This study will examine how two important brain circuits - one involving the subthalamic nucleus (STN) and one involving the ventral intermediate nucleus of the thalamus (VIM) - contribute to learning and producing speech sequences. Participants will include two groups: 1. individuals with Parkinson's disease who have deep brain stimulation (DBS) devices targeting the STN and 2. individuals with essential tremor who have DBS devices targeting the VIM.

Participants will complete speech tasks involving the learning and repetition of novel sound sequences. During some parts of the study, DBS stimulation will be temporarily turned on or off in a controlled research setting. This will allow researchers to examine how stimulation affects both the learning of new speech sequences and the production of previously learned sequences. All STN participants and most VIM participants will also be equipped with a cutting-edge DBS system, the Percept PC, which will enable the recording of deep brain activity during the tasks.

The results of this study will improve our understanding of how different brain circuits support speech learning and production. In particular, this study will help to differentiate the roles of the STN and VIM in learning the ordering of speech sounds within a syllable from learning of speech sequences containing multiple syllables. This knowledge may help guide future approaches to optimizing DBS settings to improve both movement and speech outcomes in individuals with neurological disorders, as well as provide greater general insight into how these brain structures contribute to speech production and learning.

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

All planned analyses will be carried out by research staff who are already facilitating the study.