Perceptual Coherence : Hearing and Seeing

Let me summarize at this point. The evidence is overwhelming that listeners
can use auditory properties to identify a wide variety of events and
objects. There seems to be enough information in the waveform to allow
judgments about a wide variety of physical attributes of the source of an
auditory event. It is tempting to attribute the acoustic dimensions to different
classes of receptor cells. The attack time dimension could be traced to
cells that have different frequency sweep rates and different sweep directions
(see figure 2.25). The spectral frequency dimension could be traced to
auditory cortical cells discussed in chapter 2, which have multiple interleaved
excitation and inhibition regions (e.g., M. L. Sutter et al., 1999). The
spectral frequency variation dimension could be traced to cells that are sensitive
to amplitude and frequency modulation (L. Li et al., 2002) or differences
in spectral distributions (Barbour & Wang, 2003). The combination
of such cells can account for the importance of the distribution of spectral
components during the attack and steady parts of the sound. However, I do
not believe correlating perceptual features to cells whose receptive fields
seem to match those features is a useful approach to understanding timbre
(or any other perceptual outcome). The receptive fields of cells are extremely
labile and change dramatically in different contexts, as discussed
in chapter 2. Carried to an extreme, every feature would demand a unique
receptive field. I believe that the perceptual features arise from the interaction
of cells with all types of receptive fields.
The evidence also is overwhelming that the experience and capability of
the listener will determine the level of performance. The acoustic and cognitive
factors are not wholly separable. These sounds have meaning; they
seem to have some sort of psychological structure, if not exactly a grammar
and syntax, which can affect identification. Below I consider some of the
cognitive factors in more detail.
Cognitive Factors
The Perception of Quality: Auditory Timbre 367
Although it would make identification much easier if each event or source
had a unique sound, that is not the case. Ballas (1993) investigated one
half of this ambiguity, namely that several sources can produce the same
sound, which he termed causal uncertainty. Ballas uses the example of a
click-click sound that could have been generated by a ballpoint pen, a light
switch, a camera, or certain types of staplers. If a sound can come from
many possible sources, then it seems obvious that it should be harder to
identify the actual source. To measure the causal uncertainty, Ballas and
colleagues used information theory to measure the uncertainty of the distribution
of responses to one stimulus (as described in chapter 3). The
first step was to develop a set of categories such that each category represented
similar events. One category could be impact sounds, another water