Perceptual Organization 191layer and a component from the opaque layer. For this to hap-
pen, the components due to the transparent layer must be the
same. Metelli (1974), Gerbino (1994), and Anderson (1997)
have precise, quantitative theories of the conditions for trans-
parency to be perceived. Violating these constraints also
blocks perceived transparency (Figure 7.14; E).
Figural Scission
Yet another example of visual interpolation isfigural scission:
the division of a single homogeneous region into two overlap-
ping figures of the same color, one in front of and occluding the
other. This phenomenon, illustrated in Figure 7.15, has many
interesting features. One is that there is no local sensory infor-
mation that requires the single region to be split at all. The
visual system constructs illusory contours where the closer
figure occludes the farther one. The visual system alsocom-
pletesthe portions of the farther figure that are occluded by the
closer one. But because the stimulus conditions do not deter-
mine which figure is in front and which behind, either possi-
bility can be perceived. Indeed, if you view such displays
for awhile, the depth relations of the two parts spontaneously
reverse.
Parts and Wholes
Assuming that objects are indeed perceived as structured into
something like a hierarchy of objects, parts, subparts, and so
on (cf. Palmer, 1977; Reed & Johnsen, 1975), a question that
naturally arises is whether parts are perceived before wholes
or wholes before parts. Although Gestaltists never posed the
question in precisely this form, their approach to perception
suggests that wholes may be processed first in some impor-
tant sense. Most other approaches to perception imply the
opposite: that wholes are constructed by integrating local
information into increasingly larger aggregations. Even
physiological evidence seems to support a local-first view.
Retinal receptors respond to exceedingly tiny regions of
stimulation, and as one traces the path of neural information
processing, synapse by synapse, deeper into the brain, the
receptive fields of visual neurons become ever larger and
responsive to ever more complex stimulus configurations
(e.g., Van Essen & De Yoe, 1995).
There are problems in accepting this line of argument
as settling anything about perceptual experience,however.
First, the order in which processing is initiatedmay not be
nearly as relevant for perceptual experience as the order in
which it is completed. Although it is clear that neural pro-
cessing is initiated in a local-to-global order, it is by no means
clear that it is completed in this order. Indeed, there is strong
evidence that the flow of neural information processing is not
unidirectional from the sensory surface of the retina to higher
centers of the brain. Massive backward projections from
higher to lower cortical areas suggest that a great deal of
feedback may occur, although nobody yet knows precisely
what form it takes or even what functions it serves. The exis-
tence of feedback raises the possibility that the order in which
perceptual experience arises is not given by the simplistic
reading of the physiological facts given in the previous para-
graph. Moreover, evidence from psychological experiments
suggests that perception of global objects often precedes that
of local parts.Global PrecedenceNavon (1976) asked about the priority of wholes versus parts
by studying discrimination tasks with hierarchically struc-
tured stimuli: typically, large letters made of many appropri-
ately positioned small letters. On some trials subjects were
shownconsistent configurationsin which the global and
local letters were the same, such as a large Hmade of many
smallHs or a large Smade of many small Ss. On others, they
were shown inconsistent configurationsin which the global
and local letters conflicted, such as a large Hmade of many
smallSs or a large Smade of many small Hs. They were cued
on each trial whether to report the identity of the letter repre-
sented at the global or the local level. Response times and
error rates were measured.
The results of Navon’s experiment strongly supported the
predictions of global precedence:the hypothesis that ob-
servers perceive the global level of hierarchical stimuli be-
fore the local level. Response times were faster to global than
to local letters, and global inconsistency interfered when sub-
jects were attending to the local level, but local inconsistency
did not interfere when they were attending to the global level.
The data thus appear to indicate that perceptual processesFigure 7.15 Figural scission, in which a single homogeneous region is
sometimes perceptually divided into two overlapping objects, one of which
partly occludes the other. Source: From Palmer, 1999