Bernal S D_2010.pdf - University of Plymouth

2.3. ILLUSORY AND OCCLUDED CONTOURS
established directions, and is considered one of the milestones of perceptual organization. A
similar concept, known as relatability, has been suggested as the guiding principle for contour
completion in the visual system. It is based on smtwlhness principles such as contours being
differentiable at least once, monotonic, and having bending angles below 90" (Kellman 2003).
An important tool to explain contour completijin is the bipole (Neumann and Mingolla 2001.
(irossberg et al. 1997. Roelfsema 2006). Although it can have diverging definitions depending
on the author, most generally its function is to evaluate the effect a contour element at a given
location has on the likelihood of perceiving a contour at a second k)cation. The bipole is typ~
ically represented by a characteristic hgure-eighl shape which describes the coupling strength
between the centre unit and surrounding units according to their relative position and orienta­
tion. A neural implementation of this concept gives rise to bipole cells, defined as nonlinear
grouping operators which receive input from real edges falling inside the bipole lobes (see Fig­
ure 2.12). It is strongly grounded on anatomical, psychophysical and physiological data (see
Neumann and Mingolla (2001) Cor a review), and provides a biologically grounded method of
implementing good continuation and relatabilily principles.
This section discusses existing approaches and models that show how these geometric concepts
can be implemented with neural mechanisms and how they relate to the physiology and anatomy
of the visual cortex.
2.3.2.3 Clas.sjficati(in or theoretical models
Two broad theoretical categories for contour completion models have been considered. The first
one relies on feedforward processing, and is also known as base grouping. The second deals
with recursive mcxlels, placing the focus on lateral and feedback connections, and is also known
as incremental grouping (Roclfsema 2006, Neumann and Mingolla 2001),
An alternative, though compatible, classification focuses on the specific mechanisms involved
ill contour completion, and proposes three broad classes: 1) contour interpolation, which in­
wardly extends two aligned line segments; 2) contour extrapolation, which outwardly extends
a segment of a single line segment; and 3) figural feedback, whereby a high-level representa­
tion feeds back to complete missing contours (Halko et al. 2{K)8). The lirst two classes can
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