Neuronal and Network Mechanisms of Electrocortical Stimulation

Electrocortical stimulation (ECS) mapping has been used routinely for many decades to inform surgeons on brain areas to avoid when planning surgical resections for tumors or epilepsy. In particular, those areas that are "critical" for speech and language are important to identify. Yet, despite its long history of clinical use, the precise mechanisms of ECS are poorly understood. For example, it is not known how different cortical layers and cell types respond to ECS. Moreover, it is not even clear whether ECS' behavioral effects are due to affecting the underlying cortex alone, causing trans-synaptic activation of other cortices, or directly affecting both the cortex and underlying white matter. A primary objective of this project is to determine the effects of ECS on the cortical network and subcortical white matter. This objective will be tested in two ways. First, the study will determine the relationship between ECS' effects and cortical connectivity patterns. This will be accomplished using graph theory metrics to analyze electrocorticography (ECoG) and functional magnetic resonance imaging (fMRI) from people with epilepsy or brain tumors. This will allow the recording of intracranial EEG (ECoG or stereo EEG) without adding substantial extra risk from the planned clinical treatment for these conditions. The study will investigate both static and dynamic network connectivity patterns, defined using static and dynamic graph metrics that measure connectivity at different scales (local, inter-regional, global). The study hypothesizes that structural (a.k.a. anatomical) and functional connectivity patterns of a cortical site determine its significance to the language network. To test this hypothesis, the study will use measures of structural and functional connectivity to predict which nodes are labeled critical by ECS. Second, the study will test the hypothesis that ECS' behavioral effects also depend on directly perturbing cortico-cortical white matter. The study will use focal cortical cooling as a way to test the respective effects on language function of cooling (which inhibits only the cortex) compared to ECS (which may perturb cortex and underlying white matter), as well as combined ECS and cooling.