Perceptual Coherence : Hearing and Seeing

360 Perceptual Coherence
be dominant. Grey stated that longer notes yielded different multidimensional
spatial configurations in which those temporal dimensions did not
emerge.
Another measure of spectral flux essentially refers to the change in the
shape of the spectral envelope over time. This change could be due to variations
in the source excitation, different damping of the filter’s resonance
modes (see Freed, 1990, described above), or a performer’s use of vibrato,
which creates both frequency and amplitude modulations. Such variation is
easily perceived, and McAdams, Beauchamp, Meneguzzi (1999) demonstrated
that listeners consistently discriminated between simulated instrumental
sounds that contained normal spectral flux and those in which the
flux was eliminated by making the ratios among pairs of harmonics constant
across the sound duration. Moreover, Horner, Beauchamp, and So
(2004) found that for instruments with high levels of spectral flux, it was
difficult to detect added random alterations in the amplitude of the harmonics
(an expected outcome from a signal detection perspective).
However, spectral flux does not emerge consistently in the multidimensional
solutions. For example, a measure of spectral variation was found to
weakly correlate to the third dimension by McAdams et al. (1995), but
such a measure was not found by Lakatos (2000) using sets of stimuli that
might be expected to show that variation. Erickson (2003), using classically
trained mezzo-soprano and soprano singers, found that rate of vibrato
was weakly correlated to a third dimension for experienced choral directors,
but not for inexperienced judges. The reason for this inconsistency
may be the scaling method itself. The scaling procedure tries to find the
minimum number of dimensions to account for the judged similarity, and
the predominant importance of the onset timing and the spectral centroid
may hide the effect of spectral flux.
In an identification task, Kendall (1986) completely eliminated the possibility
of any spectral flux. Kendall isolated one cycle and then repeated
that cycle continuously for the entire duration of the steady-state component.
He found that identification of instruments was poorer for the singlecycle
simulations than for the natural instrument. It appears that spectral
flux is important for recognizing an instrument (or discriminating among
simulated sounds) but not as relevant for judging similarity between
sounds.
Individual Differences
In nearly all work to date, the differences between untrained and trained
listeners (e.g., musicians, classical singers, choral directors) have been
small and variable. For example, Lakatos (2000), using the same statistical
procedure as McAdams et al. (1995), did not find any effects of musical