Cowey, Alan1,2; Walsh, Vincent1
1University of Oxford, Department of Experimental Psychology, South Parks Road, Oxford OX1 3UD, UK
2Corresponding Author: Alan Cowey
Received 13 June 2000, accepted 31 July 2000
Acknowledgements: This work was supported by MRC grants G971/397/B and G9711247, by the Royal Society and by the Dr Hadwen Trust. We are very grateful to PS and GY for their enthusiastic co-operation and to Dr Amanda Ellison for her help in preparing the figures.