Bernal S D_2010.pdf - University of Plymouth

2.3. HIVSORY AND OCCLUDED CONTOURS
more lime-consuming process, which gradually strengthens the responses of features which are
perceptually grouped together.
An important concept for these models is the local association field which formalizes the Gestalt
good continuation principle. It states that contour elements which are well aligned will tend
to be grouped together, by mutually exciting and increasing each other's saliency, while non-
collinear element.', tend to inhibit each other. Several studies have shown the neural interactions
thai represent contour elements in VI and V2 are dictated by the selectivity of horizontal con­
nections which follow the local association field principle (Roelfsema 2006). The local associ­
ation field provides an alternative method of implementing the bipole principle, based on lateral
recurrent circuits (Li 2001) instead of strictly feedforward connections.
Grossberg and colleagues proposed the Boundary Contour System (BCS), one of the most
prominent models based on the above principles. The BCS is encompassed by the more general
Adaptive Resonance Theory (see Section 2.2.3), and comprises a number of stages thai perfonn
detection, competition and cooperation of boundary segments. Later versions of the model
combine both feedback, lateral competition and feedforward integration of responses through
the so-called bipole cells to achieve contour completion (Grossberg ei al. 2(X)7). Here we fo­
cus on previous models (Grossberg et al. 1997, Raizada and Grossberg 2001) that show how
contour completion is achieved by implementing the hipole propeny using strictly horizontal
connections.
Bipole cells are activated only when both sides of their receptive Held are sufficiently stimulated
as shown in the schematic diagram of Figure 2.13. Ovals represent pyramidal celts located in
layer 2/3 with collinear and co-oriented receptive fields. They are connected to each other via
excitatory long-range horizontal synapses. These connections also excite a pool of inhibitory
iniemeurons (black circles) connected via short range synapses to the pyramidal cell. The
balance of excitation and inhibition accomplishes the desired bipole property.
When only one of a pair of pacmen is present, the excitation from the inducing pyramidal
cell to the target pyramidal cell is not enough to elicit the cell's response. This is because the
excitation also targets the inhibitory neurons that balance out the excitation. On the other hand,
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