Perceptual Coherence : Hearing and Seeing

The Transition Between Noise and Structure 189
Figure 4.15. The perception of visual repetition is very difficult. It is possible to
see the fourfold repetition in (A) by identifying unique configurations in each quadrant.
Vertical (B), horizontal (C), and vertical and horizontal (D) symmetry are perceived
effortlessly.
for symmetry detection will depend on the number of different values that
must be individually tested.
Consider a way to determine the axis of symmetry. To simplify, assume
that the axis of symmetry is vertical. Suppose there are six columns composed
of dots of differing brightness (ABCCBA) so that the symmetry axis
lies between columns 3 and 4. Then, for every pair of columns, correlate
the brightness levels across rows. For random dot patterns, each column
will be perfectly correlated to only one other column: 3-4, 2-5, and 1-6. All
the other correlations should be low or zero because the arrays are generated
randomly. Notice how this process resembles discovering repetition
symmetry. Again, suppose there are six columns (ABCABC) so that the
repetition axis lies between columns 3 and 4. Then, for every pair of
columns, correlate the brightness levels across rows. In this case, the correlated
columns would be 1-4, 2-5, and 3-6.
The important point here is that statistical structure in the stimulus does
not necessarily mean that it can be picked up by the perceptual system. The