The effect of motivational instructions on P300 amplitude.

The aim of this investigation was to determine the effect on P300 amplitude of instructions aimed at increasing the subject's degree of task involvement. To this end, two different studies were carried out. In Study 1, 20 university students were tested with an auditory event-related potential (ERP) oddball paradigm (target: 1,100 Hz; standard: 1,000 Hz) in two consecutive runs, each with a different set of instructions; after the first run, subjects were verbally motivated to increase their level of performance in the second run. In Study 2 (performed 1 year later), ERPs were similarly obtained from the same subjects during two oddball runs, but this time both tests were preceded by neutral instructions. The amplitude and latency of N1 and P2 elicited by non-targets and of N2 and P3 in target waveforms were evaluated. The findings showed that following motivating instructions, P3 amplitude increased while P3 latency showed a non-significant decrease. The amplitude of P2 to non-target stimuli--which could be interpreted as P250--was also affected by the instructions provided. The overall results suggest that the presentation of motivating instructions is followed by a higher amount of attentional resources allocated to all stimuli, and a more efficient evaluation and discrimination of relevant targets. The implication of these findings for the clinical use of P300 has been discussed.