The pattern of higher FA and lower RD observed here in absence of differences in AD in the genu of the CC could be interpreted either in terms of a higher axonal density or a higher degree of myelination (cf. Beaulieu, 2002 and Jones et al., 2013). Higher axonal density, lower axonal caliber, as well as the higher selleck kinase inhibitor degree of myelination should be reflected in lower RD and therefore higher FA (cf. Jones et al., 2013). Indeed, it has been demonstrated in eight different fiber tracts in mice that myelin loss
alone (without axonal injury) can cause an increase in RD, while the AD remains unchanged (Song et al., 2002). Additionally, Song et al. (2005) evaluated the sensitivity of DTI parameters to detect the progression of myelin by testing demyelination and remyelination of corpus callosum in the mouse brain. FLT3 inhibitor Results demonstrated that radial diffusivity offers a specific assessment of demyelination and remyelination, as distinct from acute axonal damage. Thus,
a more specific disruption of myelin is implied when an increase in RD occurs without an accompanying increase in AD (cf., Madden et al., 2012). However, the interpretation of RD as indicator of myelination is not straightforward and should be avoided especially in regions of complex tissue architecture (Sasson et al., 2010 and Wheeler-Kingshott and Cercignani, 2009). We hence assume that the higher directionality of diffusion (as indicated by FA) is either due to differences in the
number of axons and/or in the degree of myelination in more intelligent men. Myelination of axons is known to increase the signal transmission speed (Waxman, 1977) and decrease the refractory time (time needed for repolarization before a new action potential can be supported by medroxyprogesterone the axon; Felts et al., 1997 and Sinha et al., 2006). Accordingly, the degree of myelination improves the integration of information across spatially distributed neural networks supporting cognitive and motor functions (Bartzokis et al., 2010, Fuster, 1999, Lu et al., 2011, Lu et al., 2013, Lutz et al., 2005, Mesulam, 2000 and Srinivasan, 1999). The higher degree of myelination in more intelligent men thus might account for more efficient brain functioning (cf., Miller, 1994). The relationship of intelligence with the efficiency of brain functioning has been studied intensely throughout the past 20 years. It led to the postulation of the neural efficiency hypothesis assuming negative IQ-brain activation relationship, cf. Neubauer and Fink, 2009a, Neubauer and Fink, 2009b and Dunst et al., 2014). This relationship, however, can be moderated by other factors such as sex and task content (Dunst et al., 2013, Jaušovec and Jaušovec, 2008, Lipp et al., 2012, Neubauer et al., 2010, Neubauer et al., 2002 and Neubauer et al., 2005).
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