Relationships between measures of brain functions and general intelligence.

Relationships between measures of general intelligence (Wechsler-Bellevue Verbal IQ, Performance IQ, and Full-scale IQ) and a measure designed to reflect adequacy of brain functions (Halstead Impairment Index) were studied using a control group and groups with left, right and generalized cerebral lesions. The results confirmed earlier findings of (1) differential levels of Verbal and Performance intelligence depending on damage of the left or right cerebral hemisphere; and (2) the greater general sensitivity of the Impairment Index than IQ values to brain damage. Comparative data suggested particularly that IQ values may underestimate brain-dependent adaptive abilities among non-brain-damaged subjects. A significant relationship between IQ values and the Impairment Index was present. This finding permitted evaluation of use of a differing cut-off Impairment Index, as an indicator of cerebral damage, depending on the subject's IQ level. The results suggested that increased accuracy in classifying subjects as brain-damaged might be achieved using a cutting Impairment Index of .4 or greater when the IQ value is 100 or more and .5 or greater when the IQ value is below 100.