Lateralization of cognitive processes in the brain

The lateralization of cognitive processes in the brain is discussed. The tr aditional view of a language-visuo/spatial dichotomy of function between th e hemispheres has been replaced by more subtle distinctions. The use of mag netic resonance imagine (MRI) to study brain morphology has resulted in a r enewed focus on the relationship between structural and functional asymmetr y. Focus has been on the role played by the planum temporale area in the po sterior part of the superior temporal gyrus for language asymmetry, and the possible significance of the larger left planum. The dichotic listening te chnique is used to illustrate the difference between bottom-up, or stimulus -driven laterality versus top-down, or instruction-driven laterality. It is suggested that the hemispheric dominance observed at any time is the sum r esult of the dynamic interaction between bottom-up and top-down processing tendencies. Stimulus-driven laterality dominance is always monitored and mo dulated through top-down cognitive processes, like shifting of attention an d changes in arousal. A model of top-down modulation of bottom-up lateralit y is presented with special reference to the understanding of psychiatric d isorders. (C) 2000 Elsevier Science B.V. All rights reserved.