One tract, two tract, old tract, new tract: A pilot study of the structural and functional differentiation of the inferior fronto-occipital fasciculus

The inferior fronto-occipital fasciculus (IFOF), a major ventral white matter pathway, has been shown to be a crucial component of semantic (Moritz-Gasser, Herbet & Duffau, 2013) and lexical/orthographic (Vandermosten, Boets, Polemans, Sunaert, Wouters & Ghesquière, 2012) processing. However, recent anatomical studies of the brain have revealed at least two differentiable components of the IFOF: a dorsal component projecting from the frontal lobe to the superior parietal lobule and a ventral component connecting the frontal lobe with the inferior occipital gyrus and posterior temporal lobe (Martino, Brogna, Robles, Vergani & Duffau, 2010). We have replicated this anatomical division using a deterministic tractography protocol in DTI Studio with high inter-rater reliability (ICC > 0.9). Furthermore, we provide the first evidence of a functional distinction between these two components. We compared diffusion measures (fractional anisotropy [FA], as well as MD, AD, RD) with reaction times on five different reading tasks: basic naming of pure exception words, regular words, and mixed exception/regular words, and go/no-go tasks involving either pseudohomophone or nonword foils. We found a functional divide in the left IFOF, whereby dorsal FA was correlated with performance on tasks that required higher levels of visual attention and response selection (go/no-go and mixed naming tasks), while ventral FA was more broadly correlated with naming performance. This suggests that the anatomical distinction described by Martino et al. (2010) is indeed mirrored by a functional distinction, and indicates that future investigations of neuroanatomical models of reading and speech production should consider the dorsal and ventral IFOF as separate entities.