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Is the brain a decomposable or nondecomposable system?
JID:PLREV AID:503 /DIS
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Available online at www.sciencedirect.com
ScienceDirect Physics of Life Reviews ••• (••••) •••–••• www.elsevier.com/locate/plrev
Comment
Is the brain a decomposable or nondecomposable system? Comment on “Understanding brain networks and brain organization” by Pessoa Evan Thompson Department of Philosophy, University of British Columbia, 1866 Main Mall, Vancouver, BC V6T 1Z1, Canada Received 31 May 2014; accepted 2 June 2014
Communicated by L. Perlovsky
Keywords: Brain; Networks; Decomposable; Nondecomposable
Pessoa’s review [4] casts new light on a deep and difficult question: is the brain a “decomposable” or “nondecomposable” system [1,5,7]? This question pertains to the functional organization of the brain as a cognitive system. In a decomposable system, each subsystem’s operation is determined by the subsystem’s intrinsic properties independent of the other subsystems, making the system’s organization strongly modular. Modularity decreases depending on how strongly the subsystems interact, especially through feedback and reentrant or recursive processes. If the subsystems are only weakly coupled, such that the causal interactions within a subsystem play a stronger role in determining its operation than do the causal interactions between it and other subsystems, then the system is “nearly decomposable.” If the subsystems are strongly coupled, then the functional organization of the system becomes less governed by the intrinsic properties of its subsystems and more governed by the ways the subsystems interact, making the system “minimally decomposable.” In a “nondecomposable” system, the coupling is such that the subsystems no longer have clearly separable operations apart from the larger context of their interdependent operation. (Note that such strong coupling can involve weak local connections, as Pessoa discusses in Section 9.1.) The current debate about whether cognitive functions can be localized to specific brain regions [2], or whether cognitive functions need to be mapped onto dynamic networks instantiated in shifting coalitions or assemblies of regions [3,6], can be regarded also as a debate about the extent to which the brain’s cognitive organization is decomposable (modular) or nondecomposable (nonmodular). Pessoa’s review [4] provides a wealth of conceptual tools and empirical data for sharpening this question specifically at the level of functional networks, in contrast to anatomical regions. Suppose, for example, that one rejects the idea of a one-to-one mapping between anatomical regions and cognitive functions, and hence the view that the brain can be treated as a cognitively decomposable system at the anatomical level. Nevertheless, the question remains DOI of original article: http://dx.doi.org/10.1016/j.plrev.2014.03.005. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.plrev.2014.06.005 1571-0645/© 2014 Elsevier B.V. All rights reserved.
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E. Thompson / Physics of Life Reviews ••• (••••) •••–•••
of whether of the brain is cognitively decomposable at the functional network level. With the increasing tendency to map cognitive functions onto functional connectivity networks [3], the assumption often seems to be that one should expect a one-to-one function-to-network mapping, thereby supporting a decompositional view of the brain’s cognitive architecture. Yet, as Pessoa points out, given the shifting dynamical and context-dependent properties of networks, one should not anticipate a one-to-one mapping when the network approach is adopted [4] (see Figs. 3D, 4, and discussion). Thus, even at the network level, the explanatory strategy of decomposition and localization of function could turn out to be limited for modeling the brain’s cognitive architecture or functional organization as a cognitive system. This possibility speaks to the importance of ongoing efforts to characterize cognitive brain dynamics with new network concepts and tools [3,6]. References [1] Bechtel W, Richardson RC. Discovering complexity: decomposition and localization as strategies in scientific research. Princeton, NJ: Princeton University Press; 1993. [2] Kanwisher N. Functional specificity in the human brain: a window into the functional architecture of the mind. Proc Natl Acad Sci USA 2010;107:11163–70. [3] Park H-J, Friston K. Structural and functional brain networks: from connections to cognition. Science 2013;342:1238411. http://dx.doi.org/10.1126/science.1238411. [4] Pessoa L. Understanding brain networks and brain organization. Phys Life Rev 2014. http://dx.doi.org/10.1016/j.plrev.2014.03.005 [in this issue]. [5] Simon H. The sciences of the artificial. Cambridge, MA: The MIT Press; 1969. [6] Singer W. Cortical dynamics revisited. Trends Cogn Sci 2013;17:616–26. [7] Thompson E. Mind in life: biology, phenomenology, and the sciences of mind. Harvard University Press; 2007.
