Visual stimuli were designed to drive a high proportion of the neurons in restricted parts of the human visual cortex. These stimuli were used to examine changes in the ongoing EEG during visual stimulation. The topographic organization of these changes was studied. It was found that the EEG from those parts of the cortex that are exposed to the stimulus is strongly reduced in amplitude. This stimulus dependency is indicative that cortical processing results itself in a reduction of the ongoing EEG, presumably due to desynchronization of neurons. The method shows that ongoing EEG can be used for functional mapping of cortical areas and is therefore valuable in situations where stimulus locked activity can not be measured.