Читать книгу The Handbook of Speech Perception - Группа авторов - Страница 23

What causes the perception of speech? A classic answer takes a linguistically significant contrast – voicing, for instance – and provides an inventory of natural acoustic correlates of a careful articulation of the contrast (e.g. Lisker, 1978). A perceptual account that reverses the method depicts a meticulous listener collecting individual acoustic correlates as they land and assembling them in a stream, thereby to tally the strength with which a constellation of cues indicates the likely occurrence of a linguistic constituent. Klatt’s retrospective survey of perceptual accounts describes many normative approaches that treat the acoustic signal as a straightforward composite of acoustic correlates. The function of perceptual organization, usually omitted in such accounts, establishes the perceiver’s compliance with the acoustic products of a specific source of sound, and in the case of speech it is the probabilistic function that finds and tracks the likely acoustic products of vocalization. However, it is clear from evidence of several sorts – tolerance of distortion, effectiveness of impossible signals, forgiveness of departures from natural timbre – that the organizational component of perception that yields a speech stream fit to be analyzed cannot collect acoustic cues piecemeal, as this simple view describes. The functions of perceptual organization act, instead, as if attuned to a complex form of regular if unpredictable spectro‐temporal variation within which the specific acoustic and auditory elements matter far less than their overall configuration.

The evolving portrait of speech perception that includes organization and analysis recasts the raw cue as the property of perception that gives speech its phenomenality, though not its phonetic effect. The transformation of natural speech to chimera, to noise‐band vocoded signal, and to sinewave replica is phonetically conservative, preserving the fine details of subphonemic variation while varying to the extremes of timbre or auditory quality. It is apparent that the competent listener derives phonetic impressions from the properties that these different kinds of signal share, and derives qualitative impressions from their unique attributes. The shared attribute, for want of a more precise description, is a complex modulation of spectrum envelopes, although the basis for the similar effect of the infinitely sharp peaks of sinewave speech and the far coarser spectra of chimerical and noise‐band vocoded speech has still to be explained. None of these manifests the cues present in natural speech despite the success of listeners in understanding the message. The conclusion supported by these findings is clear: phonetic perception does not require the sensory registration of natural speech cues. Instead, the organizational component of speech perception operates on a spectro‐temporal grain that is requisite both for finding and following a speech signal and for analyzing its linguistic properties. The speech cues that seemed formerly to bear the burden of stimulating phonetic analyzers into action appear in hindsight to provide little more than auditory quality subordinate to the phonetic stream.

An additional source of evidence is encountered in the phenomenal experience of perceivers who listen to speech via electrocochlear prostheses (Goh et al., 2001; Liebenthal et al., 2003). Intelligibility of speech perceived via cochlear implant is often excellent, rivaling that of normal hearing, and recent studies with infant and juvenile subjects (Svirsky et al., 2000) suggest that this form of sensory substitution is effective even at the earliest stages of language development (see Hunter & Pisoni, Chapter 20). The mechanism of acoustic transduction at the auditory periphery is anomalous, it goes without saying, and the phenomenal experience of listeners receiving this appliance to initiate neural activity differs hugely from the ordinary auditory experience of natural speech. Despite the absence of veridical perceptual experience of the raw qualities of natural speech, electrocochlear prostheses are effective in the self‐regulation of speech production by its users, and are effective perceptually despite the abject deficit in faithfully presenting natural acoustic elements of speech. What brings about the perception of speech, then? Without the acoustic moments, there is no stream of speech, but the stream itself plays a causal role beyond that which has been attributed to momentary cues since the beginning of technical study of speech.