OBJECTIVE: For motor activities, visual information is crucial for organizing a movement with respect to a given situation. The present study investigates how cognitive information processing is associated with this visuomotor process. METHODS: Brain dynamics in executing two perceptual-motor tasks were examined in terms of event-related synchronization (ERS) and event-related desynchronization (ERD) of EEG. Those tasks were (1) reaching toward and grasping a visual object with a pinch grip, and (2) matching the pinch grip size with respect to the perceived object size. RESULTS: According to the aperture size in the task execution, both the tasks were affected by the perceived object size inducing the Ebbinghaus illusion. The alpha-ERD patterns were associated with the movement execution and appeared to be identical in both the tasks, whilst the gamma-ERS appeared only for the grasping motion. CONCLUSIONS: These results suggest that cognitive processing was involved not only in the matching task but also in the grasping task. These ERD/ERS patterns are thought to reflect the similarity and difference in the perceptual-motor processes between the two tasks. SIGNIFICANCE: The analysis of ERD/ERS can provide insight on the qualitative feature in a visuomotor process associated with the involvement of cognitive processing.