region is to image processing, we have not yet discussed it explicitly, having
concentrated mainly on the essentially one-dimensional constructs of lines and
edges. Now we will consider another important aspect of their perceptual
function: as boundaries that define 2-D regions. Bounded regions are central to
perceptual organization because they may well define the first level of fully 2-D
units on which subsequent visual processing is based.
8.2.1 Uniform Connectedness
Palmer and Rock (1994a) provide an explicit analysis of how Wertheimer’s
presupposed elements might be formed in terms of an organizational principle
they calluniform connectedness: the tendency to perceive connected regions
of uniform image properties—e.g., luminance, color, texture, motion, and
disparity—as the initial units of perceptual organization.^1 As we will see, the
principle of uniform connectedness also forms a crucial link between the liter-
ature on edge detection and that on perceptual organization and grouping.
Let us consider the elements in Wertheimer’s original displays as examples of
how organization into regions by uniform connectedness might occur as an ini-
tial stage in perceptual organization. The dots, lines, and rectangles in figures
8.2A–8.2F are all connected regions of uniform luminance, and they correspond
to the elements to which Wertheimer appealed in his analysis of grouping. The
V’s in figure 8.3A, the lines in figures 8.2G and 8.2H, the X-shaped drawing in
figure 8.2I, and the hourglass-shaped contour in figure 8.2J are also uniform
connected regions according to Palmer and Rock’s analysis, but their relation to
Wertheimer’s ‘‘elements’’ is slightly more complex and will be considered more
fully later.
The powerful effect of uniform connectedness on perceptual organization can
be demonstrated in simple displays of dots like those used by Wertheimer, as
illustrated in figure 8.17. Part A shows that a row of uniformly spaced dots of
different luminance are seen as unitary entities, and part B shows that the same
is true for regions that are defined by differently oriented texture elements.
Parts C and D show that such regions merge into larger, more complex unitary
elements when they are connected by regions defined by the same property,
whereas parts E and F show that when they are connected by regions of differ-
ent properties, they are no longer perceived as fully unitary elements.
One might at first think that uniform connectedness is nothing more than the
principle of similarity operating on the basis of luminance and color. For ex-
ample, if the tiny patch of light falling on each retinal receptor were taken as an
element, could uniform connected regions not be explained by grouping these
elements according to similarity of luminance and color? Perhaps this is how
Wertheimer himself thought about the organization of elements. But sameness
of color is not sufficient to explain the perceptual unity of uniform connected
regions because it does not account for the difference betweenconnectedregions
of homogeneous color anddisconnectedones. That is, without the additional
constraint of connectedness, there is no basis for predicting that two black areas
within the same dot or bar are any more closely related than comparable black
areas within two different dots or bars.^2 Phenomenologically speaking, there
is no doubt that each individual dot is more tightly organized as a perceptual
208 Stephen E. Palmer