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teraction of neurons belonging to the same hypercolumn. For instance, the M and P
input from LGN is relayed by excitatory spiny stellate cells to layers 2 and 3.
The pyramidal output is also folded back into the local circuit. Axon collaterals
of pyramidal cells from layers 2 and 3 project to layer 5 pyramidal cells, whose axon
collaterals project both to layer 6 pyramidal cells and back to cells in layers 2 and
3. Axon collaterals of layer 6 pyramidal cells project back to layer 4C inhibitory
smooth stellate cells.
Although many details of the connectivity of such local circuits are known, the
exact function of these circuits is far from being understood. Some possible func-
tions could be the aggregation of simple features to more complex ones, as hap-
pens in V1 with the aggregation from center-surround to linear oriented to phase-
invariant oriented responses. Furthermore, local gain control and the integration of
feed-forward and feedback signals are likely functions of such circuits.
In addition to local recurrent computation and vertical interactions, there is also
heavy lateral connectivity within a cortical area. Figure 2.10 shows a layer 3 pyrami-
dal cell that connects to pyramidal cells of similar orientation within the same func-
tional column and with similarly oriented pyramidal cells of neighboring aligned
hypercolumns. These specific excitatory connections are supplemented by unspe-
cific inhibition via interneurons.
The interaction between neighboring hypercolumns may mediate extra-classical
effects of receptive fields. In these cases, the response of a neuron is modulated by
the presence of other stimuli outside the classical receptive field. For instance, neu-
rons in area V1 are sensitive not just to the local edge features within their receptive
fields, but are strongly influenced by the context of the surrounding stimuli. These
contextual interactions have been shown to exert both facilitatory and inhibitory ef-
fects from outside the classical receptive fields. Both types of interactions can affect
the same unit, depending on various stimulus parameters. Recent cortical models by
Stemmler et al. [220] and Somers et al. [219] described the action of the surround as
Fig. 2.10. Lateral connections in V1. Neighboring aligned columns of similar orientation
are linked with excitatory lateral connections. There is also unspecific local inhibition via
interneurons (adapted from [117]).
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