Regional anesthesia decreases postoperative pain scores and opioid consumption, and may prevent chronic pain after surgery in patients undergoing surgery. However, some patients experience an increase of pain into the severe range when the nerve block wears off, also known as rebound pain. The investigators are studying if a nerve block (numbing injection) in the arm causes hyperalgesia (increased pain) when the nerve block is wearing off.

This is a prospective, randomized study in healthy volunteers who will undergo sensory testing using quantitative sensory testing (QST), complete psychosocial and pain questionnaires, and ultrasound-guided peripheral nerve block of the arm (axillary nerve block) in a supervised, monitored setting (BWH Clinical Investigation Center). The investigators plan to assess the association between different QST modalities at different time points pre- and post-nerve block resolution. Semi-structured interviews and qualitative analysis will be used to explore patient's experiences with nerve block placement.

Inclusion Criteria:

• Age ≥ 18 years old, ≤ 65 years old
• BMI >18, < 35
• Able to speak and understand English
• Willingness to undergo psychophysical testing (e.g., QST, pain questionnaires)
• Willingness to have nerve block performed

Exclusion Criteria:

• Ongoing acute or chronic pain in upper extremities
• Skin or tissue infection affecting upper extremities
• Previous hypersensitivity to mepivacaine or lidocaine
• Previous neuropathy (numbness, paresthesia, or motor weakness) in either upper extremity
• Loss of any limb
• Bleeding issues or bleeding disorder
• History of alcohol or drug abuse
• Currently pregnant or breastfeeding
• History of seizure or epilepsy