The ability to detect and correct errors during and after speech production is essential for maintaining accuracy and avoiding disruption in communication. Thus, it is crucial to understand the basic mechanisms underlying how the speech-motor system evaluates different errors and correspondingly corrects them. This study aims to explore the impact of three different features of errors, introduced by formant perturbations, on corrective and adaptive responses: (1) magnitude of errors, (2) direction of errors, and (3) extent of exposure to errors. Participants were asked to produce the vowel /ε/ in the context of consonant-vowel-consonant words. Participant-specific formant perturbations were applied for three magnitudes of 0.5, 1, 1.5 along the /ε-æ/ line in two directions of simultaneous F1-F2 shift (i.e., shift in the ε-æ direction) and shift to outside the vowel space. Perturbations were applied randomly in a compensation paradigm, so each perturbed trial was preceded and succeeded by several unperturbed trials. It was observed that (1) corrective and adaptive responses were larger for larger magnitude errors, (2) corrective and adaptive responses were larger for errors in the /ε-æ/ direction, (3) corrective and adaptive responses were generally in the /ε-ɪ/ direction regardless of perturbation direction and magnitude, (4) corrective responses were larger for perturbations in the earlier trials of the experiment.

More than a century of research has investigated the etiology of dyslexia, coalescing around ‘phonological awareness’ – the ability to recognize and manipulate phonemes – as a trait typically deficient in reading disorders. Meanwhile, the last few decades of research in neuroscience have highlighted the brain as a predictive organ, which subliminally calibrates sensory expectations according to experience. Do the brains of adults with dyslexia respond differently than those of matched controls to expected tones and unexpected omissions? While auditory oddball paradigms have previously been used to study dyslexia, these studies often interpret group differences to indicate deficit auditory discrimination rather than deficit auditory prediction. The current study takes a step toward fusing theories of predictive coding and dyslexia, finding that event-related potentials related to auditory prediction are attenuated in adults with dyslexia compared with typical controls. It further suggests that understanding dyslexia, and perhaps other psychiatric disorders, in terms of contributory neural systems will elucidate shared and distinct etiologies.

Parkinson's Disease (PD) is a progressive neurodegenerative disorder that affects movement and balance control. Falls are a common and often debilitating consequence of PD, and reactive balance control is critical in preventing falls. This dissertation aimed to determine the adaptability and neural control of reactive balance responses in people with PD. Aim 1 investigated whether people with PD at risk for falls can improve their reactive balance responses through a 2-week, 6-session training protocol. The study found that reactive step training resulted in immediate and retained improvements in stepping, as measured by the anterior-posterior margin of stability (MOS), step length, and step latency during backward stepping. The second aim explored the neural mechanisms behind eliciting and learning reactive balance responses in PD. The study investigated the white matter (WM) correlates of reactive stepping and responsiveness to step training in PD. White matter was not significantly correlated with any baseline stepping outcomes. However, greater retention of step length was associated with increased fractional anisotropy (FA) within the left anterior corona radiata, left posterior thalamic radiation, and right and left superior longitudinal fasciculi. Lower radial diffusivity (RD) within the left posterior and anterior corona radiata were associated with retention of step latency improvements. These findings highlight the importance of WM microstructural integrity in motor learning and retention processes in PD. The third aim examined the role of the somatosensory system in reactive balance control in people with PD. The tactile and proprioceptive systems were perturbed using vibrotactile stimulation during backward feet-in-place balance responses. The results showed that tactile and proprioceptive stimulation had minimal impact on reactive balance responses. Small effects were observed for delayed tibialis anterior (TA) onsets with proprioceptive stimulation at a medium intensity. Overall, this dissertation provides insights into improving reactive balance responses and the underlying neural mechanisms in PD, which can potentially inform the development of targeted interventions to reduce falls in people with PD.

