Persistent developmental stuttering affects more than three million people in the United States, and it can have profound adverse effects on quality of life. Despite its prevalence and negative impact, stuttering has resisted explanation and effective treatment, due in large part to a poor understanding of the neural processing impairments underlying the disorder. The overall goal of this study is to improve understanding of the brain mechanisms involved in speech motor planning and how these are disrupted in neurogenic speech disorders, like stuttering. The investigators will do this through an integrated combination of experiments that involve speech production, functional MRI, and non-invasive brain stimulation. The study is designed to test hypotheses regarding the brain processes involved in learning and initiating new speech sound sequences and how those processes compare in persons with persistent developmental stuttering and those with typical speech development. These processes will be studied in both adults and children. Additionally, these processes will be investigated in patients with neurodegenerative speech disorders (primary progressive aphasia) to further inform the investigators understanding of the neural mechanisms that support speech motor sequence learning. Together these experiments will result in an improved account of the brain mechanisms underlying speech production in fluent speakers and individuals who stutter, thereby paving the way for the development of new therapies and technologies for addressing this disorder.

Inclusion Criteria

• Healthy individuals with no history of neurological, speech, or hearing disorders (other than stuttering in studies that involve adults who stutter).
• To maximize the uniformity of prior exposure to the speech stimuli that will be used, only native speakers of American English will be recruited, and only those with limited exposure to a second language will be enrolled.
• All adult participants will also pass a standard pure-tone hearing screening at a 25dB hearing level threshold at 500, 1k, 2k, and 4kHz frequencies.
• All participating children will pass a hearing screening at a 20 dB threshold at 500, 1k, 2k, and 4k Hz.
• Participants in experiments that require them to read orthographic stimuli must have normal or corrected-to-normal vision (MRI-safe corrective glasses are available at the Boston University Cognitive Neuroimaging Center for use during neuroimaging).
• Participating children will complete additional speech, language, hearing, and cognitive tests to ensure that they are within normal performance ranges for their age with the exception of stuttering for children in the children who stutter (CWS) group.
• Persons who stutter will be evaluated formally by a speech-language pathologist to assess stuttering severity and to ensure the absence of other speech or language disorders. PWS will have no history of neurological disorder other than stuttering, and will demonstrate very mild to severe stuttering according to the Stuttering Severity Instrument for Children and Adults - 4th Edition (SSI-4: PRO-ED, Inc.), that is confirmed by clinical reports and expressed concern by the subject and/or guardian.
• Participants with primary progressive aphasia (PPA) will have been diagnosed through the Massachusetts General Hospital Frontotemporal Disorders Unit (MGH-FTD) by an experienced neurologist in coordination with a speech-language pathologist.
• Participants with PPA will have a score of 1.0 or lower on the Clinical Dementia Rating scale (i.e., mild cognitive impairment or mild dementia) to ensure cognitive levels are sufficient to complete the task.
• All participants with PPA must have a recent clinical assessment and T1 structural neuroimaging scan through the MGH-FTD Unit for eligibility for this study.

Exclusion Criteria

• Participants in studies that involve tDCS or MRI scanning will have no contraindications specific to those procedures. For the tDCS study, this includes individuals who have a metallic implant in the head or electrically sensitive devices implanted in the body, a history of seizures, significant scalp lesions, or pregnancy.
• For MRI studies, this includes a history of seizures, severe claustrophobia, the presence of magnetically or mechanically active implant, ferromagnetic material embedded in any part of the body, or pregnancy).
• All participants will perform a standardized nonword repetition pre-test (the Dollaghan and Campbell Nonword Repetition Task) to assess working memory performance. Participants who perform more than 2 standard deviations below the norm for their age range will be deemed to be unable to perform the experimental task and released from further participation.
• Participating children will have no history of neurological disorder other than stuttering, and will demonstrate very mild to severe stuttering according to the Stuttering Severity Instrument for Children and Adults, 4th Edition, that is confirmed by clinical reports and expressed concern by the subject and/or guardian.
• Children under the age of 6 and over the age of 8 will not enrolled in this study.
• Participants with PPA will not be eligible for this study if they are taking any medications that would be expected to affect speech or language.