Bernal S D_2010.pdf - University of Plymouth

3.4. EXISTING MODELS
Bollom-up cues
Frorn illuminalea B1
part of the face
High-level representalion
with aDsIract fealures
Low-level re prose nia lion
with high resolution
Top-down foe d back
B2 recognizes shadowed
edge as part of the fece
I I P(X3/X4)
x,= P(x^x,) • P(x,;x.)
X,= P(X,/X,)- P(X^Xj)
Xi
X,= P(X^X,1- P(X,/X,1
T
P(X2/X3)
P(X,/X2)
Figure 3.J J: Bayesian belief prupagat ion archileclure applied lo the visual system, a) Inilially,
bottom-up cues from the illuminated part of the face (Bl) cause a face hypothesis
iLi hci'ome aclivaicil ai ihc higher levels. Then inrormaiiim about ihe likely
features and propiirliuns of a faue is amveyed through top-down feedback (B2j
lo Ihe lower-level high resoluiion buffer. Re-examiriaiion of ihc data results in a
reinierprelalion ol" ihf Faint edj;c in the shadtiwed area as an iiTiportani part of the
face contour b] Hacli area conipuios a .set of beliefs, Xj, based on boitoin-up sensory
data (X,-])and top-down priors (/'fX/X+i). which are iniegraied according
lo ihe Bayesian inference equation. Beliefs rue continually updated according to
changes in earlier and higher areas to obtain ibc niosl probable distribution of
causes al each level. Adapicd from Lee and Mumford (2003).
practical implementaiion of this theoretical approach was provided by the authors.
Nonetheless, this theoretical paper has strongly inspired and motivated ihc present thesis, and
provides an intuitive example which allows one to heller understand the concept of how belief
propagalitin can be applied to visual processing. Consider the shadowed face example shown in
Figure 3.13. Initially, bottom-tip cues from the illuminated part of the face cause a/ocf hypoth­
esis to become activated at the higher levels. Then informalion about the likely features and
proportions of a face is conveyed through top-down feedback to the lower-level high resolution
buffer. Re-examination of the data resuhs in a reinlerpretaiion of the faint edge in the shadowed
area as an important part of the face contour. This new detailed information can then be used
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