Abstract
Understanding the relationship between genotype and neuronal circuit phenotype necessitates an integrated view of genetics, development, plasticity and learning. Challenging the prevailing notion that emphasizes learning and plasticity as primary drivers of circuit assembly, in this Perspective, we delineate a tripartite framework to clarify the respective roles that learning and plasticity might have in this process. In the first part of the framework, which we term System One, neural circuits are established purely through genetically driven algorithms, in which spike timing-dependent plasticity serves no instructive role. We propose that these circuits equip the animal with sufficient skill and knowledge to successfully engage the world. Next, System Two is governed by rare but critical ‘single-shot learning’ events, which occur in response to survival situations and prompt rapid synaptic reconfiguration. Such events serve as crucial updates to the existing hardwired knowledge base of an organism. Finally, System Three is characterized by a perpetual state of synaptic recalibration, involving continual plasticity for circuit stabilization and fine-tuning. By outlining the definitions and roles of these three core systems, our framework aims to resolve existing ambiguities related to and enrich our understanding of neural circuit formation.
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Acknowledgements
The authors thank K. Miller and H. Baier for the fruitful discussions. This work was supported by New Science Microgrant to D.L.B. and by the National Institutes of Health (NIH) Grant U19NS104653, NIH Grant 1R01NS124017, National Science Foundation Grant IIS-1912293, and Simons Foundation SCGB 542973 to F.E.
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D.L.B. and F.E. contributed to all aspects of the preparation of the manuscript. A.F.C. made a substantial contribution to the discussion of the content of the article and to the review and editing of the manuscript before submission.
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Barabási, D.L., Ferreira Castro, A. & Engert, F. Three systems of circuit formation: assembly, updating and tuning. Nat. Rev. Neurosci. 26, 232–243 (2025). https://doi.org/10.1038/s41583-025-00910-9
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DOI: https://doi.org/10.1038/s41583-025-00910-9