Spinal Cord Associative Plasticity Study

NON-INVASIVE Inclusion Criteria: (All participants) * Age between 18-80 years. * Must have stable prescription medication for 30 days prior to screening * Must be able to: abstain from alcohol, smoking and caffeine consumption on the day of each experiment; abstain from recreational drugs for the entirety of the study; commit to study requirements (i.e., 7 visits); provide informed consent. (Able-bodied participants) * No known central or peripheral neurological disease or injury. (SCI participants - including patients scheduled for intraoperative procedures) * Score of 1-4 (out of 5) on manual muscle testing of finger extension, finger flexion, or finger abduction in left or right hand. Exclusion criteria: (All participants) * Personal or extensive family history of seizures; * Ventilator dependence or patent tracheostomy site; * Use of medications that significantly lower seizure threshold, such as amphetamines, neuroleptics, dalfampridine, and bupropion; * History of stroke, brain tumor, brain abscess, or multiple sclerosis; * History of moderate or severe head trauma (loss of consciousness for greater than one hour or evidence of brain contusion or hemorrhage or depressed skull fracture on prior imaging); * History of implanted brain/spine/nerve stimulators, aneurysm clips, ferromagnetic metallic implants in the head (except for inside mouth); cochlear implants; cardiac pacemaker/defibrillator; intracardiac lines; currently increased intracranial pressure; or other contraindications to brain or spine stimulation; * Significant coronary artery or cardiac conduction disease; recent history of myocardial infarction and heart failure with an ejection fraction of less than 30% or with a New York Heart Association Functional Classification of Class III or IV; * Recent history (within past 6 months) of recurrent autonomic dysreflexia, defined as a syndrome of sudden rise in systolic pressure greater than 20 mm Hg or diastolic pressure greater than 10 mm Hg, without rise in heart rate, accompanied by symptoms such as headache, facial flushing, sweating, nasal congestion, and blurry vision (this will be closely monitored during all screening and testing procedures); * History of significant hearing problems; * History of bipolar disorder; * History of suicide attempt; * Active psychosis; * Recent history (\>1 year) of chemical substance dependency or significant psychosocial disturbance; * Heavy alcohol consumption (greater than equivalent of 5oz of liquor) within previous 48 hours; * Open skin lesions over the face, neck, shoulders, or arms; * Pregnancy; and * Unsuitable for study participation as determined by study physician. INTRA-OPERATIVE Inclusion Criteria: * Clinical indication for cervical spine surgery. Exclusion criteria: (For experiments involving cortical stimulation) * Epilepsy; * A history of skull surgery with metal implants; * Cochlear implants; * Patients with aneurysm stents in neck or brain blood vessels; * Evidence of skull shrapnel; (For experiments involving spinal cord stimulation) * Stimulation devices in the neck or chest (e.g., vagal nerve stimulation, cardiac patients with pacemakers)