Bernal S D_2010.pdf - University of Plymouth

2.3. ILLUSORY AND OCCLUDED CONTOURS
Results showed the representation of boundaries was highly precise and accurate only when
the contour could be predicted by local relaiahility cues. However, when completion had lo be
predicted from long-range contextual infbrmalion. such as from global symmetry patterns, the
precision strongly diminished. This suggests feedback alone can provide widespread activity
indicating the presence of subjective contours, but is insufficient to perform accurate contour
completion.
Inlegration uf multiple mechanisms Recent reviews of illusory contour formation suggest
that ii results from the interaction between the different proposed mechanisms, i.e. extrapo­
lation, interpolation and ligural feedback, i-vidence suggests they all play a role in subjec­
tive contour perception, although the signihcance of their contribution may vary according lo
the conditions of the stimuli. This addresses previous conflict between evidence in supp*)rt uf
boltom-up versus top-down processing. Feedforward and horizontal connections would be in­
volved in interpolation and extrapolation processes, which interact with the high-level figural
feedback, as supported by psychophysical, physiological and anatomical data. For a detailed
review see Haiko et al. (2008).
This interaction of mechanisms is consistent with models where the input image is preattcn-
tiveiy segmented based on Geslalt principles and subsequently processed following high-level
focal attention (Grossberg and Raizada 2(X)0, Marr 1982). Under this perspective the initial
feedforward base grouping would generate the high-level percept which would then attention-
ally guide, from coarse to fine scale, the local incremental grouping process that leads to illusory
contour formation. This could in turn provide more detailed representation, which could im­
prove higher level object recognition (Hochslein and Ahissar 2002). For example, the fact that
perceptual grouping does not occur during the inaitentional blindness condition (inability lo per­
ceive something that is within one's direct perceptual field as a result of a lack of attention) of
an image provides further support for this conclusion (Roelfsema 2()0f)). Overall, the different
approaches described in this section can be integrated into a common global recurrent pnKess
spanning several regions of the visual system, each of which interacts in parallel to achieve the
completion effect.