190 Visual Perception of Objects
people perceive two distinct objects, rather than one that is
completed behind it. Finally, Figure 7.12 (D) shows an
example in which there are no relatable surfaces, yet amodal
completion is perceived. These two example therefore show
that relatable surfaces are neither necessary nor sufficient
for perceiving amodal completion. Tse (1999) has argued
persuasively from such examples that completion is ulti-
mately accomplished by merging inferred three-dimensional
volumes.
Illusory Contours
Another important form of visual interpolation produces a
striking illusion in which contours are seen that do not actu-
ally exist in the stimulus image. This phenomenon of illusory
contours(also called subjective contours) was first described
almost a century ago (Schumann, 1904), but modern interest
in it was sparked by the elegant demonstrations of Kanizsa
(1955, 1979). One of the best known examples is the so-
calledKanizsa triangleshown in Figure 7.13. The white
triangle so readily perceived in this display is defined by il-
lusory contours because the stimulus image consists solely
of three pac-man–shaped figures. Most observers report see-
ing well-defined luminance edges where the contours of the
triangle should be, with the interior region of the triangle
appearing lighter than the surrounding ground. These edges
and luminance differences simply are not present in the op-
tical image.
Recent physiological research has identified cells in corti-
cal area V2 that appear to respond to the presence of illusory
contours. Cells in area V2 have receptive fields that do not
initially appear much different from those in V1, but careful
testing has shown that about 40% of the orientation selective
cells in V2 also fire when presented with stimuli that induce
illusory contours in human perception (von der Heydt,
Peterhans, & Baumgartner, 1984; Peterhans & von der
Heydt, 1991). Sometimes the orientational tuning functions
of the cells to real and illusory contours are similar, but often
they are not. Exactly how the responses of such cells might
explain the known phenomena of illusory contours is not yet
clear, however.
Perceived TransparencyAnother phenomenon of visual interpolation is perceived
transparency:the perception of objects as being viewed
through a closer, translucent object that transmits some
portion of the light reflected from the farther object rather
than blocking it entirely. Under conditions of translucency,
the light striking the retina at a given location provides infor-
mation about at least two different external points along
the same viewer-centered direction: one on the farther
opaque surface and the other on the closer translucent surface
(Figure 7.14; A).
Perception of transparency depends on both spatial and
color conditions. Violating the proper relations of either sort
is sufficient to block it. For example, transparency will be
perceived if the translucent surface is positioned so that re-
flectance edges on the opaque surface behind it can be seen
both through the translucent surface and outside it, as illus-
trated in Figure 7.14 (A). When this happens, a phenomenon
calledcolor scissionorcolor splittingoccurs, and the image
colors in the regions of the translucent surface are perceived
as a combination of one color belonging to the background
and one color belonging to the translucent surface. Color
scission will notoccur, however, if the translucent surface
lies completely within a single reflectance region, as illus-
trated in Figure 7.14 (B). It can also be blocked by destroying
the unity of the translucent region (Figure 7.14; C) or merely
weakening it (Figure 7.14; D).
When color scission occurs, the perceived color in each re-
gion of overlap is split into a component from the transparentFigure 7.13 Illusory contours in a Kanizsa triangle. Source: After
Kanizsa, 1955.
ABCD Ew xyzFigure 7.14 Conditions for perceiving transparency (see text). Source:
From Palmer, 1999.