The non-invasive sensing and stimulation of electrical signals in the brain, with high spatial and temporal specificity, has long been a strategic goal in neuroscience. Such a tool would open pathways to understanding the brain and its disorders. To date, the focal detection and stimulation of electrical signals in the brain has been impossible due to the inability to non-invasively reach deep brain areas with high spatial specificity. This thesis proposes a new hybrid modality capable of detecting and stimulating electrical signals in the brain with improved spatial specificity, through utilization of the acoustoelectric interaction.

The underlying physics of the acoustoelectric interaction is investigated, leading to a new mathematical model which encapsulates the vector nature of the interaction. This acoustoelectric mathematical model predicts that heterodyning will occur between the electric and acoustic field, enabling precise signal recovery using techniques previously established in radio communications. Simulation and acoustoelectric phantom characterization demonstrate how this technique can be used to create a focal low frequency electrical signal deep in the brain for neuromodulation and demodulate complex electrophysiological signals at the focus of the ultrasound for acoustoelectric neural recording. Moving to in vivo validation using electrophysiology in a rodent model, we acoustoelectrically demodulated visual evoked potentials around the carrier frequency of the ultrasound for the first time, with associated artefact tests. Finally, the first in vivo evidence of acoustoelectric neuromodulation is reported and an evidence-based argument developed as to why the acoustoelectric interaction is a contributing mechanism to ultrasound brain stimulation. Acoustoelectric neuromodulation has a clear mechanism of action, harnessing the focality of ultrasound and the well-understood direct electrical pathway developed by the Hodgkin-Huxley model, giving it a unique set of advantages to target deep areas of the brain.