Introduction
depression
In 2000, the report that ketamine administered intravenously at a subanesthetic dose (0.5 mg/kg) could rapidly alleviate depression symptoms in a handful of treatment resistant patientsKetamine, which has been FDA-approved for medical use since 1970, is increasingly used and researched as a therapeutic approach for depression. Ketamine is a non-competitive N-methyl-D-aspartate receptor antagonist which predominantly affects the glutamatergic systemSeveral reviews have been written recently focusing on the action of ketamineNeuroimaging studies are generally costly and complex. A magnetic resonance imaging (MRI) study must be conducted in the specific location where these expensive devices are installed. This severely limits the ability to run large studies, let alone large and longitudinal studies. However, this is exactly what is needed to make progress in establishing neuroimaging biomarkers of etiology and treatment—recruiting hundreds to thousands of patients and imaging them at several key points in the course of treatment including: at baseline, during the administration of a RAAD, and at several intervals following treatment. While the present study is not aiming to achieve this ideal, it is a proof of concept that such a study is on the horizon.Recently, Scholkmann and Vollenweider highlighted the “great potential of using optical neuroimaging with functional near-infrared spectroscopy (fNIRS) to further explore the changes in brain activity induced by psychedelics”TD-fNIRS systems have been considered the gold standard for optical neuroimaging—they can retrieve more information than the continuous wave (CW-) systems, and in particular they can achieve absolute measurements and higher sensitivity to deeper tissue layers on the human headThe current study is a proof of principle of using TD-fNIRS in clinical settings, although this study is not itself a clinical study. We therefore extensively characterized the experience of healthy participants with the system, which they wore while undergoing an intramuscular injection of a saline solution (in their first session), and of a ketamine solution (in their second session). We report on changes in cerebrovascular hemodynamics that we extracted from the recordings: physiological manifestations of ketamine effects on cardiac and respiratory features; absolute hemoglobin concentrations; changes in brain activity as measured by the fractional amplitude of low frequency fluctuations (fALFF)