Investigating Individual Differences in Speech Motor Skills in Neurotypical Speakers and Persons With Disordered Speech

This study aims to understand how people use different types of feedback to control their speech. When an individual speaks, the brain relies on several systems at the same time, such as sensory systems that monitor an individuals own voice and the movements of their speech muscles, and a motor system that builds and reads out learned motor patterns. The investigators are studying how these systems work together and how they differ across individuals. Investigators will test 90 adults between 18 and 50 years old, including people who stutter, people with dyslexia, and people with typical speech and reading development. Participants will complete several short speech tasks in which the sounds they hear or the movements of their jaw or larynx are briefly changed. These responses will be used to measure each person's speech motor skills and to estimate the settings of a computer model called "SimpleDIVA," which simulates how the brain controls speech. Participants will also complete an MRI scan so investigators can measure the structure and connectivity of different brain regions. These measures will help investigators understand how individual differences in the brain relate to the speech motor control skills we observe. Participants will also complete sessions with noninvasive brain stimulation (transcranial current stimulation, or tCS) to examine how stimulation of specific areas of the brain affects responses during the speech tasks. The knowledge gained from this study will help researchers understand why speech motor skills vary across people and how differences in neural function may contribute to conditions such as stuttering and dyslexia.

Inclusion Criteria: * Native speakers of American English * Adults age 18-50 * Age-appropriate cognitive and receptive vocabulary skills * Age-appropriate hearing * Adults with dyslexia will have a history of dyslexia or report of ongoing reading difficulties that will be confirmed at the first screening visit * Adults who stutter will have a history of stuttering that will be confirmed at the first screening visit Exclusion Criteria: * History of neurological disorder, including a history of seizures * Major brain injury, brain surgery, or stroke * Orthodontia or atypical oral structure (e.g., cleft palate) that interferes with speech * Fluency disorder (except those in the persons who stutter cohort), apraxia of speech, or dysarthria * Language or reading disorder (except those in persons with dyslexia cohort) * Standardized score below 80 on the Kaufman Brief Intelligence Test * Standardized score below 1 standard deviation on the NIH Toolbox Picture Vocabulary Test * Pregnancy * Severe claustrophobia * Presence of magnetically or mechanically active implant, or other ferromagnetic material embedded in any part of the body * Significant scalp lesions that would prevent transcranial direct stimulation