Perceptual Organization 183Is Grouping an Early or Late Process?
If perceptual organization is to be understood as the result of
computations, the question of where grouping occurs in the
stream of visual processing is important. Is it an early process
that works at the level of two-dimensional image structure or
does it work later, after depth information has been extracted
and perceptual constancy has been achieved? (Perceptual
constancy refers to the ability to perceive the unchanging
properties of distal environmental objects despite variation in
the proximal retinal images caused by differences in viewing
conditions; see Chapter 4.) Wertheimer (1923) discussed
grouping as though it occurred at a low level, presumably cor-
responding to what is now called image-based processing (see
Palmer, 1999). The view generally held since Wertheimer’s
seminal paper has been that organization must occur early to
provide virtually all higher level perceptual processes with
discrete units as input (e.g., Marr, 1982; Neisser, 1967).
Rock and Brosgole (1964) reported evidence against
the early-only view of grouping, however. They examined
whether the relevant distances for grouping by proximity are
defined in the two-dimensional image plane or in perceived
three-dimensional space. They showed observers a two-
dimensional rectangular array of luminous beads in a dark
room either in the frontal plane (perpendicular to the line of
sight) or slanted in depth, so that the horizontal dimension was
foreshortened to a degree that depended on the angle of slant.
The beads were actually closer together vertically, so that when
they were viewed in the frontal plane, observers saw them
grouped into vertical columns rather than horizontal rows.
The crucial question was what would happen when the
same lattice of beads was presented to the observer slanted
in depth so that the beads were closer together horizontally
when measured in the retinal image, even though they are
still closer together vertically when measured in the three-
dimensional environment. When observers viewed this
slanted display with just one eye, so that binocular depth
information was not available, they reported that the beads
were organized into rows. But when they perceived the slant
of the lattice in depth by viewing the same display binocu-
larly, their reports reversed: They now reported seeing the
slanted array of beads organized into vertical columns. This
finding thus supports the hypothesis that final grouping
occurs after stereoscopic depth perception.
Rock, Nijhawan, Palmer, and Tudor (1992) addressed a
similar issue in lightness perception. Their results showed that
grouping followed the predictions of a late (postconstancy)
grouping hypothesis: Similarity grouping in the presence of
shadows and translucent overlays was governed by the per-
ceived lightnesses of the elements rather than by their retinal
luminances. Further findings using analogous methods have
shown that perceptual grouping is also strongly affected
by amodal completion (Palmer, Neff, & Beck, 1996) and by
illusory contours (Palmer & Nelson, 2000), both of which are
believed to depend on depth perception in situations of occlu-
sion (see Rock, 1983). (Amodal completion is the process by
which partly occluded surfaces of objects are perceived as
continuing behind the occluding object, as illustrated in Fig-
ure 7.10, and illusory contours are edges that are perceived
where there is no physical luminance gradient present because
the occluding surface is the same color as the occluded surface,
as illustrated in Figure 7.13. See section entitled “Visual Inter-
pretation” for further information.) Such results show that
grouping cannot be attributed entirely to early, preconstancy
visual processing, but they are also compatible with the possi-
bility that grouping is a temporally extended process that in-
cludes components at both early and later levels of processing
(Palmer, in press-a). A provisional grouping might be deter-
mined at an early, preconstancy stage of image processing, but
might be overridden if later, object-based information (from
depth, lighting conditions, occlusion, etc.) required it.
Before leaving the topic of early versus late grouping, it is
worth noting that Wertheimer (1923) discussed a further fac-
tor in perceptual grouping that is seldom mentioned: past
experience. The idea is that elements that have been previ-
ously grouped in certain ways will tend to be seen as grouped
in the same way when they are seen again. According to
modern visual theory, such effects would also support the
hypothesis that grouping effects can occur relatively late in
perception, because they would have to happen after contact
has been made between the information in the stimulus dis-
play and representations in memory.
Figure 7.5 provides a particularly strong demonstration of
the effects of prior experience. People who have never seenFigure 7.5 Effects of past experience on perceptual organization (see text).
Source:Original photograph by R. C. James.