Page 71 - Foundations of Cognitive Psychology : Core Readings

70 Jay L. McClelland, David E.Rumelhart, and Geoffrey E.Hinton

Each processing unit received activation whenever both of the points the unit
stood for contained dots.So far, then, units for both real and spurious corre-
spondences would be equally activated.To allow the mechanism to find the
right correspondences, they pointed out two general principles about the visual
world: (a) Each point in each view generally corresponds to one and only one
point in the other view, and (b) neighboring points in space tend to be at nearly
the same depth and therefore at about the same disparity in the two images.
While there are discontinuities at the edges of things, over most of a two-
dimensional view of the world there will be continuity.These principles are
called the uniqueness and continuity constraints, respectively.
Marr and Poggio incorporated these principles into the interconnections be-
tween the processing units.The uniqueness constraint was captured by inhibi-
tory connections among the units that stand for alternative correspondences of
the same dot.The continuity principle was captured by excitatory connections
among the units that stand for similar offsets of adjacent dots.
These additional connections allow the Marr and Poggio model to ‘‘solve’’
stereograms like the one shown in the figure.At first, when a pair of patterns is
presented, the units for all possible correspondences of a dot in one eye with a
dot in the other will be equally excited.However, the excitatory connections
cause the units for the correct conjunctions to receive more excitation than units
for spurious conjunctions, and the inhibitory connections allow the units for the
correct conjunctions to turn off the units for the spurious connections.Thus, the
model tends to settle down into a stable state in which only the correct corre-
spondence of each dot remains active.
There are a number of reasons why Marr and Poggio (1979) modified this
model (see Marr, 1982, for a discussion), but the basic mechanisms of mutual
excitation between units that are mutually consistent and mutual inhibition
between units that are mutually incompatible provide a natural mechanism for
settling on the right conjunctions of points and rejecting spurious ones.The
model also illustrates how general principles or rules such as the uniqueness
and continuity principles may be embodied in the connections between pro-
cessing units, and how behavior in accordance with these principles can emerge
from the interactions determined by the pattern of these interconnections.
Perceptual Completion of Familiar Patterns Perception, of course, is influenced
by familiarity.It is a well-known fact that we often misperceive unfamiliar
objects as more familiar ones and that we can get by with less time or with
lower-quality information in perceiving familiar items than we need for per-
ceiving unfamiliar items.Not only does familiarity help us determine what the
higher-level structures are when the lower-level information is ambiguous; it
also allows us to fill in missing lower-level information within familiar higher-
order patterns.The well-known phonemic restoration effect is a case in point.
In this phenomenon, perceivers hear sounds that have been cut out of words as
if they had actually been present.For example, Warren (1970) presented legi-
alature to subjects, with a click in the location marked by the a.Not only did
subjects correctly identify the word legislature; they also heard the missing /s/
just as though it had been presented.They had great difficulty localizing the
click, which they tended to hear as a disembodied sound.Similar phenomena