... DEVELOPMENTAL AND ADULT VARIATION IN NEURAL ORGANIZATION...

... focus can shift to the nonconserved features, the variable elements. In grappling with this variation, researchers often look for constraints and scaling principles (Striedter, 2005), and they seek to explain the variation in mechanistic terms....

... In Chapter 4, Erin Jarvis and colleagues review the segmental variation in arthropod appendages (mainly mouthparts and limbs) and its control by hox genes. They note that hox genes also control segmental variation in the motor neurons that control the various appendages. This ...

... Continuing in Chapter 5 the neuro-evo-devo theme, Luke McGowan and colleagues present results from an experiment in which they used intraventricular FGF2 injections to delay neurogenesis in the optic tectum of chicks....

... neurogenesis have likely led to cortical folding in large-brained mammals. However, the FGF2 injections also disrupt the normally smooth pattern of tectal lamination, which is unlikely to be adaptive. Intriguingly, ... disruptions are causally linked to ruptures in the overlying pia mater. Collectively, these findings imply that evolutionary increases in the size of brain regions must be coordinated with expansions of the associated pia mater, which may be difficult when neural expansion is caused by a delay in ...

... In Chapter 6, Leah Krubitzer and Adele Seelke focus on variability in cortical organization, both within species and across mammalian taxa. In addition to describing this variability, they analyze its phylogenetic pattern and underlying mechanisms. In particular, ... vary in specific ways rather than being freely variable. This finding would explain why many features of cortical organization are broadly conserved and why some variants evolved repeatedly and independently in diverse lineages. What sorts of mechanisms generate this variation and its constraints? As ... and Seelke review, both intrinsic genetic and extrinsic activity-dependent mechanisms are at play. Furthermore, variation in one part of the nervous system can induce changes in distant, ... related brain regions. For example, removal of the eyes during early development causes a dramatic reduction and functional respecification of the primary visual cortex. A similar cascade effect has been observed in blind mole rats. Thus, experimental ...

... of mammalian cortical variation, but his chapter is focused more explicitly on neocortical modules, which include cortical areas, patches, bands, stripes and interstripes, blobs and interblobs, and columns and minicolumns. Within each module, adjacent neurons tend to be activated by similar ... such boundaries can maximize the overall probability that adjacent neurons fire concordantly. As Kaas suggests, the mechanisms for topographic map and module formation seem to exist throughout mammalian neocortex but also in some other brain regions, such as the frog’s optic tectum....

... In Chapter 8, Suzana Herculano-Houzel steps back from the organizational details of mammalian brains and focuses instead on the number of...

... neurons and nonneurons (primarily glia) found in the major brain regions of various mammals. Using the isotropic fractionator method, which involves homogenizing ... regions and counting stained cell nuclei in samples from the resulting homogenate, she discovered that neuron numbers scale differently (against brain region ... ) in primates and rodents. This finding may explain why primates tend to be more intelligent than other mammals, even when brain mass is held constant: as brain size ... T. H. Huxley, if not Darwin himself. Moving beyond these findings, Herculano-Houzel proposes interesting ideas on the evolution of brain energy costs and their relationship to feeding behavior....