Biomedical subjects

Keith R Kluender

Publications and source records attributed to Keith R Kluender.

6 recordsLinked to original sources

PubMed2005

Categorical perception of speech by children with specific language impairments.

Previous research has suggested that children with specific language impairments (SLI) have deficits in basic speech perception abilities, and this may be an underlying source of their linguistic deficits. These findings have come from studies in which perception of synthetic versions of meaningless syllables was typically examined in tasks with high memory demands. In this study, 20 children with SLI (mean age = 9 years, 3 months) and 20 age-matched peers participated in a categorical perception task. Children identified and discriminated digitally edited versions of naturally spoken real words in tasks designed to minimize memory requirements. Both groups exhibited all hallmarks of categorical perception: a sharp labeling function, discontinuous discrimination performance, and discrimination predicted from identification. There were no group differences for identification data, but children with SLI showed lower peak discrimination values. Children with SLI still discriminated phonemically contrastive pairs at levels significantly better than chance, with discrimination of same-label pairs at chance. These data suggest that children with SLI perceive natural speech tokens comparably to age-matched controls when listening to words under conditions that minimize memory load. Further, poor performance on speech perception tasks may not be due to a speech perception deficit, but rather to a consequence of task demands.

Child↗

PubMed2005

The relative importance of spectral tilt in monophthongs and diphthongs.

Ito et al. [J. Acoust. Soc. Am. 110, 1141-1149 (2001)] demonstrated that listeners can reliably identify vowel stimuli on the basis of relative formant amplitude in the absence of, or in spite of, F2 peak frequency. In the present study, formant frequencies and global spectral tilt are manipulated independently in synthetic steady-state vowels. Listeners' identification of these sounds demonstrate strong perceptual effects for both local (formant frequency) and global (spectral tilt) acoustic characteristics. Subsequent experiments reveal that effects of spectral tilt are attenuated in synthetic stimuli for which formant center frequencies change continuously. When formant peaks are kinematic, perceptual salience of the relative amplitudes of low- and high-frequency formants (as determined by spectral tilt) is mitigated. Because naturally produced English vowels are rarely spectrally static, one may conclude that gross spectral properties may play only a limited role in perception of fluently produced vowel sounds.

Humans↗

PubMed2005

Pattern playback revisited: unvoiced stop consonant perception.

Among the most influential publications in speech perception is Liberman, Delattre, and Cooper's [Am. J. Phys. 65, 497-516 (1952)] report on the identification of synthetic, voiceless stops generated by the Pattern Playback. Their map of stop consonant identification shows a highly complex relationship between acoustics and perception. This complex mapping poses a challenge to many classes of relatively simple pattern recognition models which are unable to capture the original finding of Liberman et al. that identification of /k/ was bimodal for bursts preceding front vowels but otherwise unimodal. A replication of this experiment was conducted in an attempt to reproduce these identification patterns using a simulation of the Pattern Playback device. Examination of spectrographic data from stimuli generated by the Pattern Playback revealed additional spectral peaks that are consistent with harmonic distortion characteristic of tube amplifiers of that era. Only when harmonic distortion was introduced did bimodal /k/ responses in front-vowel context emerge. The acoustic consequence of this distortion is to add, e.g., a high-frequency peak to midfrequency bursts or a midfrequency peak to a low-frequency burst. This likely resulted in additional /k/ responses when the second peak approximated the second formant of front vowels. Although these results do not challenge the main observations made by Liberman et al. that perception of stop bursts is context dependent, they do show that the mapping from acoustics to perception is much less complex without these additional distortion products.

Acoustic Stimulation↗

PubMed2003

Effects of contrast between onsets of speech and other complex spectra.

Previous studies using speech and nonspeech analogs have shown that auditory mechanisms which serve to enhance spectral contrast contribute to perception of coarticulated speech for which spectral properties assimilate over time. In order to better understand the nature of contrastive auditory processes, a series of CV syllables varying acoustically in F2-onset frequency and perceptually from /ba/ to /da/ was identified following a variety of spectra including three-peak renditions of [e] and [o], one-peak simulations of only F2, and spectral complements of these spectra for which peaks are replaced with troughs. Results for three-versus one-peak (or trough) precursor spectra were practically indistinguishable, suggesting that effects were spectrally local and not dependent upon perception of precursors as speech. Effects of complementary (trough) spectra had complementary effects on perception of following stops; however, effects for spectral complements were particularly dependent upon the interval between precursor and CV onsets. Results from these studies cannot be explained by simple masking or adaptation of suppression. Instead, they provide evidence for the existence of processes that selectively enhance contrast between onset spectra of neighboring sounds, and these processes are relevant for perception of connected speech.

Adult↗

PubMed2002

Synchrony capture hypothesis fails to account for effects of amplitude on voicing perception.

Kluender et al. [J. Acoust. Soc. Am. 97, 2552-2567 (1995)] reported that overall stimulus amplitude affects perception of the voicing contrast in syllable-initial stops as a function of frequency separation between the first formant (F1) and higher formants (F2, F3). These results were offered as support for a hypothesis that [- voice] could be signaled by a shift in the temporal pattern of neural firing from synchronizing to energy at frequencies of F2 and F3 to synchronizing to energy near F1. Several predictions from this "synchrony capture hypothesis" were tested in the current study. In all cases the hypothesis was not supported. Effect of stimulus amplitude (increased voiceless responses with higher amplitude) was maintained when there was no cutback in F1 or when F2 and F1 energy bands were presented dichotically. In further tests of the hypothesis, voice-voiceless series were created that maintained periodic energy throughout the syllable (with F1 cutback signaling voicing). Energy just below the frequency of F2 and energy above F1 were presented dichotically. Thus, at the periphery there was no competition between frequencies near F2 and lower frequencies. In contrast to predictions of the "synchrony capture hypothesis," overall amplitude still had an effect on voice-voiceless identifications.

Dichotic Listening Tests↗

PubMed2002

Responses to cochlear normalized speech stimuli in the auditory nerve of cat.

Previous studies of auditory-nerve fiber (ANF) representation of vowels in cats and rodents (chinchillas and guinea pigs) have shown that, at amplitudes typical for conversational speech (60-70 dB), neuronal firing rate as a function of characteristic frequency alone provides a poor representation of spectral prominences (e.g., formants) of speech sounds. However, ANF rate representations may not be as inadequate as they appear. Here, it is investigated whether some of this apparent inadequacy owes to the mismatch between animal and human cochlear characteristics. For all animal models tested in earlier studies, the basilar membrane is shorter and encompasses a broader range of frequencies than that of humans. In this study, a customized speech synthesizer was used to create a rendition of the vowel [E] with formant spacing and bandwidths that fit the cat cochlea in proportion to the human cochlea. In these vowels, the spectral envelope is matched to cochlear distance rather than to frequency. Recordings of responses to this cochlear normalized [E] in auditory-nerve fibers of cats demonstrate that rate-based encoding of vowel sounds is capable of distinguishing spectral prominences even at 70-80-dB SPL. When cochlear dimensions are taken into account, rate encoding in ANF appears more informative than was previously believed.

Acoustics↗