Bernal S D_2010.pdf - University of Plymouth

2.3. ILLUSORY AND OCCLUDED CONTOURS
typically attributed to high-level regions, as suggested by the active blackboard and high-
resolulion buffer hypotheses?
• What are the neural correlates of explicit visual perception (or visual awareness) and
are these guided by feedback effects (for example when focusing on the high-resolulion
details of an object) as suggested by the Reverse Hierarchy Theory?
• What effects does the inactivalion of the different higher visual areas (V4, IT, MT) have
on lower level representations? How do these compare under conditions of simple artifi­
cial stimuli (e.g. gratings), natural stimuli, and highly cluttered/illusory/occluded stimuli?
How do these correlate to subjective visual perception and the performance of vision-
related tasks?
2.3 Illusory and occluded contours
Despite living in a cluttered world where the majority of objects we see are partially occluded,
we do not have the impression of constantly being surrounded by object fragments. Our visual
system appears to have developed the appropriate lilling-in or completion mechanisms that cru­
cially allow us to perceive complete objects and make sense of the world. These mechanisms,
which compensate for mi.ssing or ambiguous information in the retinal image, can be divided
into two categories: modal and auKxial completion.
Modal completion, the induced perception of contours and surfaces in empty regions, results
in perceptually salient effects, such as illusory contours. In contrast, amodal completion, the
continuation of contours and surfaces behind occluders, has no visually salient manifestation.
Although neither of them have a physical counterpan in the retina, they both show clear neural
correlates at different levels of the visual system. Both of these phenomena are closely re­
lated to other conspicuous aspects of visual prtx;eKsing, such as feature binding and perceptual
grouping. This section provides an overview of the existing experimental evidence for both
illusory and occluded contours. The different theoretical approaches and computational models
are discussed in the subsequent section.