180 Visual Perception of Objects
just part-whole structure, but it seems to be the single most
central issue.
The first problem, therefore, is to understand what part-
whole structure people perceive in a given scene and how it
might be characterized. Logically, there are limitless possible
organizations for any particular image, only one (or a few) of
which people actually perceive. A possible part-whole struc-
ture for the leopard image in Figure 7.1 (A) is given in Fig-
ure 7.1 (C). It is represented as a hierarchical graph in which
each node stands for a perceptual unit or element, and
the various labels refer to the image regions distinguished in
Figure 7.1 (B). The top (or root) node represents the entire
image. The scene is then divided into the leopard, the branch,
and the background sky. The leopard is itself a complex per-
ceptual object consisting of its own hierarchy of parts: head,
body, tail, legs, and so forth. The branch also has parts con-
sisting of its various segments. The sky is articulated into
different regions in the image, but it is perceptually uniform
because it is completed behind the leopard and branches. The
bottom (or terminal) nodes of the graph represent the millions
of individual receptors whose outputs define this particular
optical image.
The second problem is how such a part-whole hierarchy
might be determined by the visual system. This problem, in
turn, has at least three conceptual parts. One is to understand
the nature of the stimulus information that the visual system
uses to organize images. This includes not only specifying
the crucial stimulus variables, but also determining their eco-
logical significance: why they are relevant to perceiving part-
whole structure. It corresponds to what Marr (1982) called a
“computational” analysis. The second problem is to specify
the processing operations involved in extracting this infor-
mation: how a particular organization is computed from an
image via representations and processes. It corresponds to
what Marr called an “algorithmic” analysis. The third is to
determine what physiological mechanisms perform these op-
erations in the visual nervous system. It corresponds to what
Marr called an “implementational” analysis. As we shall see,
we currently know more about the computational level of
perceptual organization than about the algorithmic level, and
almost nothing yet about the neural implementation.Perceptual GroupingThe visual phenomenon most closely associated historically
with the concept of perceptual organization is grouping:the
fact that observers perceive some elements of the visual field
as “going together” more strongly than others. Indeed, per-
ceptual grouping and perceptual organization are sometimes
presented as though they were synonymous. They are not.
Grouping is one particular kind of organizational phenom-
enon, albeit a very important one.Principles of GroupingThe Gestalt psychologist Max Wertheimer first posed the
problem of perceptual organization in his groundbreaking
1923 paper. He then attempted a solution at what would now
be called the computational level by asking what stimulusA
B
C
Figure 7.1 A natural image (A), its decomposition into uniform connected
regions (B), and a hierarchical graph of its part-whole structure (C).
Source:From Palmer, 2002.