Project Summary Intracranial electrical brain stimulation (EBS) remains a central method in the clinic as well as for research in several animal model systems. However, little is actually known about the ensembles of neurons activated by typical and clinical intracranial EBS protocols. These stimulation protocols often require a trial-and-error learning period (during and after invasive neurosurgery) to determine what stimulation parameters are effective, if any at all are effective. It remains mysterious why some stimulation patterns work in the clinic while others do not, and what underlying ensembles are activated by various stimulation patterns. It is known that focal electrical microstimulation activates nearby excitable membranes, including neural somas, dendrites, and axons. It is also known that the recruited ensemble of neurons may be locally non- homogenous and that clinical effects may rely more on axons of passage than somatic stimulation. Efforts to model EBS cannot overcome our current gaps in knowledge about the homogeneity of local propagation and brain-wide extent of activation. This ambiguity demands a more detailed understanding of local electric field propagation, particularly in the in vivo mammalian brain. Utilizing recent technological advances, we propose to fill these gaps empirically with high density electrophysiological monitoring and temporally precise fluorescent labeling methods, to quantify clinically relevant activation patterns with high spatial resolution and cell-type specificity. Here we propose an experimental study in mice, based on a biophysically realistic model of mouse cerebral cortex, of the spatial and temporal propagation of activation via focal EBS. The study will test the hypothesis that local electrical stimulation is non-isotropic and cell-type specific. We propose to measure EBS stimulation with more than 1000 electrodes arranges in three dimensions around a site of stimulation, in combination with genetic cell type identity through optotagging (Aim 1). To agnostically isolate brain-wide ensembled activated by EBS, we couple a fluorescent reporter to electrical stimulation for ex vivo whole-brain tissue clearing and light sheet imaging (Aim 2).

Project Narrative Intracranial electrical stimulation of the brain is a central method in the neurosurgery clinic, approved or in trials for treatment of Parkinson's disease, obsessive compulsive disorder, epilepsy, and others. However, the basic mechanism of action of intracranial stimulation remains unclear. This work uses recent advances in electrophysiology (Neuropixels) and optogenetic tools to study the local and brain-wide effects of electrical stimulation.