182 Visual Perception of Objects
them. Synchrony is related to the classical principle of com-
mon fate in the sense that it is a dynamic factor, but, as this
example shows, the “fate” of the elements does not have to be
common: Some dots get brighter and others get dimmer. Syn-
chrony grouping can even occur when the elements change
along different dimensions, some changing brightness, others
changing size, and still others changing orientation.
One might think that grouping principles are mere text-
book curiosities only distantly related to anything that occurs
in normal perception. On the contrary, they pervade virtually
all perceptual experiences because they determine the objects
and parts we perceive in the environment. Dramatic exam-
ples of perceptual organization going wrong can be observed
in natural camouflage. The leopard in Figure 7.1 (A) is not
camouflaged against the uniform sky, but if it were seen
against a mottled, leafy backdrop, it would be very difficult
to see—until it moved. Even perfect static camouflage is un-
done by the principle of common fate. The common motion
of its markings and contours against the stationary back-
ground causes them to be strongly grouped together, provid-
ing an observer with enough information to perceive it as a
distinct object against its unmoving background.
Successful camouflage also reveals the ecological ratio-
nale for the principles of grouping: finding objects. Camou-
flage results when the same grouping processes that would
normally make an organism stand out from its environment
as a separate object cause it to be grouped with its surround-
ings instead. This results primarily from similarity grouping
of various forms, when the color, texture, size, and shape of
the organism are similar enough to those of the objects in its
environment to be misgrouped.
Integrating Multiple Principles of Grouping
The demonstrations of continuity and closure in Fig-
ure 7.2 (I and J) illustrate that grouping principles, as formu-
lated by Wertheimer (1923/1950), areceteris paribusrules:
They predict the outcome of grouping with certainty onlywhen
everything else is equal—that is, when no other grouping fac-
tor opposes its influence. We saw, for example, that continuity
governs grouping when the elements do not form a closed fig-
ure, but continuity can be overcome by closure when they do
(Figure 7.2; I vs. J). The difficulty with ceteris paribus rules is
that they provide no scheme forintegratingmultiple factors
into an overall outcome—that is, for predicting the strength of
their combined effects. The same problem arises for all of the
previously mentioned principles of grouping as well. If prox-
imity influences grouping toward one outcome and color simi-
larity toward another, which grouping will be perceived
depends heavily on the particular degrees of proximity and
color similarity (e.g., Hochberg & Silverstein, 1956).
Recent work by Kubovy and his colleagues has begun to
address this problem. Kubovy and Wagemans (1995) mea-
sured the relative strength of different groupings in dot lat-
tices (Figure 7.4; A) by determining the probability with
which subjects reported seeing them organized into lines in
each of the four orientations indicated in Figure 7.4 (B). After
seeing a given lattice for 300 ms, subjects indicated which of
the four organizations they perceived so that, over many tri-
als, the probability of perceiving each grouping could be es-
timated. Consistent with the Gestalt principle of proximity,
their results showed that the most likely organization is the
one in which the dots are closest together, with other organi-
zations being less likely as the spacing between the dots
increased.
More precisely, Kubovy and Wagemans (1995) found that
their data were well fit by a mathematical model in which the
attraction between dots decreases exponentially as a function
of distance:f(v)e(va1),wheref(v) is the attraction between two elements in the lat-
tice as a function of the distance, v, between them, is a scal-
ing constant, and ais shortest distance between any pair of
elements. Further experiments using lattices in which the
dots differed in color similarity as well as proximity showed
that the rule by which multiple grouping factors combine is
multiplicative (see Kubovy & Gepstein, 2000). This finding
begins to specify general laws by which multiple factors can
be integrated into a combined result.closest farthest
Figure 7.4 Dot lattice stimuli (A) and possible groupings (B) studied by
Kubovy and Wagemans (1995). Source:From Palmer, 1999.