Hemispheric lateralisation in the human brain has been a focus of interest in different fields of neurosciences since a long time (Galaburda, LeMay, Kemper, & Geschwind, 1978; Rubino, 1970).
One of the most studied and earliest observed lateralised brain functions is language. Reported in the nineteenth by the French physician and anatomist Paul Broca (1861) and by the German anatomist and neuropathologist Carl Wernicke (1874), language was found to be more impaired following tumours or strokes in the left hemisphere.
In recent years, a number of studies have employed diffusion tensor imaging (DTI) to characterize left hemisphere language-related white matter pathways (Barrick, Lawes, Mackay, & Clark, 2007; Bernal & Altman, 2010; Catani et al., 2007; Glasser & Rilling, 2008; Hagmann et al., 2006; Parker et al., 2005; Propper et al., 2010; Upadhyay, Hallock, Ducros, Kim, & Ronen, 2008; Vernooij et al., 2007). In addition, lesion and fMRI studies in healthy subjects have indicated that speech comprehension and production are lateralised to the left brain hemisphere (A. U. Turken & Dronkers, 2011).
The main aim of the present doctoral work is to better delineate the relationship between anatomical and functional correlates of hemispheric dominance in the perisylvian language network. To this purpose a multi-modal neuroimaging approach including DTI and fMRI on a population of 23 healthy individuals was applied.
In the first study, a virtual in vivo interactive dissection of the three subcomponents of the arcuate fasciculus was carried out and measures of perisylvian white matter integrity were derived from tract-specific dissection. Consistently with previous studies (Barrick, et al., 2007; Buchel et al., 2004; Catani, et al., 2007; Powell et al., 2006), a significant leftward asymmetry in the fractional anysotropy (FA) value of the long direct segment of the arcuate fasciculus (AF) has been found. In addition, I found another significant leftward lateralisation in the streamlines (SL) of the posterior segment and a rightward distribution of the SL index of the anterior segment of the AF. Finally, I found no evidence of a significant relationship between the leftward lateralisation indeces and any measures of language and verbal memory performance in my group.
In the second study, I implemented functional connectivity analysis to test whether leftward lateralisation of connectivity indeces between perisylvian regions can be observed in individuals performing a language-related task. The main finding of the functional connectivity analysis is a significant rightward lateralisation (left, 0.347 ± 0.183; right, 0.493 ± 0.228; P = 0.037) in the anterior connection, between the the inferior frontal gyrus (IFG) and the inferior parietal lobe (IPG).
In the third study, I combined DTI and fMRI data to examine whether a significant relationship is present between these measures of perisylvian connectivity and it significantly differs between hemispheres. The correlation analysis demonstrated significant negative relations between the mean FA values in the long segment of the AF and the strength of inter-regional coupling between the IFG and the middle temporal gyrus (MTG) in the left hemisphere, and between the mean FA values in the anterior segment of the AF and the strength of regional coupling between IFG and IPL in the right hemisphere. Finally, there were no significant correlations between laterality indices estimated on FA and functional connectivity values.
