MENTAL COMPARISON OF VISUALLY PRESENTED 2-DIGIT NUMBERS - A P300 STUDY

Comparison processes were investigated in a multiple-stimulus paradigm with a pseudo-random sequence of visually presented numbers 11 to 20. The subjects' task was to compare each current number with the preced ing one and to indicate whether it was larger or smaller. ERPs were se lectively averaged for the 10 number stimuli, for three functional con ditions according to the information the numbers provide about the ens uing response and for the differences between consecutive numbers. P30 0 amplitude averaged for each number stimulus showed a U-shaped trend with largest amplitudes for the numbers 11 and 20. It was found that P 300 amplitudes change with the amount of information the current numbe r is delivering for the ensuing response. This information delivery is related to the processing in the subsequent trial, as revealed by a n egative correlation between P300 in the current trial and RT in the en suing trial. Reaction times decreased significantly with increasing di fference between the current number and the preceding one. This symbol ic distance effect was not found for P300 parameters. The dissociation between RT and P300 data provides evidence for the assumption that un der the present experimental conditions informational transaction lead ing to the distance effect occur after the elicitation of P300.