Bernal S D_2010.pdf - University of Plymouth

2.3. ILLUSORY AND OCCLUDED CONTOURS
implement bipole units.
A similar model which also falls into die base-grouping category focuses on the feed-forward
integration of end-stopped responses (Heitger ct al, 1998). This approach extends the classi­
cal simple and complex cell nuxiel to include end-stopped neurons, cells which decrease iheir
response when the stimulus size exceeds the classical receptive held, The response of the ori­
ented end-stopped cells is important as it is associated with figure-ground segregation, lividencc
shows that partial occlusion typically generates abrupt terminations at the side of the occluded
surface, which can be accounted for by ligure-ground segregation mechanisms. Furthermore.
the model end-slopped neurons also distinguish and encode the direction of contrast between
figure and ground surfaces. It was found that neurons in V2 that respond to illusory contours
are sensitive to the direction of contrast, and that this usually matches the occlusion direc­
tion. Thus, the model performs contour interpolation of subjective contours by integrating over
die end-stopped cells according to the bipole weighting function, and taking into accouni the
contrast-direction selectivity of these cells.
This feedforward processing stage is sometimes referred to as the preattenlive phase, and al­
though some highlight the exclusive involvement of feedl'orward as an advantage, the general
consensus is that feedforward processing by itself is insuflicient to perform contour completion,
specially across large areas of the visual field. For example, the size of receptive fields in lower
level regions, where contour completion effects are observed, is insuflicient lii cover the visual
field distance (> lO'') between the inducer line segments of large illusory figures (Angelucci and
Bullier 2003. Sterzer et al, 2006). However, as pointed out al the end of this section, preallentive
processing can also be underslotxJ as an important initial stage in the more global completion
process, which also involves recurrent processing.
Horizontal and feedback models A second category of models, also denoted as incremental
grouping models (Roelfsema 2UU6), lake into account the context of the elcmenls involved
by making use of horizonial and feedback connectivity. Under this network scheme, neurons
stimulated directly from luminance-defined contours provide facilitalive interactions to neurons
which do not receive direct retinal stimulation. It is usually considered an attentive, and thus
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