Studies using transcranial direct current stimulation (tDCS) to enhance motor training areoften irreproducible. This may be partly due to differences in stimulation parameters across studies, but it is also plausible that uncontrolled placebo effects may interact with the true ‘treatment’ effect of tDCS. Thus, the purpose of this study was to test whether there was a placebo effect of tDCS on motor training and to identify possible mechanisms of such an effect. Fifty-one participants (age: 22.2 ± 4.16; 26 F) were randomly assigned to one of three groups: active anodal tDCS (n=18), sham tDCS (n=18), or no stimulation control (n=15). Participant expectations about how much tDCS could enhance motor function and their general suggestibility were assessed. Participants then completed 30 trials of functional upper extremity motor training with or without online tDCS. Stimulation (20-min, 2mA) was applied to the right primary motor cortex (C4) in a double-blind, sham-controlled fashion, while the control group was unblinded and not exposed to any stimulation. Following motor training, expectations about how much tDCS could enhance motor function were assessed again for participants in the sham and active tDCS groups only. Results showed no effect of active tDCS on motor training (p=.67). However, there was a significant placebo effect, such that the collapsed sham and active tDCS groups improved more during motor training than the control group (p=.02). This placebo effect was significantly influenced by post-training expectations about tDCS (p=.0004). Thus, this exploratory study showed that there is a measurable placebo effect of tDCS on motor training, likely driven by participants’ perceptions of whether they received stimulation. Future studies should consider placebo effects of tDCS and identify their underlying mechanisms in order to leverage them in clinical care.

Speech and music are traditionally thought to be primarily supported by different hemispheres. A growing body of evidence suggests that speech and music often rely on shared resources in bilateral brain networks, though the right and left hemispheres exhibit some domain-specific specialization. While there is ample research investigating speech deficits in individuals with right hemisphere lesions and amusia, fewer investigate amusia in individuals with left hemisphere lesions and aphasia. Many of the fronto-temporal-parietal regions in the left hemisphere commonly associated with speech processing and production are also implicated in bilateral music processing networks. The current study investigates the relationship between damage to specific regions of interest within these networks, and an individual’s ability to successfully match the pitch and rhythm of a presented melody. Twenty-seven participants with chronic-stroke lesions were given a melody repetition task to hum short novel piano melodies. Participants underwent structural MRI acquisition and were administered an extensive speech and cognitive battery. Pitch and rhythm scores were calculated by correlating participant responses and target piano notes. Production errors were calculated by counting trials with responses that don’t match the target melody’s note count. Overall, performance varied widely, and rhythm scores were significantly correlated. Working memory scores were significantly correlated with rhythm scores and production errors, but not pitch scores. Broca’s area lesions were not associated with significant differences in any of the melody repetition measures, while left Heschl’s gyrus lesions were associated with worse performance on pitch, rhythm, and production errors. Lower rhythm scores were associated with lesions including both the left anterior and posterior superior temporal gyrus, and in participants with damage to the left planum temporale. The other regions of interest were not consistently associated with poorer pitch scores or production errors. Although the present study does have limitations, the current study suggests lesions to left hemisphere regions thought to only affect speech also affect musical pitch and rhythm processing. Therefore, amusia should not be characterized solely as a right hemisphere disorder. Instead, musical abilities of individuals with left hemisphere stroke and aphasia should be characterized to better understand their deficits and mechanisms of impairment.

The brain uses the somatosensory system to interact with the environment and control movements. Additionally, many movement disorders are associated with deficits in the somatosensory sensory system. Thus, understanding the somatosensory system is essential for developing treatments for movement disorders. Previous studies have extensively examined the role of the somatosensory system in controlling the lower and upper extremities; however, little is known about the contributions of the orofacial somatosensory system. The overall goal of this study was to determine factors that influence the sensitivity of the orofacial somatosensory system. To measure the somatosensory system's sensitivity, transcutaneous electrical current stimulation was applied to the skin overlaying the trigeminal nerve on the lower portion of the face. After applying stimulation, participants' sensitivity was determined through the detection of the electrical stimuli (i.e., perceptual threshold). The data analysis focused on the impact of (1) stimulation parameters, (2) electrode placement, and (3) motor tasks on the perceptual threshold. The results showed that, as expected, stimulation parameters (such as stimulation frequency and duration) influenced perceptual thresholds. However, electrode placement (left vs. right side of the face) and motor tasks (lip contraction vs. rest) did not influence perceptual thresholds. Overall, these findings have important implications for designing and developing therapeutic neuromodulation techniques based on trigeminal nerve stimulation.

