A within subjects crossover design will be used to address Aims 1, 2a and 2b. Subjects will complete aerobic exercise or stretching (training) before engaging in Study Only or Retrieval Practice (learning). Subjects will be counterbalanced within and across conditions. Sixteen healthy older adults and 16 individuals with aphasia, age 65-89 years, will be recruited. The study will comprise 18 sessions over 12 weeks. In Week 1, subjects will undergo baseline blood draws, a physical assessment, cognitive and language assessments, and a practice word learning task. In Weeks 2-12, subjects will complete the four conditions. Each condition will last two weeks, with a one-week break between conditions. In each condition, training and learning sessions will take place on Monday, Wednesday and Friday. Training: Subjects will engage in 30 minutes of moderate-intensity cycling or gentle upper and lower limb stretching (control) prior to engaging in a novel word learning task. Learning: Familiar objects (e.g., cup) will be paired with nonwords (e.g., flark). In the Study Only paradigm, object/nonword pairs will be presented three times across three days (Monday, Wednesday, Friday). In the Retrieval Practice paradigm, object/nonword pairs will be presented during the first session (Monday). In subsequent sessions (Wednesday, Friday), subjects will be asked to recall the nonword name of presented objects. Corrective feedback will be provided on each trial. Testing: Recognition and recall testing without corrective feedback will be administered immediately after learning (short-term memory), before training in the second and third learning session (overnight consolidation) and one week after learning (long-term memory). Blood Draws: Three blood samples will be taken at baseline (rest, 30 minutes, 60 minutes). In each of the four conditions, blood samples will be taking during the first and third learning session (at rest, after training, and after learning). Whole blood will be collected in plain tubes and in anticoagulant-treated tubes for subsequent serum BDNF and plasma dopamine analysis by ELISA. Samples will be analyzed in triplicate to increase reliability.