Bernal S D_2010.pdf - University of Plymouth

3.4. EXISTING MODELS
Section 3.4,2). Similariathc previous mapping, cortical columns implemenl the different nodes
or variables {e.g. coding for a specific region of Ihe input image), while minicolumns represent
the different possible states or features at that location {e.g. different orientations).
According to the authors, projections from lower cortical to layer 4 levels are responsible for
the storage and detection of coincidence patterns. The synaptic connections between these lay­
ers represent the co-ocurrence of palicms on its inputs. Layer 4 then projects onto layer 2/3
pyramidal ceUs which are assumed to behave as complex ceils which respond to invariant fea­
tures or motion sequences. Thus, layer 2/3 is responsible for the calculation of the feedforward
Markov chains' (groups nr sequences) states as suggested by the high density of lalerai con­
nections. At the same time, anatomical connections suggest these neurons project the Markov
chain information to higher cortical levels, and incorporate high-level information, received via
layer 1 projections, into Ihc computation of the Markov chains. Layer 5 pyramidal neurons wiih
dendriies in layers 1. 3 and 4 are responsible for the Belief calcuiaiion, hinally, layer 6 neurons
with dendrites in layer 5 compute the feedback messages for hiwer regions.
Further inspection of the proposed mappings reveals several key similarities and differences
between them, which are summarized in Figure 3.14. Each variable or graph node Is roughly
understood as a cortical functional column, containing smaller functional units or minicolumns,
which correspond to the dlffcrenl variable states (George and Hawkins 2(K)9. Litvak and Ullman
2009). Feedforward outgoing mes,sages from a node are assumed to originate from pyramidal
cells in layer 2/3 (George and Hawkins 2009, Litvak and Ullman 2009, Lee and Mumford 2003,
Friston et al. 2006). Feedback outgoing messages originate from pyramidal cells In the infra-
granular layers (Frision cl al, 2006), either layer 5 (Lee and Mumford 2003) or layer 6 (George
and Hawkins 2009). Feedforward incoming messages from lower cortical areas target layer 4
neurons (Frision et al. 2006. Litvak and Uliman 2009, George and Hawkins 2009).
However, ihere are two dlffereni neural popuhiions that could polenlially encode the incom­
ing feedback messages from higher-levels; neurons in supragranular layers (Liivak and Ullman
2009, Friston 2010), more precisely, in layer 1 according to George and Hawkins (2009); or
neurons in infragranular layer 6 (Litvak and Ullman 2009, Friston et al. 2006). Both of them
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