Perceptual Coherence : Hearing and Seeing

210 Perceptual Coherence
Figure 5.7. The brightness ratios
between different surfaces
determine whether the surfaces
are perceived as transparent
(i.e., equal ratios) or whether
one surface is perceived as occluding
and covering another.
influenced by knowledge of the rules determining images formed in natural
settings. This is essentially the same argument made by Yuille and
Grzywacz (1998), as well as Tse et al. (1998). Strictly computational models
do not capture the implicit knowledge about how natural events are
organized.
The auditory parallel to moving plaids, or generally to coherentincoherent
visual stimuli, are interleaved melodies in which one takes two
well-known tunes (e.g., “Mary Had a Little Lamb” and “London Bridge”)
and alternates the notes, keeping an equal time interval between the onsets
of each pair of adjacent notes (coming from different melodies). The
experimental question is what conditions lead to the perception of one
coherent but meaningless melody and which ones lead to the perception
of the two separate melodies, exactly the identical question asked about
plaids.
If the sound quality of the notes is identical and the notes come from the
same scale region so that it is impossible to isolate the notes of each
melody by pitch, it is extremely difficult to pick out each melody. The similarity
in pitch creates one coherent tonal sequence that does not resemble
either melody. If the notes of one melody are progressively raised in pitch
until there is no overlap in the notes of each melody, the two melodies separate
to form separate coherent melodies, and it becomes easy to identify
either one (Dowling, 1973; Hartmann & Johnson, 1991). Furthermore, Bey
and McAdams (2003) showed that differences in the timbre between the