Background: \- Developmental dyscalculia is a learning disability in which individuals have difficulty learning or comprehending mathematics or other number concepts (such as keeping score during games, measuring time, or estimating distance). Developmental dyscalculia affects certain parts of the brain that are required for processing numbers. Research has shown that a form of brain stimulation called transcranial direct current stimulation (tDCS), applied when healthy individuals are being trained to carry out tasks with numbers, improved the ability to process numbers and solve math problems. More research is needed about whether tDCS can improve number processing in people with developmental dyscalculia. Objectives: \- To examine whether the effects of transcranial direct current stimulation can help individuals with developmental dyscalculia perform mathematical calculations. Eligibility: \- Individuals between 18 and 50 years of age who have been diagnosed with developmental dyscalculia, or are healthy volunteers without dyscalculia. Design: * Participants will have a screening visit and seven study visits. The screening visit and six of the study visits will take place consecutively over the course of 6 days, and the final visit will take place 3 months after the initial participation. * Participants will be screened with a medical history, physical and neurological examination, and a brief examination to test for dyscalculia and determine the participant's dominant hand. * Participants will be randomly assigned to one of two groups for the study. One group will receive tDCS during training to perform a task with numbers, and the other group will receive the same training with sham stimulation. Participants will not know which group they are in. * During the study visits, participants will be trained on number tasks on 6 consecutive days. Before the tDCS or sham stimulation is applied at the beginning of the experiment and at the end of each training day, participants will perform other tasks with numbers. Participants will be evaluated based on the accuracy and speed with which they respond to the questions. * At the followup visit, participants will perform the same number tasks they completed during the study visits. No tDCS will be performed at this visit.
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