We briefly mentioned in the Modeling Cortical ... sectionabout possible relations between the lateral connections in our model and the horizontal connections in the visual cortex. It is known that the horizontal connections originate from pyramidal cells, which are of excitatory type [21,18]. Furthermore, pyramidal cells are the principal target of the horizontal connections [18]. The structure of our model (Figure 1b) is consistent with the anatomy of the horizontal connections. The excitatory element of the oscillator, x, could be interpreted as a local group of pyramidal cells. The inhibitory element, y, could be interpreted as a local group of inhibitory cells, such as smooth stellate cells [18,31]. The local group of pyramidal cells and the local inhibitory cell group together could form a single oscillator [7,56], which could be anatomically interpreted as a vertical column. The lateral connections between the excitatory elements of our oscillator network could thus be interpreted as the orizontal connections.
Although the horizontal connections can span quite a long range, up
to a diameter of 6-8 mm in cat primary visual cortex
[19,20,18], these connections do not quite
cover the entire primary visual cortex. In primates, horizontal
connections can extend to 2-3 mm in area V1
[40],
but the entire surface of V1 can span at least 35 mm
[60]. A similar relation between the extent
of horizontal connections and the extent of the entire area holds
true for area V2
[37] and for the primary visual cortex of rats,
[2,58].
Since the horizontal connections in the visual cortex are not quite
all-to-all, they do not provide a firm basis for those theoretical
models that require all-to-all connections
[55,63]
.
An important feature of the horizontal connections is that they are not evenly distributed within a certain radius, but form discrete clusters, and these clusters mainly connect pyramidal cells with similar orientation specificity [19,20,18]. The clustered connections are well consistent our view of how sensory segmentation is mediated by locally coupled neural oscillators: These connections would support the perception of straight boundaries and smooth curves. Though the examples in Figure 6 and Figure 7 emphasize region-based segmentation, contour-based approaches must be part of a more complete system of sensory segmentation.