Perceptual Coherence : Hearing and Seeing

The Transition Between Noise and Structure 169
consistent orientations within subregions of the field. Matched dots rarely
are the closest ones. Looking at any small area through an aperture destroys
the pattern, because the global correspondences are lost. All that remains
are inconsistent accidental pairings. The individual features and the global
pattern are independent, because the pattern emerges even if short line segments
at different orientations replace the dots. Glass patterns resemble
continuous, global natural textures. Both the perception of Glass patterns
and segmented texture regions from different micropatterns occur very rapidly
(about 100 ms) without eye movements, and thus classify as effortless
and preattentive. 6
R. K. Maloney, Mitchison, and Barlow (1987) measured the strength of
the perceived global pattern by adding unpaired random dots to the dot configuration
until that pattern disappeared and the dot texture began to look
random. They found that Glass patterns were remarkably stable. Consider a
dot and its match, generated by a small rotation. Concentric patterns were
visible even if there were 6 to 10 random dots lying closer to an original dot
than its real match. Several examples are shown in figure 4.11. Thus, the visual
system does not depend on a nearest neighbor analysis; instead, it pools
local orientations over an extended region. Variable-orientation pairings that
emerge by chance in small regions must be disregarded. The important unresolved
problem is the scale of that integration.
Theoretically, each type of Glass pattern should be equally discriminable
because the autocorrelation between matched points will be identical
no matter how the patterns are duplicated and superimposed: the local statistics
are identical. However, this is not the case because rotation (concentric)
and magnification (radial) transformations are easier to perceive than translation
(parallel) transformations, and vertical translations are easier than horizontal
translations (Jenkins, 1985; see figure 4.10). In addition, although the
autocorrelation is still the same, the Glass pattern does not emerge if the
pattern and its duplicate are opposite contrasts to the background (e.g., gray
background, black [original] and white [duplicate] dots; Glass & Switkes,
1976), if the elements of a pattern and its duplicate differ strongly in energy
6. Glass patterns are similar to auditory iterated rippled noise stimuli (Yost, Patterson, &
Sheft, 1996). To construct these stimuli, a segment of random noise is delayed by d ms and
added back to the same noise. If this process is repeated n times, the autocorrelation of the amplitudes
at the delay d equals n/n + 1. The resulting sound has two components: a tonal component
with a buzzy timbre sounding like an airplane propeller with a pitch at 1/d ms and a noise
component sounding like a hiss. The strength of the tone component is proportional to the autocorrelation,
and after 16 iterations the sound is nearly completely tonal. The parsing of the sound
into tone and noise is effortless and irresistible, and another example of preattentive figureground
organization. Thus, for both Glass patterns and iterated rippled noise, the original stimulus
is duplicated at a fixed distance or time interval and then added to the original. The strength
of the tonal percept increases with the number of iterations and the strength of the global visual
pattern would also increase with the number of spatial shift and superimposed iterations.