The basic element of the acoustic event and the auditory figure

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An introduction to auditory objects, events, figures, images and scenes

Now we need to turn, briefly, to the auditory pre-processor that coverts an acoustic event into an auditory event, to add some essential details to the analogy between ‘the pattern of light reflected from an object that gives rise to a visual figure’ and ‘the pattern of sound emitted by a source that gives rise to an auditory figure’. The animal syllables shown in Figure 2 are typical of the sounds that fish, frogs and mammals use to communicate at a distance, to declare their territories and attract mates. They are pulse-resonance sounds (Patterson et al., 2007) and they are ubiquitous in the natural world, and in the modern human environment. The structures used to produce pulse-resonance sounds can be quite elaborate, but the mechanism is conceptually very simple as illustrated in Figure 3: The animal develops some means of producing an abrupt pulse of mechanical energy which causes structures in the animal’s body to resonate. From the signal processing perspective, the pulse marks the start of the communication and the resonance provides distinctive information about the shape and structure of the sounders in the sender’s body, and thus, distinctive information about the species producing the sound. The animal syllables in Figure 2 are streams of regularly timed pulses, each of which carries a copy of the resonance to the listener. The syllables are on the order of 200–800 ms in duration, with pulse rates in the region 20 to 400 Hz, and they typically have 20 to 80, pulse-resonance cycles.

The vowels of speech are pulse resonance sounds and one of the communication sounds in Figure 2 is, in fact, a syllable with a long vowel. The microstructure of the typical vowel, is illustrated in Figure 4b; it shows a short segment of the vowel in the word “mama.” It is a stream of glottal pulses and each pulse carries a resonance showing how the vocal tract responded to that pulse. Perceptual experiments suggest that the preprocessor in the auditory system produces a scale-shift invariant (Irino and Patterson, 2002), or scale-shift covariant (Patterson, van Dinther, and Irino, 2007) representation of pulse-resonance sounds, by isolating and normalizing the neural patterns produced by individual cycles of the sound in the auditory nerve; the processes is illustrated with videos in the next section. The experiments suggest that it is the pulse-resonance cycle that is the basic element of the acoustic event, and it is the individual cycles that generate the auditory figures of the auditory event. It is also the individual cycle of the pulse-resonance sound that corresponds to the instantaneous pattern of light reflected from an object in the analogy with vision. So, in the expression for auditory perception (Eq. 8), the pattern, pe, at the heart of the acoustic event, ea, is the individual cycle of the pulse-resonance sound, and it is the cycle that gives rise to the auditory figure, FA, in the auditory event, EA.

    IA  [ EA{ FAn(ea)}]   |<=A|    ea [ ea{ cen(sa)}] . 					(8)

Here, then, is a nested expression that provides a definition for the important acoustic events in our environment and how they are related to the auditory events that we hear in response to those acoustic events.

Making the auditory figure scale-shift covariant
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