Читать книгу The Handbook of Speech Perception - Группа авторов - Страница 17

Because explanations of speech perception have depended on an unspecified account of perceptual organization, it has been natural for auditory scene analysis to be a theory of first resort for understanding the perceptual solution to the cocktail party problem (Cherry, 1953; McDermott, 2009), specifically, of attending to a single stream of speech amid other sound sources. However, this premise was largely unsupported by direct evidence. The crucial empirical cases that had formed the model had rarely included natural sources of sound – neither instruments of the orchestra (though see Iverson, 1995), which are well modeled physically (Rossing, 1990), nor ordinary mechanical sources (Gaver, 1993), nor the sounds of speech, with several provocative exceptions. It is instructive to consider some of the cases in which tests of perceptual organization using speech sounds appeared to confirm the applicability to speech of the general auditory account of perceptual organization.

In one case establishing grouping by similarity, a repeating series of syllables of the form CV‐V‐CV‐V was observed to split into distinct streams of like syllables, one of CVs and another of Vs, much as gestalt principles propose (Lackner & Goldstein, 1974). Critically, this perceptual organization precluded the perceptual resolution of the relative order of the syllables across stream, analogous to the index of grouping used by Bregman & Campbell (1971). In another case calibrating grouping by continuity, a series of vowels formed a single perceptual stream only when formant frequency transitions leading into and out of the vowel nuclei were present (Dorman, Cutting, & Raphael, 1975). Without smooth transitions, the spectral discontinuity at the juncture between successive steady‐state vowels exceeded the tolerance for grouping by closure – that is, the interpolation of gaps – and the perceptual coherence of the vowel series was lost. In another case examining organization by the common fate, or similarity in change of a set of elements, a harmonic component of a steady‐state vowel close to the center frequency of a formant was advanced or delayed in onset relative to the rest of the harmonics composing the synthetic vowel (Darwin & Sutherland, 1984). At a lead or lag of 32 ms, consistent with findings deriving from arbitrary patterns, the desynchronized harmonic segregated into a different stream than the synchronous harmonics composing the vowel. In consequence, when the leading or lagging harmonic split, the phonemic height of the vowel was perceived to be different, as if the perceptual estimate of the center frequency of the first formant had depended on the grouping. In each of these instances, the findings with speech sounds were well explained by the precedents of prior tests using arbitrary patterns of sound created with oscillators and noise generators.

These outcomes should have seemed too good to be true. It was as if an account defined largely through tests of ideal notions of similarity in simple auditory sequences proved to be adequate to accommodate the diverse acoustic constituents and spectral patterns of natural sound. With hindsight, we can see that accepting this conclusion requires two credulous assumptions. One assumption is that tests using arbitrary trains of identical reduplicated syllables, meticulously phased harmonic components, and sustained steady‐state vowels adequately express the ordinary complexity of speech and the perceiver’s ordinary sensitivity. A sufficient test of organization by the generic principles of auditory scene analysis is more properly obliged to incorporate the kind of spectra that defined the technical description of speech perception. A closer approximation to the conditions of ordinary listening must characterize the empirical tests. Tests composed without imposing these assumptions reveal a set of functions rather different from the generic auditory model at work in the perceptual organization of speech.

A second assumption, obliged by the generic auditory account of organization is that the binding of sensory elements into a coherent contour, ready to analyze, occurs automatically, with neither attention nor effort. This premise had been asserted, though not secured by evidence. Direct attempts at an assay have been clear. These studies showed plainly that, whether a sound is speech or not, its acoustic products, sampled auditorily, are resolved into a contour distinct from the auditory background only by the application of attention (Carlyon et al., 2001, 2003; Cusack, Carlyon, & Robertson, 2001; Cusack et al., 2004). Without attention, contours fail to form and sounds remain within an undifferentiated background. Deliberate intention can also affect the listener’s integration or segregation of an element within an auditory sensory contour, by an application of attentional focus (for instance, Billig, Davis, & Carlyon, 2018)