Perceptual Coherence : Hearing and Seeing

402 Perceptual Coherence
by columns even when the distances between the beads favored row organization
(i.e., at the observer’s eye, the distance between the beads in a row
was less than the distance between beads in a column). In contrast, if the
observers viewed the display with one eye and poor depth information so
that they were more likely to perceive the array as being squashed, the
grouping shifted to organization by rows at the angle at which the proximal
distances favored the row organization. In both cases then, the perceived
proximity inferred from the perceived orientation of the surface (in depth,
or not in depth) was the basis for grouping.
In a similar experiment, Rock, Nijhawan, Palmer, and Tudor (1992)
demonstrated that it was perceived lightness, not the proximal luminance,
that determined grouping. In the control condition, there were five columns,
each with four squares of equal reflectance (see figure 9.15A). In the baseline
condition, the three left columns had a greater reflectance (68.4%) than
the two on the right (17.6%), and observers obviously grouped the three left
columns together. In the experimental condition (B), the authors placed a
translucent plastic strip over the central column so that the reflectance of the
center column equaled the two on the right (to 17.6%). Following the same
logic as before, if the observers grouped on the basis of the physical energy
reaching their eyes, they would group the central column with the two on
the right. However, they continued to group the middle column with the two
on the left. The observers first compensated for the effect of the filter, and
then grouped on the basis of perceived lightness. They were able to compensate
for the darkening effect of the filter because the filter extended the
entire length of the column and simultaneously darkened the background
and boundary. The grouping was indirect, following the adjustment for the
Figure 9.15. Perceived lightness, not proximal lightness, determines organization.
Observers were able to compensate for the translucent shadow shown in (B), and
judged the middle column to group with the two left-most columns. Adapted from
“Grouping Based on Phenomenal Similarity of Achromatic Color,” by I. Rock,
R. Nijhawan, S. E. Palmer, and L. Tudor, 1992, Perception, 21, 779–789.