ences like those in figure 8.3A can be perceived by deliberate scrutiny involving
focused attention, but such processes appear to be different from normal ef-
fortless grouping such as occurs in viewing figure 8.3B.
Gestalt psychologists also described several further factors that influence
perceptual grouping of linelike elements. Symmetry (figure 8.2G) and parallel-
ism (figure 8.2H), for example, are factors that influence the grouping of indi-
vidual lines and curves. Figure 8.2I illustrates the important factor ofgood
continuation(orcontinuity) of lines or edges: All else being equal, elements that
can be seen as smooth continuations of each other tend to be grouped together.
Its effect is manifest in this figure because observers perceive it as containing
two continuous intersecting lines rather than as two angles whose vertices meet
at a point. Figure 8.2J illustrates the further factor ofclosure: All else being
equal, elements forming a closed figure tend to be grouped together. Note that
this display shows that closure can overcome continuity because the very same
lines that were organized as two intersecting lines in part I are organized as
two angles meeting at a point in part J. According to Wertheimer’s analysis,
this is because the noncontinuous segments now constitute parts of the same
closed figure.
The demonstrations of continuity and closedness in figures 8.2I and 8.2J
illustrate an important limitation in current knowledge about grouping prin-
ciples. As formulated by Gestalt psychologists, they areceteris paribus rules,
which means that they can predict the outcome of grouping with certainty only
when everything else is equal—that is, when there is no other grouping factor
influencing the outcome. We saw, for example, that continuity governs group-
ing when the elements do not form a closed figure, but it can be overcome by
closure when they do.
The difficulty with ceteris paribus rules is that they provide no general pur-
pose scheme for integrating several potentially conflicting factors into an over-
all outcome—that is, for predicting the strength of their combined influences.
The same problem arises for all the previously mentioned principles of group-
ing. If proximity influences grouping toward one outcome and similarity in
color toward another, the grouping that will be perceived depends heavily on
the particular example. Figure 8.4A shows a case in which proximity is strong
enough to overcome color similarity, whereas figure 8.4B shows one in which
color similarity dominates. The visual system clearly integrates over many
grouping factors, but we do not yet understand how it does so. Later in this
Figure 8.3
Degrees of grouping. Not all factors are equally effective in producing grouping. In part A, elements
that differ by 180in orientation are not strongly grouped, whereas those in part B that differ by
only 45produce strong grouping.
194 Stephen E. Palmer