Simultaneous recordings of the EEG at subdural and scalp electrodes often show very different activities. Large amplitude activity with maximum power between 15 and 30 c/sec can be often observed on the subdural electrodes together with smaller amplitude lower frequencies, whereas on the scalp only a small part of this high frequency activity is seen and the lower frequencies dominate. The impedance between cortex and scalp has been shown to be similar for low and high EEG frequencies and the high attenuation of the beta activity at scalp electrodes is believed to be due to summation of polyphasic cortical activity. The weighted summation of this polyphasic activity across a limited cortical area (spatial average) is similar to the activity of a non-recursive filter between cortex and scalp and has a low pass characteristic.