This study aims to learn how tVNS (transcutaneous vagus nerve stimulation) affects the size of the pupil in healthy people and those who had a stroke. The vagus nerve is a nerve that helps control heart rate and digestion. In this study, small electrodes will be placed on the outer ear to gently stimulate the nerve. This may help us understand how the nerve affects the pupil, which is the black part of your eye that lets in light. The main goal is to see if using the stimulation on both ears (bilateral) causes a bigger change in pupil size compared to one ear (unilateral). Participants will also wear a device on their finger to track their heart rate. The study uses a safe, mild electrical current that shouldn't hurt. Each person will get a mix of real and pretend (sham) stimulations in different orders while resting.

• This study is for people aged 18-60 and will involve multiple sessions.
• Participants must not have certain health problems or implanted devices.
• Participants will not feel pain from the stimulation.