Brain morphology and intelligence

Of differing methods, disputed estimates and discordant interpretations: the meta-analytical multiverse of brain volume and IQ associations

Brain size and IQ are positively correlated. However, multiple meta-analyses have led to considerable differences in summary effect estimations, thus failing to provide a plausible effect estimate. Here we aim at resolving this issue by providing the largest meta-analysis and systematic review so far of the brain volume and IQ association (86 studies; 454 effect sizes from k = 194 independent samples; N = 26 000+) in three cognitive ability domains (full-scale, verbal, performance IQ). By means of competing meta-analytical approaches as well as combinatorial and specification curve analyses, we show that most reasonable estimates for the brain size and IQ link yield r-values in the mid-0.20s, with the most extreme specifications yielding rs of 0.10 and 0.37. Summary effects appeared to be somewhat inflated due to selective reporting, and cross-temporally decreasing effect sizes indicated a confounding decline effect, with three quarters of the summary effect estimations according to any reasonable specification not exceeding r = 0.26, thus contrasting effect sizes were observed in some prior related, but individual, meta-analytical specifications. Brain size and IQ associations yielded r = 0.24, with the strongest effects observed for more g-loaded tests and in healthy samples that generalize across participant sex and age bands.

Meta-analysis of associations between human brain volume and intelligence differences: How strong are they and what do they mean?

Positive associations between human intelligence and brain size have been suspected for more than 150 years. Nowadays, modern non-invasive measures of in vivo brain volume (Magnetic Resonance Imaging) make it possible to reliably assess associations with IQ. By means of a systematic review of published studies and unpublished results obtained by personal communications with researchers, we identified 88 studies examining effect sizes of 148 healthy and clinical mixed-sex samples (>8000 individuals). Our results showed significant positive associations of brain volume and IQ (r=.24, R(2)=.06) that generalize over age (children vs. adults), IQ domain (full-scale, performance, and verbal IQ), and sex. Application of a number of methods for detection of publication bias indicates that strong and positive correlation coefficients have been reported frequently in the literature whilst small and non-significant associations appear to have been often omitted from reports. We show that the strength of the positive association of brain volume and IQ has been overestimated in the literature, but remains robust even when accounting for different types of dissemination bias, although reported effects have been declining over time. While it is tempting to interpret this association in the context of human cognitive evolution and species differences in brain size and cognitive ability, we show that it is not warranted to interpret brain size as an isomorphic proxy of human intelligence differences.

Volumetric and voxel-based morphometry findings in autism subjects with and without macrocephaly

This study sought to replicate Herbert et al. (2003a), which found increased overall white matter (WM) volume in subjects with autism, even after controlling for head size differences. To avoid the possibility that greater WM volume in autism is merely an epiphenomena of macrocephaly overrepresentation associated with the disorder, the current study included control subjects with benign macrocephaly. The control group also included subjects with a reading disability to insure cognitive heterogeneity. WM volume in autism was significantly larger, even when controlling for brain volume, rate of macrocephaly, and other demographic variables. Autism and controls differed little on whole-brain WM voxel-based morphometry (VBM) analyses suggesting that the overall increase in WM volume was non-localized. Autism subjects exhibited a differential pattern of IQ relationships with brain volumetry findings from controls. Current theories of brain overgrowth and their importance in the development of autism are discussed in the context of these findings.

