Predictors of neuropsychological change following anterior temporal lobectomy: Role of regression toward the mean

The purpose of this investigation was to identify the determinants of changes in visual-spatial, language, and memory function following anterior temporal lobectomy (ATL). Demographic (age, education, and IQ), epilepsy (age at onset), and surgical variables (laterality of resection and surgical outcome) were examined for their ability to predict postoperative changes in neuropsychological function. Particular interest was focused on the ability of an additional variable, initial level of performance, to predict postoperative cognitive change. This latter variable reflected the effects of a statistical artifact (regression toward the mean). The predictive ability of these variables was examined in three data sets involving overlapping groups of patients who underwent dominant or nondominant ATL. Results indicated that initial level of performance was the most powerful predictor of postoperative cognitive change on measures of cortical ability (visual-spatial and language function), whereas laterality of resection was not a significant predictor of postoperative cognitive decline. With regard to postoperative changes in memory function, initial level of performance remained a significant predictor. However, laterality of resection was also a consistent and significant predictor of postoperative impairments in memory ability. These results suggest that regression toward the mean exerts a significant effect on pre- to postoperative neuropsychological evaluations and needs to be taken into consideration in research with groups of patients as well as when interpreting the results of test findings from individual patients.