Abstract
With the advent of powerful parallel computers, efforts have commenced to simulate complete mammalian brains. However, so far none of these efforts has produced outcomes close to explaining even the behavioral complexities of animals. In this article, we suggest four challenges that ground this shortcoming. First, we discuss the connection between hypothesis testing and simulations. Typically, efforts to simulate complete mammalian brains lack a clear hypothesis. Second, we treat complications related to a lack of parameter constraints for large-scale simulations. To demonstrate the severity of this issue, we review work on two small-scale neural systems, the crustacean stomatogastric ganglion and the Caenorhabditis elegans nervous system. Both of these small nervous systems are very thoroughly, but not completely understood, mainly due to issues with variable and plastic parameters. Third, we discuss the hierarchical structure of neural systems as a principled obstacle to whole-brain simulations. Different organizational levels imply qualitative differences not only in structure, but in choice and appropriateness of investigative technique and perspective. The challenge of reconciling different levels also undergirds the challenge of simulating and hypothesis testing, as modeling a system is not the same thing as simulating it. Fourth, we point out that animal brains are information processing systems tailored very specifically for the ecological niches the respective animals live in.
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Notes
We understand the hierarchical organization of the nervous system as one mediated primarily by part-whole relations, where what is considered a whole at one level (say, a cell) is a part at another level (say, the tissue level). This is a distinction from other conceptions of hierarchical organization, such as van Essen and Maunsell’s (1983) hierarchy of functional streams in the visual cortex, and from David Marr’s (1982) trilevel distribution of the algorithmic, computational, and implementational levels. The chief difference here is that these conceptions of level focus on the transmission of information between functional units, rather than track compositional relations between units of nature.
One important issue that appears here concerns when abstraction (removing detail until a desired grain of description is attained) crosses the line into an idealization (actively distorting factual details of a system). Though both idealization and abstraction play positive, even necessary roles in science, there are distinct issues when one or the other is pursued. Presumably, modelers and simulators of whole brains are interested in abstraction, given their claims of producing “accurate” models of neural systems (see especially the preceding discussion). Nonetheless, we’d like to point out that such aims may pass into the realm of idealization, where a different set of issues crop up (see especially Potochnik 2017). Many thanks to an anonymous reviewer for bringing this to our attention.
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Acknowledgments
We thank the reviewers of the initial version of this manuscript, our colleagues at the Konrad Lorenz Institute, Drs. Greg Cohen, Jonathan Tapson, and the participants of the 2014 Telluride Neuromorphic Cognition Engineering Workshop for helpful discussion.
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Stiefel, K.M., Brooks, D.S. Why is There No Successful Whole Brain Simulation (Yet)?. Biol Theory 14, 122–130 (2019). https://doi.org/10.1007/s13752-019-00319-5
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DOI: https://doi.org/10.1007/s13752-019-00319-5