<p>Brain computation depends on intricately connected yet highly distributed neural networks. Owing to the absence of the requisite technologies, causally testing fundamental hypotheses on inter-areal processing has remained largely out of reach. Here, we developed a two-photon holographic mesoscope capable of simultaneously reading and writing neural activity patterns with near-single-cell resolution across large regions of the mouse cortex. We demonstrate the precise photoactivation of spatial and temporal sequences of neurons in one or multiple cortical areas while reading out the downstream effects in several other regions. Thus, we have established mesoscale two-photon holographic optogenetics as a platform for mapping functional connectivity and causal interactions across distributed cortical areas with high resolution.</p>