This study explores how ketamine, a medication, affects thinking and feeling in people with Major Depressive Disorder (MDD). MDD is a serious condition that makes people very sad. Ketamine is given in a small, safe dose, not enough to make you sleepy. Researchers will use simple tests and brain scans to see how ketamine changes emotions and decisions. People with MDD will take part in the full study. Healthy volunteers, who don’t have depression, will help by doing some tests and a brain scan, but won't take ketamine. The study looks at brain chemicals, like glutamate, which may be different in people with MDD.

Key Points to Consider:

• The study involves tests and brain scans, and people with MDD might receive ketamine.
• Participants with MDD must stop antidepressants 8 weeks before joining.
• Risks include having no serious health issues and not being pregnant.