Page 70 - Foundations of Cognitive Psychology : Core Readings

The Appeal of Parallel Distributed Processing 69

Figure 4.6
Random-dot stereograms.The two patterns are identical except that the pattern of dots in the cen-
tral region of the left pattern are shifted over with respect to those in the right.When viewed stereo-
scopically such that the left pattern projects to the left eye and the right pattern to the right eye,
the shifted area appears to hover above the page.Some readers may be able to achieve this by
converging to a distant point (e.g., a far wall) and then interposing the figure into the line of sight.
(From Foundations of Cyclopean Perception, p.21, by B.Julesz, 1971, Chicago: University of Chicago
Press.Copyright 1971 by Bell Telephone Laboratories, Inc.Reprinted by permission.)

In a simple stereogram such as the one shown here, one pattern is an exact copy
of the other except that the pattern of dots in a region of one of the patterns
is shifted horizontally with respect to the rest of the pattern.Each of the two
patterns—corresponding to two retinal images—consists entirely of a pattern
of random dots, so there is no information in either of the two views considered
alone that can indicate the presence of different surfaces, let alone depth rela-
tions among those surfaces.Yet, when one of these dot patterns is projected
to the left eye and the other to the right eye, an observer sees each region as
a surface, with the shifted region hovering in front of or behind the other,
depending on the direction of the shift.
What kind of a mechanism might we propose to account for these facts? Marr
and Poggio (1976) began by explicitly representing the two views in two arrays,
as human observers might in two different retinal images.They noted that cor-
responding black dots at different perceived distances from the observer will be
offset from each other by different amounts in the two views.The job of the
model is to determine which points correspond.This task is, of course, made
difficult by the fact that there will be a very large number of spurious corre-
spondences of individual dots.The goal of the mechanism, then, is to find those
correspondences that represent real correspondences in depth and suppress
those that represent spurious correspondences.
To carry out this task, Marr and Poggio assigned a processing unit to each
possible conjunction of a point in one image and a point in the other.Since the
eyes are offset horizontally, the possible conjunctions occur at various offsets or
disparities along the horizontal dimension.Thus, for each point in one eye,
there was a set of processing units with one unit assigned to the conjunction of
that point and the point at each horizontal offset from it in the other eye.