Bernal S D_2010.pdf - University of Plymouth

6.1. ANALYSIS OF RESULTS
6.1.2.5 Feedback fnim S3 to SI
Feedback emerging from the S3 layer was also able to generate the illusory contours in the lower
layers as illustrated in Figure 5.31. This demonsiraies ihai Teedback is able to reconstruct the
C2 square representation from the S3 layer inl'ormation. In this case the up-down instead of the
upwards belief update method was implemented as ii pnxluces a cleaner lower level response
by reducing the accumulated noise from Jt messages. Importantly, this setup can be understood
a-s a hypothetical scenario whei^ the Kanizsa figure is correctly categorized as a square and due
to some higher level mechanism, such as f{K;uscd attention (Gilbert and Sigman 2007, Reynolds
and Chelazzi 2004). only the square prototype is fed back, similitr lo a winner-take-all network.
Another important property, which was present in previous results but is more obvious here, is
that the similarity of the internal representation of each layer lo any lixed evidence (e.g. input
image or high-level square representation) is proportional to the distance to the layer containing
the evidence. In other words, lower layers show an inlernal representation close to the Kanizsa
ligure, whereas the representation in higher layers is closer to that of a square.This observation
is consistent with evidence suggesting high-level activity generated by objects containing illu­
sory contours is notably similar lo the activity of complete objects (Stanley and Rubin 2003,
Maertens el al. 2008. Sary et al. 200K). Furlhermore, il is also in consonance with evidence
showing the illusory contour response is weaker and only appears in a fraction of V1/V2 cells,
in relation to that of real contours, and thai VI lends lo show an even weaker illusory contour
response than V2 (Lee 2003, Maertens el al. 2008. Seghier and Vuilleumier 2006, Halgren et al.
2003).
The temporal sequence of illusory contour fonnalion in the model is also substantiated by ex­
perimental evidence showing that the l.OC/IT region is the first lo signal the appearance of the
illusory contour, which then gradually spreads lo lower regions (Murray et al. 2002, Halgren
ei al. 2003).
With respect to the mechanisms responsible for contour completion, Haiko et al. (2008) af­
firmed in a receni review that illusory contours result from the interaction between high-level
figural feedback and interpolation/extrapolation processes related to lateral connections. Figu-
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