First, given that exact connectivity is not known, we characterized the interneuronal connectivity statistically by requiring a fixed number of connections per neuron. As in any other theoretical analysis, we make several major assumptions. We show that the competing requirements for high connectivity and short conduction delay may lead naturally to the observed architecture of vertebrate brain as seen in mammalian neocortex and bird telencephalon.
Longer delays are detrimental because fewer computational steps can be performed within the time frame imposed on animals by the environment, making the brain a less powerful computational machine. In turn, this requires longer wiring, which, for the same conduction velocity, introduces longer delays. To see that high connectivity and short conduction delay are competing requirements, note that adding wiring to the network increases not only its volume, but also the distance between neurons. and Murre and Sturdy, including local and global connections, but minimized the conduction delay, i.e., the time that takes a signal (such as action potential and/or graded potential) to travel from one neuron's soma to another. In this paper, we adopted the model of connectivity introduced in Ruppin et al. Although wiring volume minimization is an important factor in the evolution of brain design, their results remain inconclusive because predictions of the volume minimization model for the present problem are not robust and are difficult to compare with empirical observations (see Discussion). They modeled the brain by a network consisting of local and global connections, which give rise to gray and white matter correspondingly. and Murre and Sturdy, have proposed that the segregation of white and gray matter could be a consequence of minimizing the wiring volume. The cost of wiring is due to metabolic energy required for maintenance and conduction, guidance mechanisms in development, conduction time delays and attenuation, and wiring volume. However, increasing connectivity requires adding wiring to the network, which comes at a cost. Brain functionality must benefit from higher synaptic connectivity, because synaptic connections are central for information processing as well as learning and memory, thought to manifest in synaptic modifications. We started with the assumption that evolution “tinkered” with brain design to maximize its functionality. Since such design is not observed, and invoking an evolutionary accident as an explanation has agnostic flavor, we searched for an explanation based on the optimization approach, which is rooted in the evolutionary theory. What is the evolutionary advantage of such segregation ? Networks with the same local and global connectivity could be wired so that global and local connections are finely intermixed. White matter contains global, and in large brains mostly myelinated, axons that implement global communication. Gray matter contains neuron somata, synapses, and local wiring, such as dendrites and mostly nonmyelinated axons. The theory not only provides a possible explanation for the structure of various brain regions such as cerebral cortex, neostriatum, and spinal cord, but also makes several testable predictions such as the scaling estimate of the cortical thickness.Ī ubiquitous feature of the vertebrate brain is its segregation into white and gray matter ( ). Using this postulate, they show quantitatively that the existence of many fast, long-range axons drives the segregation of the brain into gray and white matter. What is the evolutionary advantage of segregating the brain into white and gray matter rather than intermixing them? In this study, the authors postulate that brain functionality benefits from high synaptic connectivity and short conduction delays-the time required for a signal from one neuron soma to reach another. White matter contains long-range axons that implement global communication via often myelinated axons. Gray matter contains local networks of neurons that are wired by dendrites and mostly nonmyelinated local axons. Vertebrate brains generally contain two kinds of tissue: gray matter and white matter.
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