Bernal S D_2010.pdf - University of Plymouth

2.2. HSGH-LEVEL FEEDBACK
feature analysis modules. Results show this type of network can learn invariance to translation,
size, rotation and contrast, achieving good generalization to new objects even using only a small
training datasel (Wiskott and Sejnowski 2(H)2, Malhias et al. 2008).
The slow feature principle is closely related to the trace rule employed in the Visnet model
(Rolls and Milward 2000) previously described. In contrast, the main advantage of slow feature
analysis is thai it is not limited to extracting a single invariant representaiion. i.e. object iden­
tity, but also maintains a structured representation of other parameters such as object position,
rotation angles and lighting direction.
2.2 High-level feedback
The previous section acts as an introduction to the visual system and in particular to object
recognition. This provides the context to discuss the role of high-level feedback in percep­
tion, exposing many of the phenomena which remain unexplained and challenging some of the
existing classical concepts. To avoid misinterpreialion, wc define feedback as activity origi­
nating in a high-level region targeting a lower-level region, which therefore excludes inlralevel
inlerluminar activity.
2.2.1 Experimental evidence
2.2.1.1 Anatomical pcrspeclivc
From the anatomical point of view, feedback connections extensively outnumber feedforward
sensory pathways (Felleman and Van Essen 1991, Macknik and Marlinez-C'onde 2007). The
great majority of connections between regions shown in Figure 2. la are reciprocal, which pro­
vides lower areas with massive feedback from higher cortical areas. For example, cat LGN
intemeurons receive 25Cf of their inputs from the retina, while 37% come from cortex; for
LGN relay cells, the corresponding percentages are 12% and 5S% (Montero 1991). The same is
true for thalamic relay nucleus (TRN), which mediates the transfer of information to the cortex,
where Ihe largest anatomical projection is from connections of cortical feedback and not die
ascending collaterals of relay cells (Sillito and Jones 2008). For LGN relay cells it is generally
believed that feedback exerts a mtnluiatory influence, whereas cortical feedback to TRN is more
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