Mapping the structural and functional architecture of the brain

Abstract

It is intolerable that we do not have [a connectional map] for the human brain. Without it there is little hope of understanding how our brains work except in the crudest way.Francis Crick and Edward Jones, 1993 ([1Crick F. Jones E. Backwardness of human neuroanatomy.Nature. 1993; 361: 109-110Crossref PubMed Scopus (182) Google Scholar], p. 110) Although in vivo neuroimaging tools were already around at the time when Crick and Jones lamented the ‘backwardness of human neuroanatomy’ [1Crick F. Jones E. Backwardness of human neuroanatomy.Nature. 1993; 361: 109-110Crossref PubMed Scopus (182) Google Scholar], a connectional map of the sort they envisioned was nowhere in sight. However, some of the technical advances that Crick and Jones were calling for were already under way. The first diffusion MRI (dMRI) images of the human brain had been published nearly a decade earlier [2Le Bihan D. et al.MR Imaging of intravoxel incoherent motions: application to diffusion and perfusion in neurologic disorders.Radiology. 1986; 161: 401-407PubMed Google Scholar] and, in the decades that followed, dMRI became the key method for mapping the connectional structure of the brain at the macroscale. The year 1995 saw the publication of the first resting-state or intrinsic functional connectivity study [3Biswal B. et al.Functional connectivity in the motor cortex of resting human brain using echo-planar MRI.Magn. Reson. Med. 1995; 34: 537-541Crossref PubMed Scopus (6984) Google Scholar], and resting-state functional MRI (rfMRI) has since become a key tool for examining the functional connectome (Smith et al., this issue). Concurrent developments in electrophysiological recording methods in humans (both invasive and non-invasive) have provided a powerful toolbox for mapping brain connectivity. The term ‘connectome’ was introduced in 2005 to describe ‘the set of all neural connections’ (Hagmann, P. 2005. Doctoral Dissertation, École Polytechnique Fédérale de Lausanne, p. 108) or ‘the comprehensive structural description of the network of elements and connections forming the human brain’ ([4Sporns O. et al.The human connectome: a structural description of the human brain.PLoS Comput. Biol. 2005; 1: e42Crossref PubMed Scopus (2096) Google Scholar], p. 0245). Although it is a matter of debate whether the term ‘connectome’ can be felicitously applied to the patterns of functional connectivity in the brain [5Sporns O. The human connectome: origins and challenges.Neuroimage. 2013; 80: 53-61Crossref PubMed Scopus (246) Google Scholar], the field of human connectomics currently encompasses efforts to produce both structural and functional connectional maps of the brain. The articles in this issue address the human structural and functional connectome at the macroscale by identifying connectional patterns within and between anatomically distinct brain regions at the millimeter scale. They examine some core questions in human macroscale connectomics: how structural and functional connectivity are related (Uddin, this issue); how large-scale networks develop (Menon, this issue), how they evolved (Buckner and Krienan, this issue), and how they break down in psychopathology (Barch, this issue; Rubinov and Bullmore, this issue); what information can be extracted from functional networks (Haremelech and Malach, this issue); what information-processing principles the brain utilizes (Singer, this issue); what the role of anatomical hubs in large-scale brain organization and communication is (van den Heuvel and Sporns, this issue); and what the properties of task-related networks are (Zanto and Gazzaley, this issue). However, part of the excitement – and promise – of connectomics is the concerted effort to map the connectivity architecture of the brain at different scales, from the micro- (neurons and their synaptic connections) to the meso- (neural populations) to the macroscale. How to vertically integrate insights that arise from research across scales is a key challenge (Kim et al., this issue). The articles collected here provide a partial snapshot of current efforts to map the structural and functional architecture of the brain. They are complemented by several other articles we have published over the past few years on the connectome, which are collected on our website (http://www.cell.com/trends/cognitive-sciences/Connectome). This special issue is also my farewell issue, the last issue I put together as the editor of TiCS. And what better way to exit than with an issue that celebrates the shift from trying to decipher the brain by looking at its parts to probing brain function by considering it as an integrated system. I would like to thank the authors and reviewers who contributed to this issue and hope that you, the readers, will share in the excitement these contributions convey about the prospects of a fully resolved human connectome.