tVNS Responsiveness Testing With Pupil Size

Inclusion Criteria: * 18-89 years old across all races, genders, and ethnicities. * Post-stroke subjects will have persistent hemiparesis on the upper extremity (UE) with some residual UE voluntary movement. Exclusion Criteria: Able-bodied subjects: * Younger than 18 years old or older than 89 years old * Current or history of cardiac disease * Have a vision problem not corrected by glasses or contact lenses * Have an implanted device such as a neurostimulator or cochlear implant * Current or history of tympanic membrane perforation * Had a stroke or lesion (including tumor) in your brain * Had a head injury or brain surgery * Suffer from frequent or severe headaches * Had a fainting spell or syncope * Have any metal in the head such as shrapnel, surgical clips, or fragments from welding or metal work * Have any implanted device such as cardiac pacemakers, medical pumps, or intra-cardiac lines * Had any brain-related conditions (i.e. multiple sclerosis, Parkinson, Alzheimer) * Had any illness that caused brain injury (i.e. meningitis, aneurysm, brain tumor) * Had any head trauma that was associated with a loss of consciousness or diagnosed as a concussion * Being treated for any psychiatric condition (i.e. depression, anxiety, PTSD, schizophrenia) * Suspected of pregnancy Post-stroke subjects: * Younger than 18 years old or older than 89 years old * Current or history of cardiac disease * Have a vision problem not corrected by glasses or contact lenses * Have any implanted devices such as a neurostimulator or cochlear implant * Current or history of tympanic membrane perforation * No persistent hemiparesis on the upper extremity (UE) * No residual UE voluntary movement * A first stroke less than 4 months prior to the participation * Serious uncontrolled medical conditions * Excessive pain in any joint of the more affected extremity * Unable to stand independently for 2 min., transfer independently to and from the toilet or perform sit-to-stand * Suffer from frequent or severe headaches * Had a fainting spell or syncope * Have any metal in the head such as shrapnel, surgical clips, or fragments from welding or metal work * Have any implanted device such as cardiac pacemakers, medical pumps, or intra-cardiac lines * Had any illness that caused brain injury (i.e. meningitis, aneurysm, brain tumor) * Had any head trauma that was associated with a loss of consciousness or diagnosed as a concussion * Being treated for any psychiatric condition (i.e. depression, anxiety, PTSD, schizophrenia) * Suspected of pregnancy * A score of less than 24 on the Folstein Mini-Mental State Examination * Excessive frailty or lack of stamina (e.g., cannot attend to instructions, stay awake, engage in functional activities, etc.) * Excessive pain in any joint of the more affected extremity that could limit ability to cooperate with the intervention