Electrophysiologic Mechanisms of Ketamine Antidepressant Response

Inclusion Criteria: * Admitted to the Emory EMU for clinically indicated sEEG monitoring for epilepsy. * At least mild depressive symptoms at baseline, defined as MADRS ≥7 or BDI-II ≥14. * Electrode coverage that includes midline regions sufficient for enrollment and analyses. * Ability to provide informed consent and to complete bedside tasks/questionnaires in English. * Expected to remain under inpatient monitoring through the planned \~24-hour post-infusion follow-up unless clinical needs dictate otherwise. Exclusion Criteria: * Documented IQ \<70, intellectual disability, severe cognitive impairment, delirium, severe aphasia, or other condition that prevents valid consent or task participation. * Current or past schizophrenia-spectrum disorder or other psychotic disorder. * Current manic/hypomanic episode or bipolar-spectrum illness judged by study psychiatrist/investigator to increase risk or confound interpretation. * Active alcohol or substance abuse/dependence within the past 3 months. * Imminent suicide risk or active suicidal intent requiring urgent intervention as determined by clinical assessment. Passive ideation without imminent intent may be considered on a case-by-case basis with psychiatric approval and enhanced monitoring. * Contraindication to subanesthetic ketamine, including uncontrolled hypertension, unstable cardiovascular disease, aneurysm/vascular malformation or similar condition conferring unacceptable risk, pregnancy, breastfeeding, or other clinically significant medical instability. * Clinical team determination that ketamine administration or study timing would interfere with epilepsy care or perioperative management. * Sedative or other medication exposure at a level that, in the judgment of the clinical team, precludes safe ketamine administration or reliable behavioral assessment (participant may be deferred rather than permanently excluded if the issue resolves).