PROJECT SUMMARY Hallucinations are a common, distressing symptom of psychosis-spectrum disorders, which can be resistant to first-line psychiatric treatments. This symptom is also associated with abnormal audiovisual integration (AVI) in psychosis, which undermines a holistic perceptual experience of reality. Strong evidence from a causal lesion network mapping study, supported by the functional neuroimaging literature, indicates that the right superior temporal sulcus (rSTS) plays a causal role in the brain network underlying both auditory and visual hallucinations. The rSTS is similarly implicated in AVI deficits, suggesting it could be an ideal target for biomarker-informed treatment approaches. One such approach is transcranial direct current stimulation (tDCS), a non-invasive brain stimulation method for modulating the electrical activity of a cortical region. High-definition tDCS (HD-tDCS), a newly developed and refined version of tDCS, makes precise targeting of the rSTS possible. As lesion network mapping shows that the rSTS is negatively connected to the brain network underlying hallucinations, a clinical trial led by fellowship sponsor, Dr. Paulo Lizano, aims to reduce rSTS activity by using cathodal HD-tDCS, which lowers the underlying membrane potential of the stimulated neural region. In the current research proposal, I will use electroencephalography (EEG) and clinical measures to assess engagement of rSTS by HD-tDCS and clinical efficacy of this intervention through the following aims. Aim 1 will determine if neural correlates of AVI are engaged by cathodal HD-tDCS to the rSTS. I will use my expertise in EEG to extract biomarkers of audiovisual integration from neural responses to repetitive, frequency-modulated auditory and visual stimuli. I hypothesize that cathodal HD-tDCS, relative to sham, will improve the strength of these biomarkers, demonstrating improved AVI and successful engagement of the rSTS after a 5-day treatment and sustained 1 month following treatment. Aim 2 will investigate if cathodal rSTS stimulation, relative to sham, may provide acute and long-term reduction of hallucinatory experiences. I will employ my clinical evaluation skills to measure frequency and severity of hallucinations in participants. I hypothesize that cathodal HD-tDCS, relative to sham, will reduce the frequency and severity of hallucinations in clinician-rated, self-report, and behavioral measures after a 5-day treatment and sustained at 1-month follow up. During the course of this fellowship, I will learn how to perform tDCS and learn about other transcranial electrical stimulation techniques through training provided by Dr. Lizano and Dr. Matcheri Keshavan at Beth Israel Deaconess Medical Center, supported by a consultant, Dr. Robert Reinhart. I will also learn how to plan, perform, analyze, and publish data from a clinical trial through supervision and clinical coursework. Training in professional development will further prepare me for t...