David R. R. Smith

The processing and perception of size information in speech sounds.

There is information in speech sounds about the length of the vocal tract; specifically, as a child grows, the resonators in the vocal tract grow and the formant frequencies of the vowels decrease. It has been hypothesized that the auditory system applies a scale transform to all sounds to segregate size information from resonator shape information, and thereby enhance both size perception and speech recognition [Irino and Patterson, Speech Commun. 36, 181-203 (2002)]. This paper describes size discrimination experiments and vowel recognition experiments designed to provide evidence for an auditory scaling mechanism. Vowels were scaled to represent people with vocal tracts much longer and shorter than normal, and with pitches much higher and lower than normal. The results of the discrimination experiments show that listeners can make fine judgments about the relative size of speakers, and they can do so for vowels scaled well beyond the normal range. Similarly, the recognition experiments show good performance for vowels in the normal range, and for vowels scaled well beyond the normal range of experience. Together, the experiments support the hypothesis that the auditory system automatically normalizes for the size information in communication sounds.

The interaction of glottal-pulse rate and vocal-tract length in judgements of speaker size, sex, and age

Glottal-pulse rate (GPR) and vocal-tract length (VTL) are related to the size, sex, and age of the speaker but it is not clear how the two factors combine to influence our perception of speaker size, sex, and age. This paper describes experiments designed to measure the effect of the interaction of GPR and VTL upon judgements of speaker size, sex, and age. Vowels were scaled to represent people with a wide range of GPRs and VTLs, including many well beyond the normal range of the population, and listeners were asked to judge the size and sex/age of the speaker. The judgements of speaker size show that VTL has a strong influence upon perceived speaker size. The results for the sex and age categorization (man, woman, boy, or girl) show that, for vowels with GPR and VTL values in the normal range, judgements of speaker sex and age are influenced about equally by GPR and VTL. For vowels with abnormal combinations of low GPRs and short VTLs, the VTL information appears to decide the sex/age judgement.

Discrimination of speaker size from syllable phrases.

The length of the vocal tract is correlated with speaker size and, so, speech sounds have information about the size of the speaker in a form that is interpretable by the listener. A wide range of different vocal tract lengths exist in the population and humans are able to distinguish speaker size from the speech. Smith et al. [J. Acoust. Soc. Am. 117, 305-318 (2005)] presented vowel sounds to listeners and showed that the ability to discriminate speaker size extends beyond the normal range of speaker sizes which suggests that information about the size and shape of the vocal tract is segregated automatically at an early stage in the processing. This paper reports an extension of the size discrimination research using a much larger set of speech sounds, namely, 180 consonant-vowel and vowel-consonant syllables. Despite the pronounced increase in stimulus variability, there was actually an improvement in discrimination performance over that supported by vowel sounds alone. Performance with vowel-consonant syllables was slightly better than with consonant-vowel syllables. These results support the hypothesis that information about the length of the vocal tract is segregated at an early stage in auditory processing.

How the Human Brain Recognizes Speech in the Context of Changing Speakers

We understand speech from different speakers with ease, whereas artificial speech recognition systems struggle with this task. It is unclear how the human brain solves this problem. The conventional view is that speech message recognition and speaker identification are two separate functions and that message processing takes place predominantly in the left hemisphere, whereas processing of speaker-specific information is located in the right hemisphere. Here, we distinguish the contribution of specific cortical regions, to speech recognition and speaker information processing, by controlled manipulation of task and resynthesized speaker parameters. Two functional magnetic resonance imaging studies provide evidence for a dynamic speech-processing network that questions the conventional view. We found that speech recognition regions in left posterior superior temporal gyrus/superior temporal sulcus (STG/STS) also encode speaker-related vocal tract parameters, which are reflected in the amplitude peaks of the speech spectrum, along with the speech message. Right posterior STG/STS activated specifically more to a speaker-related vocal tract parameter change during a speech recognition task compared with a voice recognition task. Left and right posterior STG/STS were functionally connected. Additionally, we found that speaker-related glottal fold parameters (e.g., pitch), which are not reflected in the amplitude peaks of the speech spectrum, are processed in areas immediately adjacent to primary auditory cortex, i.e., in areas in the auditory hierarchy earlier than STG/STS. Our results point to a network account of speech recognition, in which information about the speech message and the speakers vocal tract are combined to solve the difficult task of understanding speech from different speakers.

