Perceptual Coherence : Hearing and Seeing

120 Perceptual Coherence
For example, Kubovy and Jordan (1979) built up a set of sounds, each
composed of 12 harmonically related sine waves (e.g., 600, 800, 1000,...
2800 Hz). They created 12 different tones, in which 11 of the 12 sine waves
were in phase and 1 was out of phase. Kubovy and Jordan (1979) then presented
sequences of the tones (at a presentation rate of about 3 tones per
second) so that the out-of-phase wave either increased or decreased in frequency
and listeners had to judge which direction occurred (a simple example
of upward movement using four components would be: −+++, +−++,
++−+, +++−). Listeners were able to judge the direction for sets of higherfrequency
harmonics (e.g., 2200−2800 Hz) but were unable to judge
direction for sets of lower-frequency harmonics (600–1200 Hz). It is not
clear why this difference occurred. If the 12 stimuli were presented one at a
time, no matter which sine wave was out of phase, the pitch did not change.
Listeners matched the pitch of the complex tone to the same pure tone. The
effects of phase were found only in sequences in which the phase shift
moved across the frequency components of the complex sounds.
Moreover, changing phase relationships among components of complex
sounds led listeners to segment the sounds into sets of frequency components
with constant phase relationships. Suppose we had two overlapping complex
sounds with similar but not identical fundamental frequencies. Why should
the two sounds separate? One possible answer is that the phase relationships
within each sound would be invariant, but the phase relationships between the
two sounds would shift over time because the fundamental frequencies were
not identical, and that shift could be encoded by the phase-locked response to
each sound. Similar to the work of Kubovy and Jordan (1979), it would be the
changing phase relationships that are the perceptual information. 9
I would argue that the effect of phase is identical for hearing and seeing.
For static objects, the phase relationships do not affect periodicity but do
affect quality, while for changing objects the phase relationships create stability
and movement. (In chapter 5, I consider motion created by phase
changes.) The ear is not phase-deaf and the eye is not phase-blind.
Physiological Transformations That Maximize
Information Transmission
Given the statistical structure of the environment, what should be the optimal
transformation of the proximal stimulus at the retina or at the cochlea into a
neural signal? The answer depends on what we imagine the goals of sensory
9. Listeners can make use of phase differences to localize sounds. If we present one tone
to each ear by means of headphones and then gradually increase the phase difference between
the tones, listeners report that the sound is circling their head.