How deep brain circuits help organise speech

Inclusion Criteria: * Native speakers of American English * Adults at least 18 years of age * A clinically established diagnosis of Parkinson's disease or essential tremor * Able to provide informed consent in the judgment of the investigator * Treated with deep brain stimulation of the subthalamic nucleus (Parkinson's disease) or ventral intermediate nucleus of the thalamus (essential tremor) * Stable Parkinson's disease or essential tremor medication regimen for at least one month * Stable DBS program settings for at least one month * Nominal DBS system function, including normal impedances at therapeutic DBS contacts, and adequate battery life or adequate IPG charging status for therapy * For DBS sensing, implanted with Medtronic Percept PC or Percept RC implantable pulse generator * Corrected vision adequate to easily read text presented during speech motor task Exclusion Criteria: * Cognitive impairment (Montreal Cognitive Assessment (MoCA; Nesreddine et al., 2005) score \< 25) or active psychotic or behavioral symptoms that would, in the judgment of the investigator, preclude proper participation in the study * Hearing impairment that interferes with accurate perception of the speech motor learning stimulus (25dB hearing level threshold at 500, 1k, 2k, and 4kHz frequencies) * Language impairment (aphasia) or speech articulation impairment (dysarthria) that precludes performance of the speech motor learning task * Neurological disorder that interferes with speech motor learning * Inability to tolerate symptoms when DBS is off * Orthodontia or atypical oral structure (e.g., cleft palate) that interferes with speech * Pregnancy * For participants in the sub-syllabic sequence learning study (Study C.2.1), experience with the following languages: Hebrew, Polish, Lithuanian, Romanian, Georgian, Tepehua, Hungarian, and Pima