Electrocortical stimulation (ECS) has been used for functional mapping for many decades to identify brain areas that are “critical” for speech and language (i.e., that impair function when stimulated) prior to epilepsy or tumor surgery. It also is used to modulate neural activity, e.g., in directly treating epilepsy or pain. However, despite its long history of clinical use, the precise mechanisms of ECS are poorly understood, both on neuronal and network scales. For example, it is not known how different cortical layers and cell types respond to ECS, nor whether ECS’ effects on behavior are due to affecting only the local cortex vs. underlying white matter. The long-term goal of this research is to understand how ECS interacts with the brain. The objectives of this proposal are to determine the local effects of ECS on cortical neurons, and to determine the effects of ECS on the cortical network and subcortical white matter. We will test the predictions of multiple computational modelling and indirect experimental studies. One of these that ECS preferentially activates cells in superficial cortical layers with broad projections, could help explain how focal stimulation causes widespread effects. We hypothesize that the anatomic and functional connectivity patterns of a cortical site determine its significance to the language network. That is, nodes that form connections among multiple regions are more likely to be critical. We also hypothesize that ECS causes behavioral changes by affecting both the cortex and underlying white matter. The specific aims of the project are 1) to determine the effects of ECS on a neuronal scale, 2) to determine the relationship between ECS’ effects and cortical connectivity patterns, and 3) to investigate the extent to which ECS’ effects are due to activating underlying white matter. This project is innovative in its use of nanomesh, µECoG (electrocorticography) arrays to enable simultaneous ECS and two-photon calcium imaging of neuronal responses, as well as its novel dynamic network metrics to analyze human cortical connectivity on a millisecond level. We have shown that focal cortical cooling only affects the cortex, and not the white matter, and will use cooling to probe the relative roles of cortex and white matter in ECS’ behavioral effects. Achieving our objectives will be significant because it will improve functional brain mapping and neuromodulation. We expect this to lead to better neurosurgical outcomes for a variety of neurologic disorders, including epilepsy, brain tumors, and chronic pain. We also expect this proposal will enable other studies using stimulation to investigate causality to be more precise in defining and understanding their outcomes. Finally, we anticipate this proposal will advance our understanding of how the brain encodes language.