Project Summary & Abstract Individuals with brain injuries or disorders that affect movement (such as Parkinson’s disease, cerebral palsy, amyotrophic lateral sclerosis, and many others) often have difficulties in being understood when they speak. While treatments exist, they often require substantial conscious attention to the way speech is produced, or require increased breath support to speak louder. Many individuals with speech disorders have cognitive or respiratory difficulty that renders these treatments ineffective. These individuals will benefit from alternative strategies that promote motor learning: the ability to alter motor actions through practice. One type of motor learning, sensorimotor adaptation, is a particularly promising pathway for alternative rehabilitation. In this paradigm, the auditory feedback people receive while speaking is externally perturbed, causing them to quickly change their speech to oppose these perturbations. Because of its ability to rapidly induce changes in speech production without conscious control, sensorimotor adaptation holds unique promise for rehabilitation. However, its potential clinical applicability is limited by poor understanding of key clinically-relevant features. First, existing sensorimotor adaptation paradigms do not affect speech in a way that facilitates communication. To improve rehabilitation outcomes, sensorimotor learning must target clinically-relevant speech parameters such as intelligibility. We address this barrier through a novel auditory perturbation that artificially decreases the perceived space between vowels, causing speakers to produce more vowel contrast. Critically, reduced vowel contrast is a hallmark of motor speech disorders and significantly contributes to decreased intelligibility. We determine the effectiveness of this paradigm to increase intelligibility and test how these increases are retained across multiple training sessions, how they generalize to untrained words, and how they can be elicited in complex sentences—characteristics which are key for potential clinical applications. Second, while sensorimotor adaptation is a robust effect on average, not all individuals learn to the same degree. This variability limits the potential impact to only those who show a large degree of learning. This proposal uses behavioral interventions and brain stimulation that target the hypothesized causes of this variability. By directly manipulating these factors, we can determine, for the first time, the mechanisms that underlie speech motor learning. Additionally, establishing how these factors can be modulated to increase learning would allow treatment to benefit a wider range of individuals. Although sensorimotor adaptation can quickly induce changes in speech, its current clinical applicability is limited by substantial gaps in our understanding of its mechanisms. By establishing the capacity of sensorimotor adaptation to increase speech intelligibility, characterizi...