Neuropsychological evidence for the existence of cerebral areas critical to the performance of intelligence tasks

PERCY (1964, 1969) has suggested that the theories proposed by psychologists on the nature of intelligence may be relevant to understanding how intellectual skills are organized in the cerebral cortex. If intelligence is envisaged to be made up of a general, superordinate ability ("g " factor), which enters into every Intellectual performance, and of subsidiary factors specific to each individual test or group of tests (Spearman, 1923), then on an a priori basis two hypotheses regarding the relation of intelligence to brain and brain-damage may be considered. According to the first, which is inspired by the principles of equipotentiality and mass action set out by Lashley (1929), general ability is diffusely represented on the whole cortical mantle and injury to any region of the brain produces an intellectual decrement. Its severity is dependent on the size but not on the site of the lesion, except possibly for the contribution of subordinate factors to particular performance. A hierarchical view is, however, equally compatible with the hypothesis that in the course of evolution some area of the brain has gained a predominant, or even an exclusive role in sustaining the neuronal activity which mediates the general, superordinate ability. It follows from this assumption that damage to such an area would be reflected in the disruption of any type of intellectual performance, whereas lesion of other regions would at the most impair some specific task, subserved by a subordinate ability. The main alternative to the hierarchic theory is provided by the view that intelligence consists of distinct primary mental abilities (Thurstone, 1938), for which it seems logical to admit separate cortical localizations. Injury to different areas will, consequently, result in the derangement of different intellectual performances and any attempt to compare groups with differently localized lesions and their general mental efficiency would be meaningless