Hierarchical and parallel networks are fundamental structures of the mammalian brain^{1,2,3,4,5,6,7,8}. During development, lower- and higher-order thalamic nuclei and many cortical areas in the visual system form interareal connections and build hierarchical dorsal and ventral streams^{9,10,11,12,13}. One hypothesis for the development of visual network wiring involves a sequential strategy wherein neural connections are sequentially formed alongside hierarchical structures from lower to higher areas^14,15,16,17. However, this sequential strategy would be inefficient for building the entire visual network comprising numerous interareal connections. We show that neural pathways from the mouse retina to primary visual cortex (V1) or dorsal/ventral higher visual areas (HVAs) through lower- or higher-order thalamic nuclei form as parallel modules before corticocortical connections. Subsequently, corticocortical connections among V1 and HVAs emerge to combine these modules. Retina-derived activity propagating the initial parallel modules is necessary to establish retinotopic inter-module connections. Thus, the visual network develops in a modular manner involving initial establishment of parallel modules and their subsequent concatenation. Findings in this study raise the possibility that parallel modules from higher-order thalamic nuclei to HVAs act as templates for cortical ventral and dorsal streams and suggest that the brain has an efficient strategy for the development of a hierarchical network comprising numerous areas.

分层和并行网络是哺乳动物大脑^{1,2,3,4,5,6,7,8}的基本结构。在发育过程中，低阶和高阶丘脑核和视觉系统中的许多皮层区域形成区域间连接，并建立分层的背侧和腹侧流^{9,10,11,12,13}。视觉网络布线发展的一个假设涉及一种顺序策略，其中神经连接与从低到高区域的层次结构一起顺序形成^14,15,16,17. 然而，这种顺序策略对于构建包含大量区域间连接的整个视觉网络效率低下。我们表明，从小鼠视网膜到初级视觉皮层 (V1) 或背侧/腹侧高级视觉区域 (HVA) 的神经通路通过低阶或高阶丘脑核在皮质连接之前形成平行模块。随后，V1 和 HVA 之间的皮质连接出现以结合这些模块。传播初始并行模块的视网膜衍生活动对于建立视网膜主题模块间连接是必要的。因此，视觉网络以模块化方式发展，涉及并行模块的初始建立及其后续连接。