澳大新語 • 2022 UMAGAZINE 26 22 封面專題 • COVER STORY This prompted Broca to undertake in-depth studies that suggested that different regions of the brain have different functions, contrary to the prevailing view at that time that all parts of the brain are essentially the same in terms of their functions. Broca’s argument, later known as cerebral localisation, had a profound impact on the development of brain research. When two species have certain parts that share a common evolutionary origin, these parts are called homologous. Prof Yuan’s team identified 23 pairs of homologous bundles of nerve fibres (nerve tracts) in the Broca area between humans and macaques at the voxel level, after analysing MRI data from overseas brain atlases. In other words, they discovered mapping relationships between the voxel-level tracts in the Broca areas of the two primates. The researchers then examined the structural connectivity of these homologous tracts, and found that the two primates have greater variability in the pars triangularis than in the pars opercularis. This suggests that pars triangularis is likely to be a more active area in primate evolution. When the researchers looked at the human brains only, they found greater variability in the pars triangularis than in the pars opercularis, which also holds for macaque brains. The left and right brains differ in morphological attributes, connection, and function, a phenomenon known as ‘hemispheric asymmetry’ or ‘lateralisation’, says Prof Yuan. Their team also investigated how much these homologous tracts differ in the left brain from those in the right brain. The researchers found that variabilities in the brains of humans and macaques are smaller in the pars triangularis than in the pars opercularis. At the same time, they discovered that language-related tracts in the dorsal stream (a visual pathway involved in our ability to locate objects) are quite different in the two primates, while language-related tracts in the ventral stream (a visual pathway involved in our ability to recognise objects) are rather similar. ‘These findings can lead us to a deeper understanding of why only humans have certain high cognitive skills,’ says Prof Yuan. A Cross-species Connectome To further advance connectome research, it is important to create more accurate maps that define specific areas of the brain (i.e., parcellations). To tackle this issue, Prof Yuan’s team has developed two parcellations of humans and macaques. ‘With the parcellations and our other research outcomes, we are steadily advancing towards the creation of a cross-species connectome at the whole-brain level,’ he says. ‘We will continue to investigate the neural mechanisms underlying language abilities in several projects, including mapping the brain at different scales. Breakthroughs in these fields will not only help us treat brain diseases and develop smarter technologies, but also figure out what makes our species so unique.’ 不同獼猴腦部在結構連接方面的差異性 Variability in structural connectivity between different macaque brains 熒光位置為人類與獼猴腦部的部分同源位點 The fluorescent parts are some of the homologues between human and macaque brains
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