[m3SC+; v 1.194; Prn:16/06/2014; 9:29] P.1 (1-2)
Available online at www.sciencedirect.com
ScienceDirect Physics of Life Reviews ••• (••••) •••–••• www.elsevier.com/locate/plrev
Comment
Is the brain a decomposable or nondecomposable system? Comment on “Understanding brain networks and brain organization” by Pessoa Evan Thompson Department of Philosophy, University of British Columbia, 1866 Main Mall, Vancouver, BC V6T 1Z1, Canada Received 31 May 2014; accepted 2 June 2014
Communicated by L. Perlovsky
Keywords: Brain; Networks; Decomposable; Nondecomposable
Pessoa’s review [4] casts new light on a deep and difficult question: is the brain a “decomposable” or “nondecomposable” system [1,5,7]? This question pertains to the functional organization of the brain as a cognitive system. In a decomposable system, each subsystem’s operation is determined by the subsystem’s intrinsic properties independent of the other subsystems, making the system’s organization strongly modular. Modularity decreases depending on how strongly the subsystems interact, especially through feedback and reentrant or recursive processes. If the subsystems are only weakly coupled, such that the causal interactions within a subsystem play a stronger role in determining its operation than do the causal interactions between it and other subsystems, then the system is “nearly decomposable.” If the subsystems are strongly coupled, then the functional organization of the system becomes less governed by the intrinsic properties of its subsystems and more governed by the ways the subsystems interact, making the system “minimally decomposable.” In a “nondecomposable” system, the coupling is such that the subsystems no longer have clearly separable operations apart from the larger context of their interdependent operation. (Note that such strong coupling can involve weak local connections, as Pessoa discusses in Section 9.1.) The current debate about whether cognitive functions can be localized to specific brain regions [2], or whether cognitive functions need to be mapped onto dynamic networks instantiated in shifting coalitions or assemblies of regions [3,6], can be regarded also as a debate about the extent to which the brain’s cognitive organization is decomposable (modular) or nondecomposable (nonmodular). Pessoa’s review [4] provides a wealth of conceptual tools and empirical data for sharpening this question specifically at the level of functional networks, in contrast to anatomical regions. Suppose, for example, that one rejects the idea of a one-to-one mapping between anatomical regions and cognitive functions, and hence the view that the brain can be treated as a cognitively decomposable system at the anatomical level. Nevertheless, the question remains DOI of original article: http://dx.doi.org/10.1016/j.plrev.2014.03.005. E-mail address: [email protected]. http://dx.doi.org/10.1016/j.plrev.2014.06.005 1571-0645/© 2014 Elsevier B.V. All rights reserved.
JID:PLREV AID:503 /DIS
2
[m3SC+; v 1.194; Prn:16/06/2014; 9:29] P.2 (1-2)
E. Thompson / Physics of Life Reviews ••• (••••) •••–•••
of whether of the brain is cognitively decomposable at the functional network level. With the increasing tendency to map cognitive functions onto functional connectivity networks [3], the assumption often seems to be that one should expect a one-to-one function-to-network mapping, thereby supporting a decompositional view of the brain’s cognitive architecture. Yet, as Pessoa points out, given the shifting dynamical and context-dependent properties of networks, one should not anticipate a one-to-one mapping when the network approach is adopted [4] (see Figs. 3D, 4, and discussion). Thus, even at the network level, the explanatory strategy of decomposition and localization of function could turn out to be limited for modeling the brain’s cognitive architecture or functional organization as a cognitive system. This possibility speaks to the importance of ongoing efforts to characterize cognitive brain dynamics with new network concepts and tools [3,6]. References [1] Bechtel W, Richardson RC. Discovering complexity: decomposition and localization as strategies in scientific research. Princeton, NJ: Princeton University Press; 1993. [2] Kanwisher N. Functional specificity in the human brain: a window into the functional architecture of the mind. Proc Natl Acad Sci USA 2010;107:11163–70. [3] Park H-J, Friston K. Structural and functional brain networks: from connections to cognition. Science 2013;342:1238411. http://dx.doi.org/10.1126/science.1238411. [4] Pessoa L. Understanding brain networks and brain organization. Phys Life Rev 2014. http://dx.doi.org/10.1016/j.plrev.2014.03.005 [in this issue]. [5] Simon H. The sciences of the artificial. Cambridge, MA: The MIT Press; 1969. [6] Singer W. Cortical dynamics revisited. Trends Cogn Sci 2013;17:616–26. [7] Thompson E. Mind in life: biology, phenomenology, and the sciences of mind. Harvard University Press; 2007.