The hypothesis that electric and magnetic brain activity in the gamma range is related to cognitive processing is based mainly on experiments with complex stimuli. Our report, however, demonstrates the existence of gamma responses to "extremely simple" visual stimuli in several intracranial structures of the cat brain. This implies that the functional correlates of gamma activity are diverse, encompassing cognitive as well as primary sensory processing. The ubiquity of gamma responses strongly suggests the universal importance of gamma rhythms as functionally relevant elementary signals of the brain.