Lisa M Henderson1, Robert H Taylor2, Brendan Barrett3, Philip G Griffiths4. 1. Department of Psychology, University of York, York, UK. 2. Department of Ophthalmology, York Hospital, York, UK. 3. School of Optometry and Vision Science, University of Bradford, Bradford, UK. 4. St Bernard's Hospital, Harbour Views Road, Gibraltar GX11 1AA philip.griffiths@gha.gi.