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Acknowledgements I thank Wael Asaad, Jonathan Cohen, Peter Dayan, John Duncan, Howard Eichenbaum, David Freedman, Tomaso Poggio, Maximilian Riesenhuber and Marlene Wicherski for valuable com￾ments and discussions. NATURE REVIEWS | NEUROSCIENCE VOLUME 1 | OCTOBER 2000 | 6 5 REVIEWS © 2000 Macmillan Magazines Ltd