1. Tennent Institute of Ophthalmology, Gartnavel General Hospital, 1053 Great Western Road, Glasgow G12 0YN, Scotland
2. 3 Kaimhill, Crosslee Road, Bridge of Weir, PA11 3RQ, Scotland
3. Royal Hospital for Sick Children (Yorkhill), Glasgow G3 8SJ, Scotland, and professor of vision science, Department of Vision Sciences, Glasgow Caledonian University, Glasgow G4 0BA, Scotland