1. Division of Neurophysiology, MRC National Institute for Medical Research, London NW7 1AA, UK
2. Centre for Synaptic Plasticity, School of Physiology and Pharmacology, University of Bristol, Dorothy Hodgkin Building, Whitson Street, Bristol BS1 3NY, UK
3. Laboratory for Cognitive Neuroscience, Centre for Cognitive and Neural Systesms, School of Biomedical Science, University of Edinburgh, 1 George Square, Edinburgh EH8 9JZ, UK