Speech recognition in noise and music perception is especially challenging for current cochlear implant users. The present study utilizes the residual acoustic hearing in the nonimplanted ear in five cochlear implant users to elucidate the role of temporal fine structure at low frequencies in auditory perception and to test the hypothesis that combined acoustic and electric hearing produces better performance than either mode alone. The first experiment measured speech recognition in the presence of competing noise. It was found that, although the residual low-frequency (<1000 Hz) acoustic hearing produced essentially no recognition for speech recognition in noise, it significantly enhanced performance when combined with the electric hearing. The second experiment measured melody recognition in the same group of subjects and found that, contrary to the speech recognition result, the low-frequency acoustic hearing produced significantly better performance than the electric hearing. It is hypothesized that listeners with combined acoustic and electric hearing might use the correlation between the salient pitch in low-frequency acoustic hearing and the weak pitch in the envelope to enhance segregation between signal and noise. The present study suggests the importance and urgency of accurately encoding the fine-structure cue in cochlear implants.

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Speech and melody recognition in binaurally combined acoustic and electric hearing

Speech is the primary vehicle of human social interaction. In everyday life, speech communication occurs under an enormous range of different environmental conditions. The demands placed on the process of speech communication are great, but nonetheless it is generally successful. Powerful selection pressures have operated to maximize its effectiveness. The adaptability of speech is illustrated most clearly in its resistance to distortion. In transit from speaker to listener, speech signals are often altered by background noise and other interfering signals, such as reverberation, as well as by imperfections of the frequency or temporal response of the communication channel. Adaptations for robust speech transmission include adjustments in articulation to offset the deleterious effects of noise and interference (Lombard 1911; Lane and Tranel 1971); efficient acousticphonetic coupling, which allows evidence of linguistic units to be conveyed in parallel (Hockett 1955; Liberman et al. 1967; Greenberg 1996; see Diehl and Lindblom, Chapter 3); and specializations of auditory perception and selective attention (Darwin and Carlyon 1995). Speech is a highly efficient and robust medium for conveying information under adverse conditions because it combines strategic forms of redundancy to minimize the loss of information. Coker and Umeda (1974, p. 349) define redundancy as “any characteristic of the language that forces spoken messages to have, on average, more basic elements per message, or more cues per basic element, than the barest minimum [necessary for conveying the linguistic message].” This definition does not address the function of redundancy in speech communication, however. Coker and Umeda note that “redundancy can be used effectively; or it can be squandered on uneven repetition of certain data, leaving other crucial items very vulnerable to noise. . . . But more likely, if a redundancy is a property of a language and has to be learned, then it has a purpose.” Coker and Umeda conclude that the purpose of redundancy in speech communication is to provide a basis for error correction and resistance to noise.

The Perception of Speech Under Adverse Conditions

A general account of auditory perceptual organization has developed in the past 2 decades It relies on primitive devices akin to the Gestalt principles of organization to assign sensory elements to probable groupings and invokes secondary schematic processes to confirm or to repair the possible organization Although this conceptualization is intended to apply universally, the variety and arrangement of acoustic constituents of speech violate Gestalt principles at numerous junctures, cohering perceptually, nonetheless The authors report 3 experiments on organization in phonetic perception, using sine wave synthesis to evade the Gestalt rules and the schematic processes alike These findings falsify a general auditory account, showing that phonetic perceptual organization is achieved by specific sensitivity to the acoustic modulations characteristic of speech signals

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On the perceptual organization of speech.

Human listeners are better able to identify two simultaneous vowels if the fundamental frequencies of the vowels are different. A computational model is presented which, for the first time, is able to simulate this phenomenon at least qualitatively. The first stage of the model is based upon a bank of bandpass filters and inner hair‐cell simulators that simulate approximately the most relevant characteristics of the human auditory periphery. The output of each filter/hair‐cell channel is then autocorrelated to extract pitch and timbre information. The pooled autocorrelation function (ACF) based on all channels is used to derive a pitch estimate for one of the component vowels from a signal composed of two vowels. Individual channel ACFs showing a pitch peak at this value are combined and used to identify the first vowel using a template matching procedure. The ACFs in the remaining channels are then combined and used to identify the second vowel. Model recognition performance shows a rapid improvement in ...

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Modeling the identification of concurrent vowels with different fundamental frequencies.

Three experiments and a computational model explored the role of within‐channel and across‐channel processes in the perceptual separation of competing, complex, broadband sounds which differed in their interaural phase spectra. In each experiment, two competing vowels, whose first and second formants were represented by two discrete bands of noise, were presented concurrently, for identification. Experiments 1 and 2 showed that listeners were able to identify the vowels accurately when each was presented to a different ear, but were unable to identify the vowels when they were presented with different interaural time delays (ITDs); i.e. listeners could not group the noisebands in different frequency regions with the same ITD and thereby separate them from bands in other frequency regions with a different ITD. Experiment 3 demonstrated that while listeners were unable to exploit a difference in interaural delay between the pairs of noisebands, listeners could identify a vowel defined by interaurally decorrelated noisebands when the other two noisebands were interaurally correlated. A computational model based upon that of Durlach [J. Acoust. Soc. Am. 32, 1075–1076 (1960)] showed that the results of these and other experiments can be interpreted in terms of a within‐channel mechanism, which is sensitive to interaural decorrelation. Thus the across‐frequency integration which occurs in the lateralization of complex sounds may play little role in segregating concurrent sounds.

Perceptual separation of concurrent speech sounds: Absence of across‐frequency grouping by common interaural delay

The perceptual consequences of exciting one formant (F2 or F4) of a composite syllable with a different fundamental from the others were examined in experiment 1 using a four‐formant syllable for which F1, F2, and F3 give /ru/ and F1, F3, and F4 give /li/. When all four formants are on the same fundamental, the percept is /ru/, but exciting F2 on a sufficiently different fundamental gave predominantly /li/ percepts. Smaller differences in fundamental left the phonetic percept unchanged but made F2 stand out as a separate sound source. Exciting F4 on a different fundamental also made it stand out as a separate sound source, though larger differences in fundamental were needed than for F2. Experiment 2 replicated these static effects of fundamental frequency differences on F2, but failed to find any substantial effect of coherent or incoherent fundamental frequency modulation functions on perceptual grouping or separation. Although static differences in fundamental frequency exert a strong effect on the perceptual grouping of formants both in terms of the number of sound sources heard and the perceived phonetic category, the coherence of fundamental frequency movement has almost no independent effect on the perceptual grouping of formants.

C. J. Darwin

Papers

Perceptual grouping of formants with static and dynamic differences in fundamental frequency