1. Department of Experimental Psychology, University of Bristol, Bristol BS8 1TU, UK
2. Laboratoire Psychologie de la Perception, Université Paris Descartes, 45 rue des Saints Pères, 75006 Paris, France; also Department of Psychology, Harvard University, 33 Kirkland Street, Cambridge, MA 02138, USA
3. Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge CB2 3EB, UK
4. Department of Experimental Psychology, University of Bristol, 12A Priory Road, Bristol BS81TU, UK