1. Group in Vision Science, School of Optometry, 360 Minor Hall, University of California, Berkeley, CA 94720–2020, USA
2. Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK
3. Department of Zoology, Box 351800, University of Washington, Seattle, WA 89195, USA
4. Sussex Centre for Neuroscience, School of Biological Sciences, University of Sussex, Brighton BN1 9QG, UK