Neural Representation of Auditory Size in the Human Voice and in Sounds from Other Resonant Sources

Summary The size of a resonant source can be estimated by the acoustic-scale information in the sound [1–3]. Previous studies revealed that posterior superior temporal gyrus (STG) responds to acoustic scale in human speech when it is controlled for spectral-envelope change (unpublished data). Here we investigate whether the STG activity is specific to the processing of acoustic scale in human voice or whether it reflects a generic mechanism for the analysis of acoustic scale in resonant sources. In two functional magnetic resonance imaging (fMRI) experiments, we measured brain activity in response to changes in acoustic scale in different categories of resonant sound (human voice, animal call, and musical instrument). We show that STG is activated bilaterally for spectral-envelope changes in general; it responds to changes in category as well as acoustic scale. Activity in left posterior STG is specific to acoustic scale in human voices and not responsive to acoustic scale in other resonant sources. In contrast, the anterior temporal lobe and intraparietal sulcus are activated by changes in acoustic scale across categories. The results imply that the human voice requires special processing of acoustic scale, whereas the anterior temporal lobe and intraparietal sulcus process auditory size information independent of source category.

Role of glottal‐pulse rate, vocal‐tract length, and original talker upon judgements of speaker sex and age

Glottal‐pulse rate (GPR) and vocal‐tract length (VTL) are important determinants of the perceived sex and age of the speaker [D. R. R. Smith and R. D. Patterson, J. Acoust. Soc. Am. 118, 3177–3186 (2005)]. Our previous research simulated the voices of variously‐sized speakers of both sexes by manipulating the recorded vowels of one adult male talker. The current study explored whether there are additional cues in the voices of men, women, and children that influence judgements of speaker sex and age. We manipulated the recorded vowels of an adult man, adult woman, young boy, and young girl, and determined the effect upon the distribution of sex and age responses (man, woman, boy, girl). Results show that the distribution of sex and age judgements across the GPR‐VTL plane is heavily influenced by GPR and VTL, but it is also affected by the original talker’s size (or age). The effect of original talker appears to be mainly due to the consistent difference between oral‐pharyngeal length ratios of children an...

Perception of speaker size and sex of vowel sounds

Glottal‐pulse rate (GPR) and vocal‐tract length (VTL) are both related to speaker size and sex—however, it is unclear how they interact to determine our perception of speaker size and sex. Experiments were designed to measure the relative contribution of GPR and VTL to judgements of speaker size and sex. Vowels were scaled to represent people with different GPRs and VTLs, including many well beyond the normal population values. In a single interval, two response rating paradigm, listeners judged the size (using a 7‐point scale) and sex/age of the speaker (man, woman, boy, or girl) of these scaled vowels. Results from the size‐rating experiments show that VTL has a much greater influence upon judgements of speaker size than GPR. Results from the sex‐categorization experiments show that judgements of speaker sex are influenced about equally by GPR and VTL for vowels with normal GPR and VTL values. For abnormal combinations of GPR and VTL, where low GPRs are combined with short VTLs, VTL has more influence t...

Speaker sex discrimination performance for voiced and whispered vowels at very short durations

When listening to someone’s voice what duration of stimulus is required to tell whether the person speaking is a man or a woman? Previous research studied this using voiced speech [D. R. R. Smith, Acta Psychologica 148, 81–90 (2014)]. The current study expanded this analysis by investigating what duration of stimulus is required to discriminate speaker sex when listening to whispered speech. Psychometric functions were collected plotting percent correct discrimination that a man or woman spoke, as a function of very brief vowel segment durations (up to 60 ms), for both voiced and whispered vowels. Results show that speaker sex discrimination performance is significantly impaired for whispered vowels, as compared to voiced vowels, for all durations tested. These findings are interpreted in terms of the impoverished cues to speaker sex in whispered compared to voiced speech.

Perception of speaker sex and vowel recognition at very short durations

A man or woman saying the same vowel do so with very different voices. The auditory system solves the problem of extracting what the man or woman said despite substantial differences in the acoustic properties of the carrier voice. Much of this acoustic variation is due to differences in the underlying anatomical mechanisms for producing speech. If the auditory system knew the sex of the speaker then it could correct for speaker-sex related acoustic differences thus facilitating vowel recognition. We measured the minimal stimulus duration necessary to accurately discriminate whether a brief vowel segment was spoken by a man or woman, and to accurately recognize what vowel was spoken. Results show that reliable vowel recognition precedes reliable speaker sex discrimination. Furthermore, the pattern of performance across experiments where voice pitch and resonance information were systematically varied, is markedly different depending on whether the task is speaker-sex discrimination or vowel recognition. T...