Asymmetry of white matter pathways in the brain
In book: The Two Halves of the Brain. 2010-01-01; :
Here are eight instances in which the lesion was in the posterior third of the third frontal con-volution. This number seems to me to be sufficient to give strong presumptions. And the most remarkable thing is that in all the patients the lesion was on the left side. I do not dare draw conclusions from this. —Paul Broca (1863) With these words, from a short report of a series of patients with acquired speech deficits, begins the modern period of the study of cerebral asymmetry. Despite Broca’s reticence to draw any conclusion from his clinical–anatomical observation, his words clearly allude to a concept that has stood the test of time: the asymmetrical distribution of functions in the human brain. In later writings, Broca not only vehemently defended his idea of left lateralization of speech but initiated the discussion on the anatomical correlates of cerebral dominance (Finger, 1994). This became an intensely growing field of research where anatomists focused their attention on either macro-scopic (e.g., volume of gyri) or microscopic (e.g., cytoarchitectonic) differences between the two hemispheres. However, a handful of researchers faithful to their belief on the importance of brain connections have tried to explain cerebral dominance in terms of of white matter asymmetry. Their efforts have often been limited to mere speculation , for the availability of reliable methods to trace connections in the human brain have been lacking for decades. Recent developments in magnetic resonance imaging (MRI) have introduced new methods, based on diffusion imaging tractography that can reconstruct white matter trajectories in the living human brain (Basser et al., 2000; Le Bihan, 2003). The resultant influx of information on human connectional anatomy derived from tractography is likely to fill the gap on our anatomical knowledge of human brain connections and reinvigorate models of cognition based on asymmetrical distribution of large-scale networks (Catani & Mesulam, 2008). An overview of the hodological (pathway-based) approach to cerebral dominance and its historical context, with a special focus on the perisylvian networks, constitutes the subject matter of this chapter.