Stroke is the leading cause of long-term disability in the U.S., with up to 60% of strokescausing speech loss. Individuals with severe stroke, who require the most frequent, intense speech therapy, often cannot adhere to treatments due to high cost and low success rates. Therefore, the ability to make functionally significant changes in individuals with severe post- stroke aphasia remains a key challenge for the rehabilitation community. This dissertation aimed to evaluate the efficacy of Startle Adjuvant Rehabilitation Therapy (START), a tele-enabled, low- cost treatment, to improve quality of life and speech in individuals with severe-to-moderate stroke. START is the exposure to startling acoustic stimuli during practice of motor tasks in individuals with stroke. START increases the speed and intensity of practice in severely impaired post-stroke reaching, with START eliciting muscle activity 2-3 times higher than maximum voluntary contraction. Voluntary reaching distance, onset, and final accuracy increased after a session of START, suggesting a rehabilitative effect. However, START has not been evaluated during impaired speech. The objective of this study is to determine if impaired speech can be elicited by startling acoustic stimuli, and if three days of START training can enhance clinical measures of moderate to severe post-stroke aphasia and apraxia of speech. This dissertation evaluates START in 42 individuals with post-stroke speech impairment via telehealth in a Phase 0 clinical trial. Results suggest that impaired speech can be elicited by startling acoustic stimuli and that START benefits individuals with severe-to-moderate post-stroke impairments in both linguistic and motor speech domains. This fills an important gap in aphasia care, as many speech therapies remain ineffective and financially inaccessible for patients with severe deficits. START is effective, remotely delivered, and may likely serve as an affordable adjuvant to traditional therapy for those that have poor access to quality care.

Prosodic features such as fundamental frequency (F0), intensity, and duration convey important information of speech intonation (i.e., is it a statement or a question?). Because cochlear implants (CIs) do not adequately encode pitch-related F0 cues, pre-lignually deaf pediatric CI users have poorer speech intonation perception and production than normal-hearing (NH) children. In contrast, post-lingually deaf adult CI users have developed speech production skills via normal hearing before deafness and implantation. Further, combined electric hearing (via CI) and acoustic hearing (via hearing aid, HA) may improve CI users’ perception of pitch cues in speech intonation. Therefore, this study tested (1) whether post-lingually deaf adult CI users have similar speech intonation production to NH adults and (2) whether their speech intonation production improves with auditory feedback via CI+HA (i.e., bimodal hearing). Eight post-lingually deaf adult bimodal CI users and nine NH adults participated in this study. 10 question-and-answer dialogues with an experimenter were used to elicit 10 pairs of syntactically matched questions and statements from each participant. Bimodal CI users were tested under four hearing conditions: no-device (ND), HA, CI, and CI+HA. F0 change, intensity change, and duration ratio between the last two syllables of each utterance were analyzed to evaluate the quality of speech intonation production. The results showed no significant differences between CI and NH participants in any of the acoustic features of questions and statements. For CI participants, the CI+HA condition led to significantly greater F0 decreases of statements than the ND condition, while the ND condition led to significantly greater duration ratios of questions and statements. These results suggest that bimodal CI users change the use of prosodic cues for speech intonation production in different hearing conditions and access to auditory feedback via CI+HA may improve their voice pitch control to produce more salient statement intonation contours.

Parkinson’s Disease is one of the most complicated and abundantneurodegenerative diseases in the world. Previous analysis of Parkinson’s disease has identified both speech and gait deficits throughout progression of the disease. There has been minimal research looking into the correlation between both the speech and gait deficits in those diagnosed with Parkinson’s. There is high indication that there is a correlation between the two given the similar pathology and origins of both deficits. This exploratory study aims to establish correlation between both the gait and speech deficits in those diagnosed with Parkinson’s disease. Using previously identified motor and speech measurements and tasks, I conducted a correlational study of individuals with Parkinson’s disease at baseline. There were correlations between multiple speech and gait variability outcomes. The expected correlations ranged from average harmonics-to-noise ratio values against anticipatory postural adjustments-lateral peak distance to average shimmer values against anticipatory postural adjustments-lateral peak distance. There were also unexpected outcomes that ranged from F2 variability against the average number of steps in a turn to intensity variability against step duration variability. I also analyzed the speech changes over 1 year as a secondary outcome of the study. Finally, I found that averages and variabilities increased over 1 year regarding speech primary outcomes. This study serves as a basis for further treatment that may be able to simultaneously treat both speech and gait deficits in those diagnosed with Parkinson’s. The exploratory study also indicates multiple targets for further investigation to better understand cohesive and compensatory mechanisms.