Volume 116, Issue 2, August 2004
Index of content:
- SPEECH PERCEPTION 
116(2004); http://dx.doi.org/10.1121/1.1764832View Description Hide Description
Previous studies suggest that speakers are systematically inaccurate, or biased, when imitating self-produced vowels. The direction of these biases in formant space and their variation may offer clues about the organization of the vowel perceptual space. To examine these patterns, three male speakers were asked to imitate 45 self-produced vowels that were systematically distributed in F1/F2 space. All three speakers showed imitation bias, and the bias magnitudes were significantly larger than those predicted by a model of articulatory noise. Each speaker showed a different pattern of bias directions, but the pattern was unrelated to the locations of prototypical vowels produced by that speaker. However, there were substantial quantitative regularities: (1) The distribution of imitation variability and bias magnitudes were similar for all speakers, (2) the imitation variability was independent of the bias magnitudes, and (3) the imitation variability (a production measure) was commensurate with the formant discrimination limen (a perceptual measure). These results indicate that there is additive Gaussian noise in the imitation process that independently affects each formant and that there are speaker-dependent and potentially nonlinguistic biases in vowel perception and production.
Categorization and discrimination of nonspeech sounds: Differences between steady-state and rapidly-changing acoustic cues116(2004); http://dx.doi.org/10.1121/1.1766020View Description Hide Description
Different patterns of performance across vowels and consonants in tests of categorization and discrimination indicate that vowels tend to be perceived more continuously, or less categorically, than consonants. The present experiments examined whether analogous differences in perception would arise in nonspeech sounds that share critical transient acoustic cues of consonants and steady-state spectral cues of simplified synthetic vowels. Listeners were trained to categorize novel nonspeech sounds varying along a continuum defined by a steady-state cue, a rapidly-changing cue, or both cues. Listeners’ categorization of stimuli varying on the rapidly changing cue showed a sharp category boundary and posttraining discrimination was well predicted from the assumption of categorical perception. Listeners more accurately discriminated but less accurately categorized steady-state nonspeech stimuli. When listeners categorized stimuli defined by both rapidly-changing and steady-state cues, discrimination performance was accurate and the categorization function exhibited a sharp boundary. These data are similar to those found in experiments with dynamic vowels, which are defined by both steady-state and rapidly-changing acoustic cues. A general account for the speech and nonspeech patterns is proposed based on the supposition that the perceptual trace of rapidly-changing sounds decays faster than the trace of steady-state sounds.
116(2004); http://dx.doi.org/10.1121/1.1772398View Description Hide Description
Neural-population interactions resulting from excitation overlap in multi-channel cochlear implants(CI) may cause blurring of the “internal” auditory representation of complex sounds such as vowels. In experiment I, confusion matrices for eight German steady-state vowellike signals were obtained from seven CI listeners. Identification performance ranged between 42% and 74% correct. On the basis of an information transmission analysis across all vowels, pairs of most and least frequently confused vowels were selected for each subject. In experiment II, vowel masking patterns (VMPs) were obtained using the previously selected vowels as maskers. The VMPs were found to resemble the “electrical” vowel spectra to a large extent, indicating a relatively weak effect of neural-population interactions. Correlation between vowel identification data and VMP spectral similarity, measured by means of several spectral distance metrics, showed that the CI listeners identified the vowels based on differences in the between-peak spectral information as well as the location of spectral peaks. The effect of nonlinear amplitude mapping of acoustic into “electrical” vowels, as performed in the implant processors, was evaluated separately and compared to the effect of neural-population interactions. Amplitude mapping was found to cause more blurring than neural-population interactions. Subjects exhibiting strong blurring effects yielded lower overall vowel identification scores.