Auditory Enhancement Research Articles

Function and evolution of the frill of the frillneck lizard, Chlamydosaurus kingii (Sauria: Agamidae)

Relative to body size, the frill of the Australian agamid lizard Chlamydosaurus kingii is one of the largest and most spectacular display structures seen in any animal species. More than 300 hours observation of free-ranging lizards, combined with data on museum specimens, revealed that the frill is used primarily for intraspecific communication and predator deterrence. Earlier hypotheses on alternative uses for the frill (gliding, food storage, thermoregulation or auditory enhancement) are not supported. The folded frill may also enhance camouflage, but this is probably a fortuitous effect rather than an adaptation. Male frillnecks frequently display and fight during the mating season. Male displays are highly stereotyped, and involve repeated partial erection of the frill, head-bobbing, tail-lashing, and waving of forelimbs. Both males and females erect the frill during social encounters, and in response to potential predators. Males grow larger than females and have larger heads than do females at the same body size, but no dimorphism is apparent in the relative size of the frill. The extreme development of the display structure in this species may be due to general allometric relationships, as well as to ecological features that have intensified the action of sexual selection in Chlamydosaurus.

Auditory enhancement and suppression in normal‐hearing persons with reduced speech recognition in noise

Auditory enhancement has been suggested as a possible mechanism underlying one's ability to understand speech in noise. Using a forward‐masking paradigm, enhancement effects were evaluated in two groups of normal‐hearing persons who differed in their ability to recognize high‐frequency words in noise. Masked thresholds for a 2000‐Hz probe were compared between a standard condition in which a four‐component harmonic complex including 2000 Hz was the masker and an enhancing condition in which a three‐component adapter not including 2000 Hz preceded the masker. One explanation for the enhancement effect involves an adaptation of suppression that results in an increase in the masker effectiveness in the 2000‐Hz region. Therefore, adaptation of suppression was also evaluated by measuring forward‐masked thresholds for a 2000‐Hz probe as a function of the duration of a suppressor tone added to a 2000‐Hz masker. Although all subjects showed some degree of enhancement, there were differences in the reduction in suppression that occurred with increased suppressor duration.

Effects of syllable duration on the perception of the Mandarin Tone 2/Tone 3 distinction: evidence of auditory enhancement

Mandarin syllables carrying a mid-high-rising F 0 contour (Tone 2) tend to be shorter than those carrying a low-falling-rising contour (Tone 3). It is possible that talkers use length differences to enhance the perceptual contrast between Tone 2 and Tone 3. We tested this hypothesis in Experiment 1 by comparing perceptual judgments of native Mandarin and native English speakers on various sets of Mandarin tones. A short-stimulus (350 ms) and a long-stimulus (450 ms) series were synthesized for each of the syllables /bi/, /ba/, and /bu/. Each series included 11 stimuli that varied incrementally in F 0 contour from a Tone 2 exemplar to a Tone 3 exemplar, both of which were Mandarin morphemes. Subjects (both Mandarin- and English-speaking) were first trained with feedback to assign the short and long series-endpoint stimuli to two categories based on F 0 contour alone. Next, subjects identified the entire stimulus series (both short and long) on the basis of the two training categories. For both Mandarin-speaking and English-speaking subjects, a longer syllable duration shifted the labeling boundary reliably toward more Tone 3 responses. Syllable lengthening appears to enhance auditorily the F 0 cues for the Tone 3 category. The results of Experiment 2, in which English-speaking listeners made comparative judgments of the apparent length of the Tone 2 and Tone 3 exemplars, allowed us to distinguish empirically between two possible auditory-enhancement accounts of the durational effect observed in the first experiment.

Auditory enhancement and the perception of concurrent vowels.

Listeners identified both constituents of double vowels created by summing the waveforms of pairs of synthetic vowels with the same duration and fundamental frequency. Accuracy of identification was significantly above chance. Effects of introducing such double vowels by visual or acoustical precursor stimuli were examined. Precursors specified the identity of one of the two constituent vowels. Performance was scored as the accuracy with which the other vowel was identified. Visual precursors were standard English spellings of one member of the vowel pair; acoustical precursors were 1-sec segments of one member of the vowel pair. Neither visual precursors nor contralateral acoustical precursors improved performance over the condition with no precursor. Thus, knowledge of the identity of one of the constituents of a double vowel does not help listeners to identify the other constituent. A significant improvement in performance did occur with ipsilateral acoustical precursors, consistent with earlier demonstrations that frequency components which undergo changes in spectral amplitude achieve enhanced auditory prominence relative to unchanging components. This outcome demonstrates the joint but independent operation of auditory and perceptual processes underlying the ability of listeners to understand speech despite adversely peaked frequency responses in communication channels.

