Perceptual Coherence : Hearing and Seeing

Perception of Motion 213
Braddick (1974) originally proposed the small-distance versus longdistance
distinction. Braddick and others used white-and-black random dot
patterns similar to those used originally by Julesz to study texture differences.
Typically, the random dot patterns were built from square matrices
in which 50% of the cells were filled with black dots (e.g., 288 dots in a
24 × 24 matrix). Two such patterns are alternated at the same spatial position:
One dot pattern contains the original array of dots, and the second dot
pattern is modified in some way.
1. The dots in the cells of one small region of the array are reversed by
chance. Thus, each black cell would have a 0.50 probability of shifting
to white, and each white cell would have a .50 probability of
shifting to black. If two such patterns are presented sequentially, the
small region seems to flicker or glitter, but there is no perception of
movement.
2. The dots in the cells of one small region are shifted by a certain
amount (a 4 × 3 rectangle is shifted five columns to the right). The
black-and-white pattern in the rectangle remains fixed and overwrites
the original pattern in the target columns. This leaves the cells in the
original rectangle empty, and they are filled randomly. Here, if the
two patterns are presented sequentially, the small rectangle seems to
move to the right. The visual system must be performing a global
comparison between the two arrays, because the rectangle cannot be
seen in either static view.
3. The dots in the cells of one small region are shifted by a certain
amount (as in number 2) and then the black and white cells are reversed.
If the two patterns are presented sequentially, the perceived
movement is from the second pattern to the first; the perception is the
reverse of the actual temporal sequence.
All three of these possibilities are shown in figure 5.8.
To see that a small region has been shifted laterally (or vertically) as in
numbers 2 or 3, the correspondences in the dot patterning within the region
must be recognized in the two different arrays. The perceptual problem
is that after the shift of the small region, there may be several similar regions
in the array that can act as incorrect matches. We can conceptualize this process
as having two steps. The first step isolates the local correspondences
between the dots in the two arrays: Each cell in the first array has a list of all
the possible brightness matches in the second array. Somehow, the indeterminacy
of these matches must be resolved in order to see motion. The second
step searches for sets of connected cells with identical correspondences
(e.g., correspondences equal to two steps to the right) and groups those cells
to create the perception of coherent movement. This grouping process will
inevitably misallocate cells, and there are many examples in which a true