Disclosing the origin of convolutions in the mammalian brain remains a scientific challenge. Primary folds form before we are born: they are static, well defined and highly preserved across individuals. Secondary folds occur and disappear throughout our entire lifetime: they are dynamic, irregular and highly variable among individuals. While extensive research has improved our understanding of primary folding in the mammalian brain, secondary folding remains understudied and poorly understood. Here, we show that secondary instabilities can explain the increasing complexity of our brain surface as we age. Using the nonlinear field theories of mechanics supplemented by the theory of finite growth, we explore the critical conditions for secondary instabilities. We show that with continuing growth, our brain surface continues to bifurcate into increasingly complex morphologies. Our results suggest that even small geometric variations can have a significant impact on surface morphogenesis. Secondary bifurcations, and with them morphological changes during childhood and adolescence, are closely associated with the formation and loss of neuronal connections. Understanding the correlation between neuronal connectivity, cortical thickness, surface morphology and ultimately behaviour, could have important implications on the diagnostics, classification and treatment of neurological disorders.

中文翻译：

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次生不稳定性调节哺乳动物大脑的皮质复杂性

揭示哺乳动物大脑中卷积的起源仍然是一个科学挑战。初级褶皱在我们出生之前就形成了：它们是静态的、界限分明的并且在个体中高度保存。次级褶皱在我们的整个一生中都会发生和消失：它们是动态的、不规则的并且在个体之间高度可变。虽然广泛的研究提高了我们对哺乳动物大脑初级折叠的理解，但次级折叠仍然没有得到充分研究和了解。在这里，我们表明，继发性不稳定性可以解释随着年龄的增长，我们的大脑表面变得越来越复杂。利用力学的非线性场论并辅以有限增长理论，我们探索了二次不稳定性的临界条件。我们发现，随着大脑的持续生长，我们的大脑表面会继续分化成越来越复杂的形态。我们的结果表明，即使很小的几何变化也会对表面形态发生产生重大影响。继发性分叉以及儿童期和青春期期间的形态变化与神经元连接的形成和丧失密切相关。了解神经元连接、皮质厚度、表面形态和最终行为之间的相关性，可能对神经系统疾病的诊断、分类和治疗产生重要影响。