Subjects will be randomised to one of two sequence: Placebo received first followed by GSK598809, or GKS598809 received first followed by Placebo. Between 8 and 14 days after the last dose of withdrawal medication, subjects will have their first dose of study medication and first MRI scan. At least 7 days and less than 10 days after the first dose/scanning day subjects will receive their second dosing and their second MRI scan.