Perceptual Coherence : Hearing and Seeing

386 Perceptual Coherence
Frequency
The above cues seem more relevant to the grouping of simultaneous frequency
components in a single sound than to grouping successive sounds
as found in music. The question here is whether the series of sounds comes
from one source or whether the series results from the interleaving of
sounds from different sources. The basic methodology is to continuously
recycle a small number of sounds and measure whether the listener hears
all the sounds as coming from one source or whether the sounds appear
to break into separate ongoing sound sequences, often termed streams,
such that each stream seems to come from a different source. As Bregman
(1990) has argued, the default percept is that of one stream, and switching
the percept to that of different streams is cumulative and gradual over
repetitions.
In a classic set of experiments, van Noorden (1975) presented sequences
at different presentation rates that simply alternated two pure tones at
different frequencies (also discussed in chapter 5). Listeners reported simply
whether they heard the two tones alternate like a trill or whether the
notes formed a low-note sequence and a separate high-note sequence. These
sequences place temporal proximity (adjacent notes) in conflict with frequency
similarity (alternate notes of the same frequency).
The results demonstrated an inverse relationship between rate and
frequency ratio. Increasing either the presentation rate or the frequency
separation (or both) increases the probability of hearing the two tones as
separate streams. Conversely, decreasing the rate or frequency separation
increases the probability of hearing the tones as forming a single coherent
stream. At intermediate values of rate or frequency separation, the tendency
to hear the tones as forming one or two streams can be affected by the
listener’s attention. Bregman et al. (2000) have shown that it is not the rate
per se that affects grouping; it is the gap between the offset of one note and
the onset of the next one in the same frequency region, the temporal separation.
The tendency to form a coherent stream can be enhanced by connecting
the low- and high-frequency tone by a frequency glide between the two
tones.
Bregman (1990) made an extremely important distinction about the
asymmetry in auditory grouping from the results of van Noorden’s experiment.
It always is possible to attend to either tone at all presentation
rates as long as the frequency separation between the low and high tones
allows them to be discriminated apart. In contrast, it is impossible to
maintain the perception of a single alternating sequence once the combination
of frequency separation and presentation rate reach a certain point.
There is an obligatory split into low and high tone streams. On this basis
and others, Bregman argued that we have primitive segregation processes