Googling the brain : discovering hierarchical and asymmetric network structures, with applications in neuroscience
Crofts, J.J. and Higham, D.J. (2011) Googling the brain : discovering hierarchical and asymmetric network structures, with applications in neuroscience. Internet Mathematics, 7 (4). pp. 233254.
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Abstract
Hierarchical organisation is a common feature of many directed networks arising in nature and technology. For example, a welldefined messagepassing framework based on managerial status typically exists in a business organisation. However, in many realworld networks such patterns of hierarchy are unlikely to be quite so transparent. Due to the nature in which empirical data is collated the nodes will often be ordered so as to obscure any underlying structure. In addition, the possibility of even a small number of links violating any overall “chain of command” makes the determination of such structures extremely challenging. Here we address the issue of how to reorder a directed network in order to reveal this type of hierarchy. In doing so we also look at the task of quantifying the level of hierarchy, given a particular node ordering. We look at a variety of approaches. Using ideas from the graph Laplacian literature, we show that a relevant discrete optimization problem leads to a natural hierarchical node ranking. We also show that this ranking arises via a maximum likelihood problem assiocated with a new rangedependent hirearchical random graph model. This random graph insight allows us to compute a likelihood ratio that quantifies the overall tendency for a given network to be hierarchical. We also develop a node ordering algorithm based on the combinatorics of directed walks. In passing, we note that Google’s PageRank algorithm tackles a closely related problem, and may also be motivated from a walkcounting viewpoint. We illustrate the performance of the Hierarchical organisation is a common feature of many directed networks arising in nature and technology. For example, a welldefined messagepassing framework based on managerial status typically exists in a business organisation. However, in many realworld networks such patterns of hierarchy are unlikely to be quite so transparent. Due to the nature in which empirical data is collated the nodes will often be ordered so as to obscure any underlying structure. In addition, the possibility of even a small number of links violating any overall “chain of command” makes the determination of such structures extremely challenging. Here we address the issue of how to reorder a directed network in order to reveal this type of hierarchy. In doing so we also look at the task of quantifying the level of hierarchy, given a particular node ordering. We look at a variety of approaches. Using ideas from the graph Laplacian literature, we show that a relevant discrete optimization problem leads to a natural hierarchical node ranking. We also show that this ranking arises via a maximum likelihood problem assiocated with a new rangedependent hirearchical random graph model. This random graph insight allows us to compute a likelihood ratio that quantifies the overall tendency for a given network to be hierarchical. We also develop a node ordering algorithm based on the combinatorics of directed walks. In passing, we note that Google’s PageRank algorithm tackles a closely related problem, and may also be motivated from a walkcounting viewpoint. We illustrate the performance of theHierarchical organisation is a common feature of many directed networks arising in nature and technology. For example, a welldefined messagepassing framework based on managerial status typically exists in a business organisation. However, in many realworld networks such patterns of hierarchy are unlikely to be quite so transparent. Due to the nature in which empirical data is collated the nodes will often be ordered so as to obscure any underlying structure. In addition, the possibility of even a small number of links violating any overall “chain of command” makes the determination of such structures extremely challenging. Here we address the issue of how to reorder a directed network in order to reveal this type of hierarchy. In doing so we also look at the task of quantifying the level of hierarchy, given a particular node ordering. We look at a variety of approaches. Using ideas from the graph Laplacian literature, we show that a relevant discrete optimization problem leads to a natural hierarchical node ranking. We also show that this ranking arises via a maximum likelihood problem assiocated with a new rangedependent hirearchical random graph model. This random graph insight allows us to compute a likelihood ratio that quantifies the overall tendency for a given network to be hierarchical. We also develop a node ordering algorithm based on the combinatorics of directed walks. In passing, we note that Google’s PageRank algorithm tackles a closely related problem, and may also be motivated from a walkcounting viewpoint. We illustrate the performance of the resulting algorithms on synthetic network data, and on a realworld network from neuroscience where results may be validated biologically.
ORCID iDs
Crofts, J.J. and Higham, D.J. ORCID: https://orcid.org/0000000266353461;

Item type: Article ID code: 31418 Dates: DateEvent2011PublishedKeywords: hierarchical , network structures, asymmetric , chain of command, Probabilities. Mathematical statistics Subjects: Science > Mathematics > Probabilities. Mathematical statistics Department: Faculty of Science > Mathematics and Statistics Depositing user: Pure Administrator Date deposited: 02 Jun 2011 13:24 Last modified: 23 Sep 2021 04:00 URI: https://strathprints.strath.ac.uk/id/eprint/31418