The goal of this study is to characterize an electroencephalogram (EEG) biomarker for fentanyl and understand where this signal is coming from in the brain. The investigators also aim to understand how this EEG biomarker is connected to patient perception to drug liking.

Inclusion Criteria:

1. 18 years old or above (male and female)
2. American Society of Anesthesiologists (ASA) physical status classification of I, II or III
3. Candidates scheduled for general surgical procedures under general anesthesia and receive fentanyl for surgery

Exclusion Criteria:

1. Craniofacial abnormalities
2. Known or suspected difficult intubation or mask ventilation
3. Known or suspected need for rapid sequence induction and intubation
4. Body mass index above 45 kg/m2
5. Allergies to fentanyl
6. History of obstructive sleep apnea requiring CPAP
7. History of obstructive or restrictive lung disease
8. Opiate use within 24 hours
9. History of opiate abuse within the last 3 years
10. Known or suspected severe chronic pain condition that require use of opiates or limit daily activities
11. MRI contraindications, such as presence of pacemakers, aneurysm clips, artificial heart valves, ear implants, metal fragments, or foreign objects in the eyes, skin, or body

Adult patients scheduled for general surgical procedures under general anesthesia