663 affected by a specific impairment because of the use of a global score based on different subtests. 6,7 More impor- tantly, the concept of intelligence does not change with age and tests for children and adults yield IQ quotients that are of very similar in magnitude and are highly cor- related (see the technical manual of the Wechsler scales), making this function suitable for a developmental study. Studies of the effect of age on IQ in brain lesioned cohorts have been inconsistent. Some supported the Kennard principle, 8,9 others refuted it, 10-15 some indicated that the relation depends on other factors such as the side of the lesion, 16-18 and finally many failed to find an asso- ciation between age and IQ. 8,19-25 Interpretation of these results is difficult given the many limitations of these stud- ies: (1) many investigations were not designed to address this specific question; (2) the sample sizes were often small; (3) the etiology of the lesion varied greatly from 1 study to another; (4) the comparison point differed greatly, some comparing clinical groups to each other (eg, right vs left lesions), some comparing a clinical group to a control group, and others comparing the clinical group to the test’s normative sample; and (5) age groups were also inconsis- tent. An early lesion in 1 study can be considered a late lesion in another. These studies are also mostly based on juvenile samples, whereas the Kennard principle concerns the prognosis of children compared with that of adults. To our knowledge, only 2 investigations included both pediatric and adult populations. Mosch et al 26 matched 29 children having sustained a focal brain lesion with adults having sustained a comparable lesion (in terms of etiology, site, and volume). No difference was found on F ollowing Kennard’s studies on motor function in pri- mates, 1,2 scientists and clinicians believed that a lesion causes less impairment in children than in adults (Kennard principle). Better outcome in children was attrib- uted to the child’s neuroplasticity, which enables the brain to anatomically adapt to the damage. Although this principle has been found to have many lim- its and has often been refuted, it is nonetheless a shared con- viction among professionals. Indeed, studies have shown that health care professionals believe in the Kennard principle 3,4 and that this belief may even influence legal practice in cases of injury compensation claims. 5 Previous research, including that by Kennard, has shown that the effect of age on outcome after a brain lesion depends on many other factors, including the func- tion under study. Intellectual outcome after a brain injury is particularly interesting for several reasons. It is a global and complex measure that has been found to be a good predictor of global adaptation. It is less likely to be Original Article Brain Lesions and IQ: Recovery Versus Decline Depends on Age of Onset Julie Duval, PhD, Claude M. J. Braun, PhD, Isabelle Montour-Proulx, PhD, Sylvie Daigneault, PhD, Isabelle Rouleau, MSc, and Jean Bégin, PhD A growing literature suggests that early lesions are associated with poorer IQ outcome. Those studies covered a restricted age range in pediatric populations only and did not control for important moderator variables. The present investigation stud- ied IQ change in brain-lesioned children and adults (age 0 to 84 years). Altogether, 725 cases with a documented unilateral focal lesion were gathered from hospital charts and from published cases in the literature, including 240 with repeated IQ testing. Multiple regression analyses isolated the contribution of age at lesion onset to IQ change. Important mediator variables included were lesion side, site, volume, etiology, and so on. An early lesion was significantly associated with poorer postlesion IQ in time and with decline of IQ in time. Later onset lesions were associated with better postlesion IQ and recovery in time. The so-called Kennard principle is refuted, with regard to IQ. Keywords: intelligence; brain lesion; age; Kennard princi- ple; longitudinal outcome From the Université du Québec à Montréal, Département de psychologie, (JD, CMJB, IR, JB); Children’s Hospital of Eastern Ontario, Oncology Patient Service Unit, Ottawa (IM-P); Hôpital de Montréal pour Enfants, Centre universitaire de santé McGill, Département de psychologie, Montréal, Québec, Canada (SD). Address correspondence to: Claude M. J. Braun, PhD, Centre de Neuro- sciences Cognitives, UQAM, C.P 8888, Succ. Centre-Ville, Montréal, Québec, Canada, H3C 3P8; e-mail: braun.claude@uqam.ca. Duval J, Braun CMJ, Montour-Proulx I, Daigneault S, Rouleau I, Bégin J. Brain lesions and IQ: Recovery versus decline depends on age of onset. J Child Neurol. 2008;23:663-668. 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