When the Objects of Perception Are Spoken

An analysis of the auditory enhancement theory of speech perception reveals its liabilities despite its plausibility. Four points are featured in a critique of its main premise: that the objects of speech perception are sounds, and, therefore, the perception of speech relies primarily on the categorization of auditory qualities. First, this critique notes that auditory enhancement theory is conventional, despite professed differences from the accounts which it disputes, in adopting consonant and vowel inventories as the objects of perception. Second, the critique argues that auditory enhancement theory fails three empirical tests of this equa- tion of phonetic attributes and auditory qualities. Third, the critique alleges that the auditory enhancement theory seeks support from equivocal experiments on animal training, in which the limited similarity of human and nonhuman test performance remains unexplored empirically and unexplained. Last, the critique concludes that the theory of speech perception by au...

Cues to What? A Comment on Diehl and Kluender 'On the Objects of Speech Perception'

The principle of auditory enhancement is a valuable extension of the theory of adaptive dispersion. However, neither the principle nor the theory has any bearing on what we perceive in speech. All perceptual objects, including those of speech, are abstract, amodal structures made available to cognition through one or more sensory modalities. A focus on the modalities or media of information transfer in speech perception diverts attention from two central and related aspects of speech function: perceptuomotor functional equivalence and imitation. Arguments and evidence from studies of lipreading, short-term memory, and infant behavior are put forward to support the postulation of an output unit, the phonetic gesture, at a level in the communication chain corresponding on the input side to the acoustic cue. The object of speech perception is then taken to be the amodal phonetic segment, a cohesive set of direct mappings between sound and gesture.

Evaluation of auditory enhancement effect in normal and hearing-impaired listeners

The auditory enhancement effect was evaluated in normal and hearing-impaired listeners using a forward masking paradigm similar to that used by Viemeister and Bacon [J. Acoust. Soc. Am. 71, 1502–1507 (1982)]. Masked thresholds for a 2000-Hz probe were compared between a standard condition in which the masker was a four-component harmonic complex including 2000 Hz, and an enhancing condition in which a three-component harmonic complex not including 2000 Hz preceded the four-component masker. The spectrum level of the masker was held constant at 85 dB SPL. For the normal listeners, the thresholds in the enhancing condition were approximately 30 dB greater than in the standard condition. For the hearing-impaired listeners, the enhancing condition resulted in only an 8- to 18-dB shift in threshold relative to the standard condition. One explanation for the enhancement effect in this case involves the adaptation of suppression in components surrounding the unadapted component such that there is actually an increase in intensity in the unadapted region. Suppression effects were also evaluated in both groups by measuring forward masked thresholds for a 2000-Hz probe as a function of a variable frequency suppressor tone that was added to a 2000-Hz masker. There was evidence of suppression for the normal listeners, particularly when the suppressor component was above the masker in frequency. However, for the hearing-impaired listeners, there was limited evidence of suppression as there was little change in masked threshold as a function of suppressor frequency.

On the possible role of auditory short-term adaptation in perception of the prevocalic [m]–[n] contrast

Acoustic information about the place of articulation of a prevocalic nasal consonant is distributed over two distinct signal portions, the nasal murmur and the onset of the following vowel. The spectral properties of these signal portions are perceptually important, as is their relationship (the pattern of spectral change). A series of experiments was conducted to investigate to what extent relational place of articulation information derives from a peripheral auditory interaction, viz., short-term adaptation caused by the murmur. Experimental manipulations intended to disrupt the effects of such adaptation included separation of the murmur and the vowel by intervals of silence, presentation to different ears, and reversal of order. Other tests of the possible role of adaptation included manipulation of murmur duration, murmur-vowel cross splicing, and high-pass filtering of the excised vowel onset. While the results of several experiments were compatible with the peripheral adaptation hypothesis, others did not support it. An alternative hypothesis, that the manner cues provided by the murmur are crucial for accurate place judgments, was also discredited. It was concluded that, at least under good listening conditions, the perception of spectral relationships does not depend on peripheral auditory enhancement and probably rests on a central comparison process.

Auditory enhancement of changes in spectral amplitude.

An auditory enhancement effect occurs when one component of a harmonic series is omitted for a few hundred milliseconds and then reintroduced: The reintroduced harmonic stands out perceptually. Three experiments are reported that studied a version of this effect in which several components of a harmonic series are enhanced to define the formants of a vowel. Using the accuracy of vowel identification to measure the prominence of the formant peaks in the effective auditory representation, forms of the effect were identified that are qualitatively similar to the incremental and decremental responses seen in primary auditory-nerve fibers. These results are compatible with an origin for the enhancement effect in peripheral auditory adaptation. However, an additional mechanism is required to account for the demonstration [Viemeister and Bacon, J. Acoust. Soc. Am. 71, 1502-1507 (1982)] that enhancement can involve a true gain in the frequency region of the reintroduced component. These effects demonstrate one way in which the auditory system may attenuate the prominence of background noises while preserving the ability to represent changes in spectral amplitude produced by newly arriving signals.