Inhibition, response mode, and stimulus probability: a comparative event-related potential study

Objectives: In the present study, effects of response mode (finger movement or counting) and stimulus probability on inhibitory processes were studied. Methods: Electroencephalographic activity was registered in a visual go/nogo paradigm. Subjects either responded manually to go stimuli or counted silently the occurrence of each go stimulus in different conditions. In both response mode conditions, response probability was varied. Results: For finger movement and counting, similar N2 and P3 go/nogo effects were found. The amplitude of the nogo N2 and nogo P3 varied as a negative function of nogo stimulus probability. The go P3 varied as a negative function of go stimulus probability. In the manual condition, however, the descending flank of the go N2 at anterior electrode sites was more negative in the 0.50go and 0.75go probability trials than in the 0.25go probability trials. Conclusions: The results of the present study confirm the hypothesis that differences between go and nogo event-related potentials are not dependent on overt movement-related potentials. It could be speculated that the probability effect on the N2 amplitude in go trials in the manual condition has to be explained in terms of a modulation of the strength of motoric preparation processes varying as a positive function of the probability of the go stimulus.