We recorded motor cortical activity using highdensity electrocorticogram (ECoG) from three patients during awake craniotomy. Subjects repeatedly executed hand flexion/extension tasks according to auditory instructions. Clear event-related desynchronization (ERD) in beta band (8-32) Hz and event-related synchronization (ERS) in gamma band (60-200) Hz were observed. High frequency band (HFB: 60-200 Hz) activation was found to be more localized compared to low frequency band (LFB: 8-32 Hz) activation in all subjects. Local spatial correlation maps in LFB and HFB were constructed by computing the correlation between channels. Local spatial correlation dropped more in the ERD/ERS areas consistently in two subjects. The results indicate that ERD/ERS patterns are more spatially uncorrelated and denser ECoG electrode is necessary within these areas to map uncorrelated 'sources'. High resolution electrodes might improve both clinical functional mapping and brain machine interface outcomes in the near future.