The  Weizmann  Institute  of  Science
                  Faculty of Mathematics and Computer Science

                          Vision and Robotics Seminar

                Amiram Grinvald, Dirk Jancke and Fredric Chavane
                           Department of Neurobiology

                                 will speak on

                          Imaging cortical correlates
                      of illusion in primary visual cortex

Exploring visual illusions may reveal fundamental principles of cortical processing
as they directly show discrepancies between the outside world and internal cortical
representations. Illusionary motion perception of non-moving stimuli has long been 
described by Gestalt psychologists, however, the underlying neuronal mechanisms remain 
unknown. To explore cortical mechanisms underlying the line-motion illusion, we employed 
real time optical imaging, known to be exquisitely sensitive to subthreshold neuronal 
activity. We examined, in primary visual cortex of the anaesthetized cat, responses to
the following four stimuli:  stationary small square and a long bar; moving small square 
and finally the well-established line-motion illusion: a small stationary square, 
preceding a long stationary bar presentation by 60 to 100 msec. Whereas flashing the 
square or bar alone evoked the expected localized, short latency, high amplitude activity
patterns, presenting a square before a bar induced activity patterns resembling that of
a moving object. We demonstrate how the preceding cue, even though physically non-moving,
creates a propagating gradient of subthreshold neuronal activity that account for this 
perceptual illusion of motion and discuss its general implications for cortical processing.

                      The lecture will take place in the
                     Lecture Hall, Room 1, Ziskind Building
                         on Thursday, February 6, 2003
                                  11:00 - noon