Mental rotation and the right hemisphere.

Mental rotation may be considered a prototypical example of a higher-order transformational process that is nonsymbolic and analog as opposed to propositional. It is therefore a paradigm case for testing the view that these properties are fundamentally right-hemispheric. Evidence from brain-imaging, unilateral brain lesions, commissurotomy, and visual-hemifield differences in normals is reviewed. Although there is some support for a right-hemispheric bias, at least for the holistic rotation of relatively simple shapes, it is unlikely that this bias approaches the degree of left-hemispheric dominance for language-related skills. An evolutionary scenario is sketched in which the characteristically symbolic mode of the left hemisphere evolved relatively late and achieved the quality of recursive generativity only in the late stages of hominid evolution. This forced an increasingly right-hemispheric bias onto analog processes like mental rotation. Such processes nevertheless remain important and are integral even to those processes we think of as highly symbolic, such as language and mathematics.