Neural and Psychological Mechanisms of Pain Perception

* INCLUSION CRITERIA: * Healthy * Between 18 and 50 years old * Fluent in English * Able to provide written informed consent. EXCLUSION CRITERIA (all sub-studies): * Unable to comply with study procedures or follow-up visits. * Has a major medical condition or medical history that in a clinician's assessment could affect heat sensitivity, pain thresholds, or ability to comply with study procedures. This may include cardiovascular, autonomic, or neurological conditions, including stroke, blindness, deafness, a history of brain damage, or a chronic systemic disease (e.g., diabetes). * Has a current mood disorder, anxiety disorder, or substance use disorder, or has a history of psychosis, hospitalization for a mental health condition, or recurrent psychiatric episodes. * Has a medical condition that in a clinician's assessment might affect somatosensation (e.g., Raynaud's disease, peripheral neuropathy, or circulatory disorder). * Has a current chronic pain condition or has had chronic pain in the past (painful condition lasting more than six months). * Has a dermatological condition affecting the testing region such as scars, burns, or recent tattoos that might influence cutaneous sensibility. * Regular use of prescription medication that has a significant effect on pain or heat perception. Excluded medications include central-acting agents such as opiates (morphine, tramadol), antidepressants (amitriptyline, duloxetine, milnacipran), anticonvulsants (gabapentin, pregabalin), anxiolytics (barbituates, benzodiazepines), hypnotics (zolpidem, sodium oxybate), antipsychotics (valproate, lithium, olanzapine), antimigraine agents (sumatriptan, ergotamine), and muscle relaxants (cyclobenzaprine, carisoprodol). Use of analgesic medications, such as non-steroidal anti-inflammatories, salicylates, and acetaminophen, taken on an "as needed" basis is acceptable as long as the last dose taken was within 5 half-lives of testing. * Is pregnant. * NIH staff member who is a subordinate/relative/co-worker of any investigator on the protocol. EXCLUSION CRITERIA (fMRI sub-studies): * Individuals with conditions that could pose a risk relating to the safety of the fMRI procedure or pain stimulation will be excluded from the MRI portion of the protocol, but may participate in the non-fMRI sessions (with the exception of pregnant women). Such conditions include: * Those with ferromagnetic metal in the cranial cavity or eye, e.g. aneurysm clip, implanted neural stimulator, cochlear implant, ocular foreign body. * Those with an abnormality on a structural MRI. * Those with an implanted cardiac pacemaker or auto-defibrillator. * Those with an insulin pump. * Those with irremovable body piercing. * Pregnant women (based on urine test completed within 24 hours prior to scan). * Individuals who are left-handed (based on self-report or score on handedness questionnaire) will be excluded from fMRI substudies. EXCLUSION CRITERIA (placebo analgesia sub-studies): -Participation in an NIH study of analgesia, as gleaned from CRIS.