C) 2001 Elsevier Science B.V. All rights reserved.