This paper deals with the functional correlates of the gamma response of the brain. A critical review of the literature findings reveals the existence of two types of gamma responses: an early gamma that fulfills sensory functions and a late gamma that fulfills perceptual-cognitive functions. However, even the early gamma shows individual differences. Such a finding points to the existence of top-down influences on sensory processes and to a parallel-processing model for brain function. (C) 2001 Elsevier Science B.V. All rights reserved.