Bernal S D_2010.pdf - University of Plymouth

6.1. ANALYSIS Ob RESULTS
update methods during loopy belief propagation as shown in Figure 5,2^. The different methods
were explained in detail in Section 4.5.3. An initial tirsl observation is that both the upwards
method, used to obtain most of the results, and the up-down method provide similar results to
that of the rigorously correct, but computationally expensive, compti-ie update method. This
argues for the validity of these methods, suggesting that computing the beliefs of layers where
no new evidence information has arrived might be redundant and not provide any significant
contribution to the linal belief. However, it is likely that the differences between methods is
accenluated as feedback originates from higher layers, leading lo longer internal loops. Further
and more systematic research is required lo confirm the validity of these update methods.
Other interesting elTects can be observed in Figure 5.29. To start with, the SI belief seems
to show an oscillatory effect where the illusory contour gels narrower and wider. It would be
interesting lo study in more detail whether the narrowing is a consequence of feedback disam­
biguation guided by the real contours of the inpul image or an epiphenomenon derived from
some other cause. Also, the up-down method S1 belief shows the cleanest response, which is
consistent with the fact that it is updating the layers in Ihe expected sequence of propagation,
minimizing the propagation of noisy information. According to this account, the illusory con­
tour from the upwards method should be cleaner than that of the complete method, but is not.
This might be a consequence of the a.symmclry of the upwards method, which gives preference
lo the bollom-up evidence, as compared with the other two methods, where bottom-up and top-
down evidence propagate at Ihe same rate. Again, these are jusl speculative ideas ba.sed on the
limited preliminary results obtained.
The results shown in Figure 5.30 demonstrate Ihe ability of the model lo feed back informaiion
from the C2 layer. Using the infonnalion from Figure 5,23. an S2-C2 weight matrix was chosen
to try to obtain the best reconstruction possible, although this was still I'ar from an ideal square
representation. Nonetheless, the SI and Ci layer still develop activity close to the illusory
contour region.
245