Bernal S D_2010.pdf - University of Plymouth

2.2. HIGH-LEVEL FEEDBACK
PREDICTIVE CODING SHARPENING
Li J .Li 11.1 A I
HighLevRi Model High-I B\ie\ Model
I • I. \u\Al
Res dual Sharpened Response
UiX.ljl. uUll'iL
Law-level Inpul PaHem Low-level Input Patient
Figure 2.10: Comparison of predictive coding (left) and sharpening (righl)i;ffects in mediating
response reduclimi in lower levels. In predictive coding, a high-level prediction
of the expected inpui is fed back and subiracied at the inpul level. What is sent
forward is the dillerenee heiween the expected value and ihe ueiual input. With
sharpening (presenl in biased compelilion and adaptive re.sonance models), the
same high-level predieiion is fed back but is instead used loenhance those aspects
of ihe input thai are consi.sleni wilh ihc high-level percepi and reduce all other
aspects. The result, in both eases, can he a reduciion in activity. (Murray el al.
20(M).
error. While the error population will show reduction with high-level feedback, the prediction
population may show enhancement. Predictive coding theories can be misleading, as they place
a stronger focus on the error-delecting nodes and consequently under-emphasize or omit predic­
tion nodes. A second requisite to reconcile biased competition and predictive coding theories
regards the connectivity of feedback. In most biased competition mtxlels. nodes al each level
compete by inhibiting Ihe output of neighhotiring nodes, while feedback in predictive coding
typically acts on the level below. Therefore, the bia,sed conipeiiiion model thai was shown to
be mathematically equivalent to predictive coding (Spratling 2008b). required an alternative
implementation thai suppressed the inputs of neighbouring nodes,
It is not therefore surprising that predictive coding models are also compatible with theories
of feedback as attention. Both Rao (2005). by extending his original model, and .Spralling
(2008a). using the previously described architecture, demonstrated thai predictive coding could
account for spatial and feature attentional effecls. Turihermore, Spratling (2()08a) hypothe-
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