Mind, Body, World- Foundations of Cognitive Science, 2013a

element y, seen at time t + 1, is called a motion correspondence match. However, the
assignment of motion correspondence matches is underdetermined. This is illustrated
in Figure 8-7 as a simple apparent motion stimulus in which two squares
(dashed outlines) are presented at one time, and then later presented in different
locations (solid outlines). For this display there are two logical sets of motion correspondence
matches that can be assigned, shown in B and C of the figure. Both
sets of matches are consistent with the display, but they represent radically different
interpretations of the identities of the elements over time. Human observers of
this display will invariably experience it as Figure 8-7B, and never as Figure 8-7C.
Why is this interpretation preferred over the other one, which seems just as logically
plausible?
The natural computation approach answers this question by claiming that the
interpretation illustrated in Figure 8-7B is consistent with additional natural constraints,
while the interpretation in Figure 8-7C is not. A number of different natural
constraints on the motion correspondence problem have been identified and then
incorporated into computer simulations of motion perception (Dawson, 1987, 1991;
Dawson, Nevin-Meadows, & Wright, 1994; Dawson & Pylyshyn, 1988; Dawson & Wright,
1989, 1994; Ullman, 1979).
A
B
C
Figure 8-7. The motion correspondence problem.
One such constraint is called the nearest neighbour principle. The visual system
prefers to assign correspondence matches that represent short element displacements
(Burt & Sperling, 1981; Ullman, 1979). For example, the two motion correspondence
matches in Figure 8-7B are shorter than the two in Figure 8-7C; they are
therefore more consistent with the nearest neighbour principle.
376 Chapter 8