BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//Talks.cam//talks.cam.ac.uk//
X-WR-CALNAME:Talks.cam
BEGIN:VEVENT
SUMMARY:Fading and Filling-in - Professor Lothar Spillmann\, Freiburg
DTSTART:20141029T130000Z
DTEND:20141029T140000Z
UID:TALK55496@talks.cam.ac.uk
CONTACT:John Mollon
DESCRIPTION:Since Troxler's original observation in 1804\, fading and fill
 ing-in phenomena have aroused the interest of researchers. However\, the q
 uestion of why stimuli imaged on the retina become invisible (fading) and 
 – vice versa – why incomplete stimuli are perceived as complete (filli
 ng-in)\, has been systematically studied only during the last 30 years. \n
 \nFading is due to local adaptation and is comparable to a stimulus lowere
 d to subthreshold. We now know that most targets\, whether static or flick
 ering\, fade into the background. Strict prolonged fixation is crucial\; a
 s soon as the eye moves\, the percept is refreshed and reappears. We assum
 e that this restoration is achieved by the edge signal arising at the bord
 er of the target due to microsaccades. This signal if laterally propagated
  could revive the brightness or color of the interior. The time course of 
 fading typically does not exceed 15 seconds. By comparison\, an image arti
 ficially stabilized on the retina disappears almost instantaneously. \n\nW
 hen a percept fades\, the void may be replaced by properties from the surr
 ounding background. This is called filling-in. Backgrounds need not be uni
 form\, a textured background or dynamic visual noise field will be as effe
 ctive. Filling-in ensures the perception of brightness\, color\, and textu
 re in a artificial scotoma\, where\, in fact\, there is no signal from the
  retina reaching the brain. Other examples are lesion scotomata and the op
 tic disk (blind spot) which are filled-in with properties of the surround 
 and therefore are invisible. \nWe have found that filling-in of the blind 
 spot requires relatively little surround information. A thin red ring hugg
 ing the boundary of the blind spot will fill in the “blind“ area unifo
 rmly and completely with color. Similarly\, a thin chromatic double contou
 r will induce a spread of “watercolor” over a large area. However\, th
 e watercolor effect requires no fixation.\n\nPsychophysical and neurophysi
 ological experiments suggest that filling-in is achieved by long-range hor
 izontal interaction beyond the classical receptive field. Candidates are v
 isual areas V1-3. \n\nLiterature: Luiz Pessoa & Peter De Weerd (eds.). Fil
 ling-In: From Perceptual Completion to Cortical Reorganization. Oxford Uni
 versity Press 2003\n\n\n
LOCATION:Kenneth Craik Room\, Craik-Marshall Building\, Downing Site
END:VEVENT
END:VCALENDAR