Abstract With the advancement of image acquisition and analysis methods in recent decades, unique opportunities have emerged to study the neuroanatomical correlates of intelligence. Traditional approaches examining global measures have been complemented by insights from more regional analyses based on pre-defined areas. Newer state-of-the-art approaches have further enhanced our ability to localize the presence of correlations between cerebral characteristics and intelligence with high anatomic precision. These in vivo assessments have confirmed mainly positive correlations, suggesting that optimally increased brain regions are associated with better cognitive performance. Findings further suggest that the models proposed to explain the anatomical substrates of intelligence should address contributions from not only (pre)frontal regions, but also widely distributed networks throughout the whole brain. Keywords: Brain, Cerebral Cortex, Corpus Callosum, MRI

Introduction The substrate of human intelligence has been a topic of considerable interest to scientists, philosophers, and physicians from the time of the ancient Greeks. The advent of modern brain imaging technologies and the development of advanced analysis methods have since provided unique opportunities to examine the biological essence of human intelligence. The goal of this article is to summarize macro-structural associations with intelligence based on magnetic resonance imaging (MRI) data from healthy individuals. However, rather than providing an exhaustive overview, we aimed to demonstrate a variety of research findings using traditional and modern approaches that allow us to analyze correlations with different degrees of regional specificity. This review encompasses studies investigating relationships between global (e.g., brain volume), regional (e.g., lobar volume), and highly localized (e.g., voxel-level) brain measurements and intelligence. Measures cover diverse brain structures (e.g., whole brain, cerebral cortex, corpus callosum) and cerebral characteristics (e.g., volume, concentration, thickness). A related goal of this article is to highlight that many questions regarding the biological substrates of intelligence remain unresolved – not only due to partly overlapping results across studies and the sparseness of research findings, but also due to limitations in the spatial resolution of the images and other acquisition parameters. Thus, conclusions with respect to the underlying neural architecture and its possible impact on higher cognitive function remain speculative.

6. Future Developments Modern neuroimaging methods have contributed considerably to our understanding of the neurobiology of intelligence. In addition to structural MRI (which constituted the main focus of this article) other imaging techniques will further contribute to elucidating the physical correlates of human intelligence and have the potential to resolve how structural, physiological, histological, or neuro-chemical characteristics of the brain are interrelated. Systematically combining structural MRI with functional MRI (fMRI) or with positron emission tomography (PET) could add information regarding whether more or less tissue in a particular region is associated with more or less activation during a given cognitive task. Moreover, applying magnetic resonance spectroscopy (MRS) in combination with structural MRI or with diffusion tensor imaging (DTI) could help to clarify how regional concentrations of particular neurometabolites are related to other white matter measurement (e.g., fractional anisotropy; concentration; volume). Last but not least, imaging genomics could provide unprecedented opportunities to establish common biological determinants for both brain structure and higher cognitive function. The development of such methods allowing cross-correlation of region-specific measurements will be a valuable step in the still-emerging field linking brain anatomy with intelligence.

Footnotes 1Obviously, intelligence is not driven by global enlargement per se but by some optimal size increase that represents the importance of size × intelligence relationships. For example, macrocephaly (without any identifiable developmental errors) occurs more frequently in autism where intellectual abilities are compromised. The discussed findings in this article are mainly based on healthy samples; thus we will abstain from using the phrase “optimally” increased (which is what we mean) to simplify matters. 2To our knowledge, only one VBM study analyzing the relationship between brain tissue and intelligence has reported significant negative correlations that were observed for gray matter density in the caudate nucleus (Frangou et al., 2004). Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.