![]() The MT +complex is located in the ventrolateral part of the posterior temporal cortex in the human brain (hMT+ Malikovic et al., 2016), but is located more dorsally in the ventral bank of the posterior superior temporal sulcus in the macaque monkey ( Paxinos et al., 2000) ( Figure 2, left panel). ![]() A particularly challenging case is presented by areas sensitive to visual motion. Although the early visual areas are present in both humans and macaques, their location and the amount of cortical territory they occupy differs in the two species ( Orban et al., 2004). We first investigated whether the connectivity blueprints could be used to identify known homologs between the two species. This method thus provides a powerful approach to comparative anatomy, and allows one to quantitatively define common principles and unique specializations in the brains of related animals. Our results show how connectivity blueprints can be used for comparative anatomy of humans and macaques, but the approach can be generalized to all higher primates where the blueprints can be identified. We demonstrate that such areas overlap with known specializations in the human and macaque lineages. Furthermore, by quantifying the distances between the blueprints of different parts of the two brains, we quantify where these brains have tended to specialize since their last common ancestor. We demonstrate that the connectivity blueprints can be used to predict the location of cortical areas across species. We illustrate this approach by comparing human and macaque cortex. ( d) These blueprints can then be compared using the KL divergence as a comparative metric indicating how each vertex' connectivity fingerprint in one brain differs from that of each vertex in an other brain. ( b) The resulting connectivity matrices were then multiplied by connectivity matrices defining the connectivity of each vertex of the grey matter to the rest of the brain, creating a full connectivity blueprint ( c) describing how each vertex is connected to each tract. ( a) 39 tracts common across both species were defined and reconstructed using probabilistic tractography. These connectivity blueprints provide a common space in which we can ask how each part of the grey matter in one species maps onto the other species. ![]() We term the matrix describing the connectivity of each vertex of the grey matter surface with each white matter tract the connectivity blueprint. This allowed us to construct a map of each of the main white matter tracts and to describe cortical grey matter organization in terms of this map ( Figure 1). The bodies of the major fiber bundles can be identified reliably in different species and allow identification of homologous fiber bundles. Thus, we can investigate neural organization using the architecture of the main white matter fibers. We exploit the idea that cortical regions can be described by their unique sets of connections to the rest of the brain ( Passingham et al., 2002), a feature that we have previously shown is useful in comparing brain organization between species ( Mars et al., 2016). We propose that common white matter pathways can be used to form blueprints of cortical connections to enable comparisons of cortical organization between higher primates. These pathways share core properties such as the broad brain areas that they connect, but differ in the details of their branching patterns, suggesting a common connectivity backbone with varying degrees of connectivity specialization. Several association pathways have been identified in humans, chimpanzees, and macaques ( Rilling et al., 2008 Hecht et al., 2013). In higher primates, white matter organization has striking commonalities between the different species ( Thiebaut de Schotten et al., 2012). Unique properties and adaptations of a species’ brain can then be understood as deviations from the template. To understand what is unique about the brain of a given species, a useful starting point is to cast it in the context of a common template. However, the study of specific adaptations cannot be performed without an appreciation of the common organizational principles of different brains. The ultimate goal of comparative and evolutionary neuroscience is to understand the organization of each species' brain as an adaptation to its unique ecological niche.
0 Comments
Leave a Reply. |