Moon Hyun Seok, Vo Thanh Tan, Ho Im Geun, Hong Seok-Jun, Kim Seong-Gi
Center for Neuroscience Imaging Research, Institute for Basic Science, Suwon 16419, Republic of Korea.
Department of Biomedical Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea.
Proc Natl Acad Sci U S A. 2025 Aug 26;122(34):e2505294122. doi: 10.1073/pnas.2505294122. Epub 2025 Aug 18.
Functional connectivity (FC), a statistical correlation of pair-wise brain signals from resting-state (RS) functional MRI (fMRI), is a widely used concept for mapping large-scale functional networks in both humans and animals. However, its underlying causal mechanism remains poorly understood, particularly for strong interhemispheric connectivity (e.g., homotopic connections) consistently observed in FC. In this study, we investigated the neural basis of RS FC in mice using fMRI with anatomically defined patterned optogenetic activation and inhibition of excitatory neurons in six cortical regions. Unlike commonly used optogenetic activation, optogenetic silencing suppresses spontaneous neural activity in a localized region, reducing RS synaptic inputs to downstream networked areas. Consequently, fMRI can track spontaneous functional connections without the neural perturbations associated with excitation. While conventional optogenetic activation of excitatory neurons in the targeted cortical areas predominantly elicited their ipsilateral functional responses in both cortical and subcortical regions, optogenetic silencing induced both intra- and interhemispheric cortical responses, which were stronger than cortical-subcortical connections. These effects more closely resembled statistically defined RS FC patterns, providing insight into the underlying mechanisms of intrinsic FC. By modeling synaptic path length-dependent connectivity patterns based on structural connectivity (SC), we found that spontaneous functional connections can be explained by polysynaptic propagation, whereas evoked activity is largely restricted to monosynaptic pathways. These findings highlight the critical role of polysynaptic pathways in shaping spontaneous connectivity, suggesting that RS FC arises from causal interactions of spontaneous ongoing neural activity.
功能连接（FC）是静息态（RS）功能磁共振成像（fMRI）中两两脑信号之间的统计相关性，是在人类和动物中绘制大规模功能网络时广泛使用的概念。然而，其潜在的因果机制仍知之甚少，特别是对于在FC中持续观察到的强烈半球间连接（如同位连接）。在本研究中，我们使用fMRI对小鼠RS FC的神经基础进行了研究，采用解剖学定义的模式光遗传学激活和抑制六个皮质区域的兴奋性神经元。与常用的光遗传学激活不同，光遗传学沉默可抑制局部区域的自发神经活动，减少RS向下游网络区域的突触输入。因此，fMRI可以追踪自发功能连接，而不会出现与兴奋相关的神经扰动。虽然在目标皮质区域对兴奋性神经元进行传统的光遗传学激活主要在皮质和皮质下区域引发同侧功能反应，但光遗传学沉默诱导了半球内和半球间的皮质反应，且这些反应强于皮质 - 皮质下连接。这些效应更类似于统计学定义的RS FC模式，为内在FC的潜在机制提供了见解。通过基于结构连接（SC）对突触路径长度依赖性连接模式进行建模，我们发现自发功能连接可以用多突触传播来解释，而诱发活动主要局限于单突触通路。这些发现突出了多突触通路在塑造自发连接中的关键作用，表明RS FC源于自发进行的神经活动的因果相互作用。
