The aim of this experiment was to evaluate a scalp distribution of processing-related negativity that was thought as an electrophysiological correlate of the mental rotation process. Event-related brain potentials (ERPs) were recorded from 16 subjects who performed a mental rotation task (to decide whether the letter presented in one of nine different orientations was in normal or mirror-reversed format). The probability of normal stimuli was manipulated between conditions to reduce the effects of P3b that was simultaneously superimposed on the rotation-related negativity. The results showed that the increase of mental rotation demand caused negative shifts of the ERPs between 400 and 800 ms latency range. Although the probability of the normal stimuli did not change the amplitudes of P3b, some differences of topography were observed between the rotation-related negativity and the P3b
