Effects of dynamic rotation on event-related brain potentials.

Event-related potentials were recorded during a mental rotation task. Subjects were shown pairs of letter-like shapes and were asked to make a parity judgment. The shape on the left was always in its canonical position and the shape on the right could either be in its canonical position or be a mirror image. Two variables were manipulated for the shape on the right. First, it could appear at different orientations (50 degrees , 100 degrees or 150 degrees ); second, it could be presented in a stationary position, in a dynamic congruent direction (the shape slowly rotating toward its normal upright position) or in a dynamic incongruent direction (the shape slowly rotating in the opposite direction to its normal upright position). Orientation- and direction-dependent modulations of a negative slow wave were found. For orientation, the typical amplitude effect over parietal sites was found, the amplitude becoming more negative as the rotational angle increased. For direction, the amplitude of the negative slow wave was larger for stationary and dynamic incongruent trials than for dynamic congruent trials at 100 degrees and 150 degrees . This result suggests that presentation of a stimulus in a dynamic congruent direction facilitates the mental rotation process. At 50 degrees , differences between dynamic incongruent trials and both stationary and dynamic congruent trials were found, suggesting that the incongruent movement elicits an obstructing effect over the mental rotation process. In summary, the present experiment provides new evidence in support of the idea that the amplitude modulation over the parietal cortex is a psychophysiological marker of the mental rotation process.