1. Chartered psychologist and rehabilitation consultant, 16 Russley Road, Bramcote, Nottingham N69 3JE, England;
2. Blind Mobility Research Unit, Department of Psychology, University of Nottingham, University Park, Nottingham, NG7 2RD, England
3. Blind Mobility Research Unit, Department of Psychology, University of Nottingham. Correspondence should be addressed to the first author