Title: Towards a comprehensive structural human connectome: mapping short association fibres in the human brain.
The extraordinary number of short association fibres (SAF) connecting neighbouring cortical areas is a prominent feature of the large gyrified human brain. The contribution of SAF to the human connectome is largely unknown because of methodological challenges in mapping them. We present a multimodal method to characterise cortico-cortical connectivity mediated by SAF combining ultrahigh resolution diffusion weighted imaging, quantitative microstructural and functional MRI. We demonstrate, that SAF have microstructural properties distinct from those of long-range fibers. We introduce the `structural connective fields' (sCF) metric which specifically quantifies information transfer and integration mediated by SAF in topologicaly organized ares. This new metric complements functional connective field metrics encompassing integrated contributions from short- and long-range white matter and intracortical fibres. Applying the method in the human early visual processing stream, we show that SAF preserve cortical functional topology. Retinotopic maps of V2 and V3 could be predicted from retinotopy in V1 and SAF connectivity. The sCF sizes increased along the cortical hierarchy and were smaller than their functional counterparts. Applying the method in language network we demonstrate that short connections within Broca area can be mapped. In summary, our SAF mapping provides insights into short-range cortico-cortical connectivity in humans comparable to tract tracing studies in animal research and is an essential step towards creating a complete human connectome.