Introduction - Hierarchical Neural Networks for Image Interpretation - page 4

(a) (b) (c)

(d) (e) (f)
Fig. 1.3. Gestalt principles of perception [125]: (a) similar stimuli are grouped together; (b)
proximity is another cue for grouping; (c) line segments are grouped based on good contin-
uation; (d) symmetric contours form objects; (e) closed contours are more salient than open
ones; (f) connectedness and belonging to a common region cause grouping.

Fig. 1.4. Kanizsa figures [118]. Four inducers produce the percept of a white square partially
occluding four black disks. Line endings induce illusory contours perpendicular to the lines.
The square can be bent if the opening angles of the arcs are slightly changed.

Other heuristics are summarized by the Gestalt principles of perception [125].
Some of them are illustrated in Figure 1.3. Gestalt psychology emphasizes the
Pragnanz of perception: stimuli group spontaneously into the simplest possible con-
figuration. Examples include the grouping of similar stimuli (see Part (a)). Proximity
is another cue for grouping (b). Line segments are connected based on good con-
tinuation (c). Symmetric or parallel contours indicate that they belong to the same
object (d). Closed contours are more salient than open ones (e). Connectedness and
belonging to a common region cause grouping as well (f). Last, but not least, com-
mon fate (synchrony in motion) is a strong hint that stimuli belong to the same
object.
Although such heuristics are correct most of the time, sometimes they fail. This
results in unexpected perceptions, called visual illusions. One example of these il-
lusions are Kanizsa figures [118], shown in Figure 1.4. In the left part of the figure,
four inducers produce the percept of a white square in front of black disks, because