Gray matter intracortical connectivity includes both branched axonal ramifications within the gray matter (typically at lengths of 02 mm (Braitenberg and Schuz, 2013), and the intrinsic horizontal connectivity that can reach up to 8 mm within the gray matter along unbranched axons that run parallel to the cortical surface (Gonzlez-Burgos, Barrionuevo, Lewis, 2000, Melchitzky, GonzlezBurgos, Barrionuevo, Lewis, 2001, Voges, Guijarro, Aertsen, Rotter, 2010, Voges, Schüz, Aertsen, Rotter, 2010, Braitenberg and Schuz, 2013). The horizontal gray matter connections can terminate in patches across brain regions, resulting in anisotropic propagation of brain activity throughout the gray matter and thus playing a role in first order processing among sensory areas (Voges and Perrinet, 2012). White matter connectivity is usually derived from dMRI and tractography analysis and includes both streamlines of lengths less than 40 mm (Bajada et al., 2019), including U fibers, known to be unreliably counted due to limitations in dMRI spatial resolution (Gigandet et al., 2008), and medium and long-range streamlines that are 80160 mm long, believed to be associated with areas involved in functional integration (Bajada et al., 2019). Note that to show U-fibers as short as 3 mm requires a specific tractography algorithm parameter of step size less than 1 mm and a high resolution MRI, which differs from the standard protocol (Song et al., 2014). In our study, we use the term local connectivity to refer to intrinsic horizontal gray matter connectivity of length 16 mm, but not including the single cortical column scale of less that 1 mm. This choice is consistent with the cortical surfaces used herein, whereby the average edge lengths are 2.9 mm, 3.1 mm and 5.6 mm for fsaverage5, cvs_avg35 and fsaverage4, respectively (see also Suppl. Figure 1C). We use the term long-range connectivity to refer to any white matter connection fibers with a minimum track length of 10 mm. This lower bound of 10 mm for a tractography step size of 2 mm is consistent with previous seminal work wherein tracks shorter than 5 mm were discarded when a tractography step size of 1 mm was used (Zalesky et al., 2010) in order to avoid the inclusion of spurious tracks (see Suppl. Figure 1B). The shortest of the white matter connections are referred to as short-range white matter connections, but should not be confused with the local gray matter connectivity described above.

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