Abstract
The organisational and architectural configuration of white matter pathways connecting brain regions has ramifications for all facets of the human condition, including manifestations of incipient neurodegeneration. Although diffusion tensor imaging (DTI) has been used extensively to visualise white matter connectivity, due to the widespread presence of crossing fibres, the lateral projections of the corpus callosum are not normally detected using this methodology. Detailed knowledge of the transcallosal connectivity of the human cortical motor network has, therefore, remained elusive. We employed constrained spherical deconvolution (CSD) tractography—an approach that is much less susceptible to the influence of crossing fibres, in order to derive complete in vivo characterizations of white matter pathways connecting specific motor cortical regions to their counterparts and other loci in the opposite hemisphere. The revealed patterns of connectivity closely resemble those derived from anatomical tracing in primates. It was established that dorsal premotor cortex (PMd) and supplementary motor area (SMA) have extensive interhemispheric connectivity—exhibiting both dense homologous projections, and widespread structural relations with every other region in the contralateral motor network. Through this in vivo portrayal, the importance of non-primary motor regions for interhemispheric communication is emphasised. Additionally, distinct connectivity profiles were detected for the anterior and posterior subdivisions of primary motor cortex. The present findings provide a comprehensive representation of transcallosal white matter projections in humans, and have the potential to inform the development of models and hypotheses relating structural and functional brain connectivity.
Highlights
The corpus callosum (CC) is by far the largest white matter fibre bundle in the human brain
To further characterise the transcallosal structural connectivity represented by these fibre pathways, we delineated separately for each node the composition of the region, in terms of the percentage of all trajectories passing through the regions of interest (ROIs) that were connected to each of the other motor regions in the opposite hemisphere (Fig. 4)
The reconstructed fibre trajectories passing through a designated ROI, as derived by tractography, do not correspond to individual neurons as traced by the injection of compounds which highlight the specific locations at which their axons terminate, it is striking that the patterns of connectivity identified in the current investigation resemble closely those derived from neuroanatomical studies conducted in non-human primates (Boussaoud et al 2005; Fang et al 2008; Marconi et al 2003; Rouiller et al 1994)
Summary
The corpus callosum (CC) is by far the largest white matter fibre bundle in the human brain. Degeneration of the corpus callosum is a consistent feature of amyotrophic lateral sclerosis (ALS) (Filippini et al 2010), and the microstructural pathology of transcallosal projections within the corticomotor network provides a candidate imaging biomarker (Chapman et al 2014). Several cortical brain regions are known to contribute to different aspects of motor planning, learning and execution, and with many functional sub-divisions emerging, there is an increasing need to understand the network interactions between motor regions in one hemisphere with both their homologous counterparts, and heterogenous regions in the opposite hemisphere. As there is increasing evidence that brain structure predicts function (e.g., Honey et al 2010), the facility to extract reliable estimates of white matter organisation is of profound importance in resolving anatomical constraints upon functional motor network connectivity
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