1. Department of Psychology, University of Essex, Wivenhoe Park, Colchester, Essex, CO4 3SQ, UK,
2. Department of Psychology, University of Essex, Wivenhoe Park, Colchester, Essex, CO4 3SQ, UK
3. Department of Electronic Systems, Engineering, University of Essex, Wivenhoe Park, Colchester, Essex, CO4 3SQ, UK
4. IPO, Center for Research on User-System Interaction, Eindhoven University of Technology Den Dolech 2, P.O. Box 513 5600 MB Eindhoven, The Netherlands