title: The hypothesis of neuronal interconnectivity as a function of brain size-a general organization principle of the human connectome.
abstract: Twenty years ago, Ringo and colleagues proposed that maintaining absolute connectivity in larger compared with smaller brains is computationally inefficient due to increased conduction delays in transcallosal information transfer and expensive with respect to the brain mass needed to establish these additional connections. Therefore, they postulated that larger brains are relatively stronger connected intrahemispherically and smaller brains interhemispherically, resulting in stronger functional lateralization in larger brains. We investigated neuronal interconnections in 138 large and small human brains using diffusion tensor imaging-based fiber tractography. We found a significant interaction between brain size and the type of connectivity. Structural intrahemispheric connectivity is stronger in larger brains, whereas interhemispheric connectivity is only marginally increased in larger compared with smaller brains. Although brain size and gender are confounded, this effect is gender-independent. Additionally, the ratio of interhemispheric to intrahemispheric connectivity correlates inversely with brain size. The hypothesis of neuronal interconnectivity as a function of brain size might account for shorter and more symmetrical interhemispheric transfer times in women and for empirical evidence that visual and auditory processing are stronger lateralized in men. The hypothesis additionally shows that differences in interhemispheric and intrahemispheric connectivity are driven by brain size and not by gender, a finding contradicting a recently published study. Our findings are also compatible with the idea that the more asymmetric a region is, the smaller the density of interhemispheric connections, but the larger the density of intrahemispheric connections. The hypothesis represents an organization principle of the human connectome that might be applied also to non-human animals as suggested by our cross-species comparison.
authors.affiliation: Division Neuropsychology, Department of Psychology, University of Zurich Zurich, Switzerland.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Hänggi
authors.affiliation: Division Neuropsychology, Department of Psychology, University of Zurich Zurich, Switzerland.
authors.forename: Laszlo
authors.initials: L
authors.lastname: Fövenyi
authors.affiliation: Division Neuropsychology, Department of Psychology, University of Zurich Zurich, Switzerland ; Research Unit for Neuroplasticity and Learning in the Healthy Aging Brain (HAB LAB), Department of Psychology, Institute of Psychology, University of Zurich Zurich, Switzerland.
authors.forename: Franziskus
authors.initials: F
authors.lastname: Liem
authors.affiliation: Research Unit for Neuroplasticity and Learning in the Healthy Aging Brain (HAB LAB), Department of Psychology, Institute of Psychology, University of Zurich Zurich, Switzerland.
authors.forename: Martin
authors.initials: M
authors.lastname: Meyer
authors.affiliation: Division Neuropsychology, Department of Psychology, University of Zurich Zurich, Switzerland ; Department of Psychology, International Normal Aging and Plasticity Imaging Center, University of Zurich Zurich, Switzerland ; Center for Integrative Human Physiology (ZIHP), University of Zurich Zurich, Switzerland ; University Research Priority Program, Dynamic of Healthy Aging, University of Zurich Zurich, Switzerland ; Department of Special Education, King Abdulaziz University Jeddah, Saudi Arabia.
authors.forename: Lutz
authors.initials: L
authors.lastname: Jäncke
keywords: corpus callosum; cross-species comparison; diffusion tensor imaging; elephant; functional specialization; hemispheric lateralization; human brain connectome; interhemispheric vs. intrahemispheric connectivity