A quantitative magnetic resonance imaging analysis of the cerebellar deficit hypothesis of dyslexia

Recent evidence suggests that the primary source of dysfunction in dyslexia is the cerebellum. To examine the cerebellar deficit hypothesis of dyslexia, 20 children with dyslexia and 20 children without dyslexia were assessed using neuropsychological testing and quantitative magnetic resonance imaging. Results demonstrated that the volumes of both hemispheres and the vermis were not statistically significantly different between groups. However, children without dyslexia demonstrated greater rightward cerebellar hemisphere asymmetry. The relationship between cerebellar morphologic structure and phonological processing was assessed. For children without dyslexia, bilateral hemisphere volume moderately correlated with phonological awareness and phonological short-term memory. Hemisphere asymmetry moderately correlated with rapid naming errors, and the anterior vermis volume moderately correlated with phonological awareness. For children with dyslexia, the only statistically significant correlation was between rapid naming errors and the left hemisphere volume. Evidence suggests that atypical cerebellar morphologic structure may have a role in dyslexia for a subgroup of individuals. Although children with and without attention-deficit/hyperactivity disorder did not differ in cerebellar morphologic structure, the anterior vermis volume moderately correlated with inattention, hyperactivity, and impulsivity, while the right hemisphere volume moderately correlated with inattention and hyperactivity. Our findings provide mixed support for the cerebellar deficit hypothesis of dyslexia. Although cerebellar morphologic structure is atypical in some individuals with dyslexia, it is inconsistently related to cognitive or motor dysfunction. In our sample, cerebellar morphologic structure may be related to about one-third of cases of dyslexia. Hence, dyslexia may be best accounted for by a combination of cortical and cerebellar contributions.

Premorbid intellectual functioning, education, and brain size in traumatic brain injury: an investigation of the cognitive reserve hypothesis

Cognitive reserve theories have been postulated in an attempt to explain individual differences in functional outcome following cerebral insult or disease. These theories suggest that higher education and psychometric intelligence may preserve functional capacity regardless of injury or disease severity. This study investigated cognitive reserve in 25 participants with traumatic brain injury (TBI) using high-resolution magnetic resonance imaging (MRI) analyses. We examined the relationships between total intracranial volume (TICV), ventricle-tobrain ratio (VBR), education level, and standardized testing obtained prior to injury with post-injury cognitive outcome. Participants with lower post-injury IQ scores had significantly lower TICV values, irrespective of injury severity, and experienced significantly greater change in IQ from pre- to post-injury. TICV and education correctly predicted participants' post-injury IQ category ( Y 90 or < 90). However, premorbid standardized testing (PST) scores did not predict cognitive outcome. The results of this study suggest that larger premorbid brain volume and higher education level may decrease vulnerability to cognitive deficits following TBI, consistent with the notion of a cognitive reserve.

Technology in the assessment of learning disability

Recent neuroradiologic and brain imaging technologies, along with methods for displaying electrophysiologic data, have promulgated active exploration in the assessment of learning disability with attempts to improve diagnostic precision and elucidate the neurobiological substrates of learning disorders. The following article reviews these techniques and explores the research that has been conducted in this area over the past two decades. Initial investigations attempted to elucidate irregularities or abnormalities of brain morphology in individuals with learning disability utilizing computerized tomography (CT). The current standard for structural imaging of the brain is magnetic resonance (MR) imaging, which has allowed for greater specificity in identifying brain abnormalities. More recently, functional magnetic resonance imaging (fMRI) has been postulated as holding some promise in distinguishing anatomic/function differences in LD. Electrophysiological (EEG) and metabolic imaging techniques offer methods by which human brain activity can be studied during cognitive processes. Computerized EEG studies including evoked potentials (EP) or event-related potentials (ERP), spectral EEG analysis, and topographic EEG brain mapping have also identified a number of brain irregularities in individuals with learning disabilities, though no consistent exemplars have emerged. Studies with positron emission tomography (PET) and single photon emission computerized tomography (SPECT) have also demonstrated a number of abnormalities and inconsistencies in individuals with learning disabilities, but, again, no systematic research has demonstrated specific diagnostic abnormalities. Though inroads and some consistent patterns have begun to emerge in the assessment of learning disability with the preceding technologies, a number of challenges persist with neuroimaging and neurophysiological and metabolic imaging techniques. To date, no diagnostic conclusions have been drawn utilizing these methods in the assessment of the neurobiologic basis to LD.