Bernal S D_2010.pdf - University of Plymouth

2.3. ILLUSORY AND OCCLUDED CONTOURS
2005) and single-cell recording (Lee and Nguyen 2001, Lee 2003). Rauschenbergeret al. (2006)
showed how the representation in early visual cortex of an occluded disc evolved from that of a
notched disc to one corresponding lo a complete disc after approximately 250 ms. The amodal
completion process shows temporal properties similar lo those of illusory conlours, i,e, a delay
of approximately 100-200 ms with respect to real contours; although the response tends to be
significantly lower Ihan for illusory contours (Lee 2003). This is consistent with the weaker,
non-visually salient, perceptual experience associated with occluded conlours.
The representation of occluded ohjecis in high-level objcci recognition areas, such as IT and
LOC, has also been repeatedly documented (Hegde et al. 2008, Murray et al, 2006. Weigelt
et al. 2007, Hulme el al. 2007). Consistenl with this observation, abundant evidence sustains
the multistage model of object processing, and shows the temporal representation of occluded
contours occurs in atop-down fashion (Murray el al. 2(K)6, Rauschenbcrger el al. 2006. Weigell
et al. 2007), This is indicative of high-level feedback being responsible for amodal completion
in lower regions. However, two diverging interpretations exist as to what exactly is represented
by higher-level neurons.
The first interpretation rests upon evidence showing ihat high-level represenlalions of occlusion
are invariant, and have similar time courses and magnitude to those of complete objects (Weigelt
ei al. 2007, Hulme et al. 2007, Rauschcnberger el al. 2006). It therefore suggests that, although
the occluded-objecl and incomplete-object interpretations are both kept alive in lower visual
areas, in ihe higher levels only ihe occluded-object interpretation persists. This means the high-
level neurons represent just the completed object, which becomes the explicit percept. This is
consistenl with the lileralure on bistable stimuU which indicates only ihe conscious percept is
represented in high-levels (Fang ei al. 2008).
Contrastingly, an fMRl study (Hegde et al- 2(X)8) reported regions in ihe LOC area which show
significantly stronger responses to occluded object,s than to unoccluded objects. Along the same
lines, Murray et al. (20O6) identified within the LOC region a specific object recognition stage
which included boundary completion processes. This would suggest ihc incomplete object is
also represented al some stage in this high-level region. The siudy pointed out thai this does not
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