This volume contains chapters derived from a N. A. T. O. Advanced Study Institute held in June 1983. As the director of this A. S. I. it was my hope that some of the e1ectrophysiologists could express the potentialities of their work for perceptual theory, and that some perceptionists could speculate on the underlying "units" of perception in a way that would engage the imagination of physio- logists. The reader will have to be the judge of whether this was achieved, or whether such a psychophysiological inter1ingua is still overly idealistic. It is clear that after the revolution prec~pitated by Hube1 and Weisel in understanding of visual cortical neurons we still have only a foggy idea of the behavioral output of any particular species of cortical detector. It was therefore particularly unfortunate that two persons who have made great strides in correlating interesting facets of cat cortical physio- logy with human psychophysics (Max Cynader and Martin Regan of Dalhousie University) were unable to attend this meeting. Never- theless, a number of new and challenging ideas regarding both spatial perception and cortical mechanisms are represented in this volume, and it is hoped that the reader will remember not only the individual demonstrations but the critical questions posed by the apposition of the two different collections of experimental facts. David Ingle April 1984 VII TABLE OF CONTENTS PREFACE V D. N. Lee and D. S. Young Visual Timing of Interceptive Action 1 J. J.