This study focuses on understanding how ketamine affects people with Major Depressive Disorder (MDD). Ketamine is a medicine often used for pain relief or sedation, but here it's used in smaller amounts than usual. The study wants to see how ketamine changes feelings and decision-making in people with MDD. Researchers will use questionnaires, brain imaging, and tasks to study these changes. Additionally, a group of healthy people will also participate, but they won't receive ketamine. They'll help by providing a comparison point.

Key Points:

• Duration: Specific length not mentioned, but involves multiple sessions.
• Participation: MDD patients must be aged 18-65, off antidepressants for 8 weeks, and not have certain health issues.
• Compensation: Not specified; contact the study team for details.