Inclusion Criteria:
* Potential subjects must be between the ages of 18-45 years old and right-handed. Aging has been shown to affect the morphology of sensory and motor nerves, conduction velocities of nerves, and number of motor neurons in the spinal cord; to avoid these confounding factors we will only examine younger-to middle-aged adults.
* There are differences in cortical function and corticospinal projections such that testing the right arm of a right-handed individual is not necessarily equivalent to testing the left arm of a left-handed individual. To eliminate this confound, we will only test right-handed individuals. All these factors will be determined with the initial screening questions.
* Covid has been found to have neurological effects in some people, but mostly the effects on sensorimotor control and neurophysiology are unknown. So we want to reduce the chances of inadvertently testing subjects who have covid. We will therefore only include individuals who are fully vaccinated (2+ weeks past their final vaccine dose) or have a negative Covid test within 4 days of testing. We will further only include individuals who report being free of Covid symptoms in week preceding testing.
Exclusion Criteria:
1. Determined with initial screening questions, pre-enrollment:
Potential subjects will be excluded for current vision problems, other than needing glasses or contacts. Subjects will also be excluded if they currently suffer from frequent severe headaches, glaucoma, heart or respiratory disease, hypertension, psychiatric conditions, or learning or attention conditions. They will also be excluded for current or past: visual, hearing, or balance impairments; stroke, seizure/epilepsy (including family history), or severe head trauma; fainting; or diabetes. Subjects will be excluded for metal implants in the head other than titanium; cochlear implants; implanted neurostimulator; cardiac pacemaker; intracardiac lines; or a medication infusion device. Because TMS does not penetrate deeply into the head, we cannot test subjects whose hair does not permit contact between the TMS coil and the scalp. We will therefore exclude subjects with dreadlocks, weaves, or hair extensions. To protect the data from extraneous peripheral influences, we will also exclude subjects who have had serious injury to the bones, joints, or muscles of either hand or arm, and have not fully recovered. For the purpose of this study, "fully recovered" means they no longer notice any pain, weakness, or loss of sensation in the injured area, and have no mobility limitations.
For the validity of our data, we will exclude subjects taking medications or drugs that are known to affect cortical excitability and possibly seizure risk in an rTMS study. These medications/drugs are (Rossi et al., 2009): imipramine, amitriptyline, doxepine, nortriptyline, maprotiline, chlorpromazine, clozapine, foscarnet, ganciclovir, ritonavir, amphetamines, cocaine, (MDMA, ecstasy), phencyclidine (PCP, angel's dust), ketamine, gamma-hydroxybutyrate (GHB), theophylline, mianserin, fluoxetine, fluvoxamine, paroxetine, sertraline, citalopram, reboxetine, venlafaxine, duloxetine, bupropion, mirtazapine, fluphenazine, pimozide, haloperidol, olanzapine, quetiapine, aripiprazole, ziprasidone, risperidone, chloroquine, mefloquine, imipenem, penicillin, ampicillin, cephalosporins, metronidazole, isoniazid, levofloxacin, cyclosporin, chlorambucil, vincristine, methotrexate, cytosine arabinoside, BCNU, lithium, anticholinergics, antihistamines, sympathomimetics.
2. Determined on each day of TMS testing:
Potential subjects will be invited to reschedule if they would otherwise be eligible (according to the initial screening), but the day of testing have drunk more than 3 units of alcohol or taken other recreational drugs in the 24 hour period prior to testing; have had more than 3 cups of coffee in the last hour; are sleep deprived (\<4 hours sleep the previous night); or have participated in another brain stimulation experiment the same day. These are standard in the TMS literature to protect the validity of the data and keep seizure risk minimal. In addition, we will invite invite subjects to reschedule if they have any of the common Covid symptoms within the last week and if they haven't been fully vaccinated or obtained a negative Covid test within the past 4 days. If they don't believe they can meet these criteria on another date, they will be excluded.
3. Determined during the Familiarization session.
After giving their consent, participants may be excluded during or after the familiarization session if they are unable to perform the reaching task or follow instructions, or if their TMS stimulation parameters cannot be reliably determined by the experimenter, or if TMS is not well tolerated.
