Inclusion Criteria:
* Complete or incomplete tetraplegia (quadriplegia), tetraparesis (quadriparesis), severe ataxia, or disabling motor impairments in both upper limbs, based on neurological exam. In addition, these motor impairments may be combined with severe motor-related speech impairment (dysarthria or anarthria), as in Locked In Syndrome (LIS) and amyotrophic lateral sclerosis (ALS), including the bulbar variant of ALS.
* Clinical diagnosis must be established for the etiology of motor impairments, including brainstem stroke\*, traumatic spinal cord injury (SCI), or progressive and irreversible neuromuscular disease, including muscular dystrophy and motor neuron disease, including amyotrophic lateral sclerosis (ALS). \*Brain stem stroke is defined as an acute onset of neurological deficit with clinical features of brain stem or cerebellar dysfunction lasting more than 24 hours together with computed tomography (CT) or magnetic resonance imaging (MRI) evidence of ischemic infarction or parenchymal hemorrhage.
* Candidates with traumatic spinal cord injury must have a complete or incomplete tetraplegia or tetraparesis (ASIA Impairment Scale A, B, C) with an injury level of C6 or higher.
* Candidates with tetraplegia or tetraparesis from traumatic SCI and other non-progressive neurological disorders must have an upper extremity motor score (UEMS, ISNCSCI) of 7 or less in each of the upper extremities. Candidate must also have less than antigravity strength (\< 3) throughout the lower limbs.
* Candidates with progressive conditions with shortened life expectancy, such as ALS, must have less than antigravity strength (\<3) throughout the upper limbs.
* Persistence of motor impairments at least 12 months prior to enrollment if due to a non-progressive neurological cause such as stroke or spinal cord injury
* Meeting surgical safety criteria, including surgical clearance by the participant's primary healthcare provider, study physicians, and any necessary consultants
* Ability to communicate reliably, such as through speech or eye movement
* Stable psychosocial support system with caregiver capable of monitoring participant throughout the study
* Ability and willingness to travel to study location up to five days per week for the duration of the study
* Ability to understand and comply with study session instructions
* Corrected visual acuity sufficient for use of computer monitor
Exclusion Criteria:
* Psychiatric conditions or cognitive impairments that would interfere with obtaining informed consent or fully participating in study activities.
* Individuals with active implanted devices, including devices that are incompatible with magnetic resonance imaging (MRI).
* Contraindications to MRI or anticipated need for an MRI during the study period
* Medical conditions contraindicating device implantation surgery (for example significant pulmonary, cardiovascular, metabolic, or renal impairments making the surgical procedure unsafe)
* Chronic anti-coagulation and medical contraindication to temporary suspension for surgery
* Medical conditions contraindicating chronic device implantation (e.g. osteomyelitis, chronic infection, poorly controlled diabetes, cancer, severe autoimmune disorder, epilepsy, poor wound healing)
* Participants with dental caries and a significant risk of dental or periodontal infection
* Chronic oral or intravenous use of steroids or immunosuppressive therapy
* Active cancer within the past year or ongoing chemotherapy
* Uncontrolled autonomic dysreflexia within the past 3 months
* Hydrocephalus with or without an implanted ventricular shunt
* Other chronic, unstable medical conditions that could interfere with subject participation.
* Persistent suicidal ideation within the past 12 months.
* History of substance use disorder within the past year
* Pregnancy (confirmation through blood test)
* Nursing an infant, planning to become pregnant, or not using adequate birth control
