Determining the Likelihood Ratio From Perceptual Attributes of Voice

PURPOSE

To provide voice experts with a method for determining the likelihood ratio (LR) from the perceptual evaluation of distinctive voice attribute scores. The proposed method aims to obtain the similarity and typicality judgments made by forensic voice experts (FVEs) during the comparison of attributes in voice pairs.

METHOD

It is based on the scoring method for LR calculation. In the first stage, 17 perceptual attributes grouped into six vocalic categories are specified. A novel graphical interface is used to obtain discriminative responses both globally and for each attribute from ten pairs of test sentences produced by the same and different speakers. The FVEs should discriminate whether the attributes are similar or different in each pair and should indicate the degree to which the attributes are present. In addition, for six specific attributes, the FVEs must decide whether the attribute is typical or atypical in the reference population. In the second stage, the mean score obtained in the first stage is converted to LR using probability density functions of listeners' responses to 1680 same/different speaker pairs discriminated for female and male speakers.

RESULTS

The responses of the FVEs to the test pairs show the discriminatory power of the attributes, the incidence of the typicality factor on the final score and the performance of each FVE. With the application of the probability density functions obtained for the responses to pairs of the same or different origin taken from the reference population, the final scores are converted into LRs that are compared with the true conditions of each pair.

CONCLUSIONS AND FUTURE WORK

The application of the developed system allows the global and discriminated evaluation of the perceptual attributes with high agreement in the comparison of pairs of voices. Obtaining the LRs allows associating the perceptual evaluation method with the automatic methods that are used nowadays. The responses of the FVEs taken as a reference, will allow training and evaluating the performance of young FVEs.

A Perceptual Method to Rate Dysphonic Voices

OBJECTIVE

To present and test a production-matching method with external references, looking at the improvement of inter-rater variability of expert evaluations.

METHOD

It consists of adjusting quality attribute levels of a synthetic vowel for a simultaneous matching with the natural patient vowel (NPV) attributes. In an initial experiment, seven speech-language pathology (SLP) experts performed this task with the new method and evaluated the same NPV with the standard method. Targets were twelve NPVs with a variety of quality attribute combinations. In a second experiment, we employed the proposed method to assess the evaluation performance of 65 SLP students.

RESULTS

Expert evaluations show less dispersion for the proposed method than those obtained using the standard rating method. Student individual responses were compared with overall responses from their own group and were cross referenced with expert responses. A Kappa index is proposed as a measure of SLP students' performance.

CONCLUSIONS

The proposed method was readily accepted by both SLP experts and students. Experts' consensus was improved. SLP students could benefit by quickly learning to discriminate complex attributes, which usually demands years of experience.

Comparison of Two Perceptual Methods for the Evaluation of Vowel Perturbation Produced by Jitter

OBJECTIVES

To explore perceptual evaluation of jitter produced by fundamental frequency (F0) variation in a sustained vowel /a/, using two different methods. One is based on listener's internal references and the other is based on external references provided by the experimenter.

METHODS

We used two methods: one is magnitude estimation-converging limits (ME-CL), which is close to the standard approach used by speech therapists when they use numerical estimations and their own standards, and other is intramodal matching procedure (IMP), where each matched stimulus is to be compared with a fixed-set matching stimuli. Systematic variations were introduced in vowel /a/ by Linear Prediction Coding synthesis using an F0 contour function obtained from a statistical jitter model. Six jitter values were used for each of two reference F0 values. Three groups of listeners were tested: expert speech therapists, speech therapy students, and naïve listeners.

RESULTS

Perceptual functions appear to be similar and linear for both methods as the theory predicts. The answers of all groups of listeners tested with ME-CL present higher standard deviations than for IMP. When subjects were tested with IMP, intrareliability and interreliability measurements show a significant improvement for both expert and naïve listeners.

CONCLUSIONS

Both intraindividual and interindividual differences for expert speech therapists could be better managed when tested with an IMP than when they use numerical estimations and internal standards to evaluate vowel perturbation produced by jitter. This procedure could be the basis for the development of a clinical evaluation tool.

Aplicación de los índices de perturbación integrado y de precisión articulatoria en pacientes con disfonía espasmódica

La disfonía espasmódica es un desorden vocal severo caracterizado por interrupciones involuntarias de la fonación, denominada también distonía focal laríngea. Este trabajo da a conocer resultados de la aplicación del índice de perturbación integrado (IPI), y de precisión articulatoria (IPA), en una población de pacientes de ambos sexos con disfonía espasmódica, y presenta la comparación con un grupo control, sujetos con diagnóstico otorrinolaringológico de cuerdas vocales móviles, sin daños estructurales en la mucosa glótica, ni alteraciones neurológicas que afecten la dinámica cordal. El índice de perturbación agrupa a cuatro parámetros: Jitter, Shimmer, relación Armónico/Ruido y la Amplitud Diferencial del Cepstrum. Los resultados muestran diferencias significativas entre los grupos de casos y controles femeninos y masculinos en los parámetros de índice de perturbación integrado y de grado de precisión articulatoria, por lo que ambos pueden considerarse indicadores apropiados para caracterizar la falta de estabilidad en la función articulatoria que se produce en las cavidades resonanciales de las pacientes con disfonía espasmódica.