Part 2 (PD Assessment) is a double blind randomized, placebo-controlled 2-period crossover design with 14-day treatment periods investigating the efficacy (cognitive performance) and tolerability (gastrointestinal [GI] side effects) of single daily dose of GSK2981710 selected from Part 1. Part 2 of the study will include the Screening period, two Baseline assessments (6-8 days before each Treatment period) and two 14-day treatment periods separated by a minimum 7-day washout period and follow-up visit of 3 to 5 days. Approximately 50 to 80 subjects will be randomized to either GSK2981710 or placebo. The PD assessments will be performed on 6 occasions for each subject: at 2 baselines (6 to 8 days before Day 1 of each treatment period), post-dose on the Day 1 of each treatment period to assess acute effects and on Day 15 of each treatment period (which is the day after the final dose) to assess chronic effects. A subject's total participation in Part 2 of the study will last approximately up to 12 weeks including screening.