Auditory free classification of nonnative speech

Abstract

Through experience with speech variability, listeners build categories of indexical speech characteristics including categories for talker, gender, and dialect. The auditory free classification task-a task in which listeners freely group talkers based on audio samples-has been a useful tool for examining listeners' representations of some of these characteristics including regional dialects and different languages. The free classification task was employed in the current study to examine the perceptual representation of nonnative speech. The category structure and salient perceptual dimensions of nonnative speech were investigated from two perspectives: general similarity and perceived native language background. Talker intelligibility and whether native talkers were included were manipulated to test stimulus set effects. Results showed that degree of accent was a highly salient feature of nonnative speech for classification based on general similarity and on perceived native language background. This salience, however, was attenuated when listeners were listening to highly intelligible stimuli and attending to the talkers' native language backgrounds. These results suggest that the context in which nonnative speech stimuli are presented-such as the listeners' attention to the talkers' native language and the variability of stimulus intelligibility-can influence listeners' perceptual organization of nonnative speech.

Keywords: classification; foreign-accented speech; native language background; perceptual similarity.

Figure 1

Two-dimensional MDS solutions for the Multi-sentence (A) and Same-sentence (B) conditions in which listeners grouped talkers by overall perceived similarity. Each point on the MDS solution represents a talker and is labeled with a unique talker ID as indicated in Appendix A. Each talker ID includes information about the talker’s native language background (indicated with the first letter of the language name). Male talkers are indicated by open triangles; filled circles indicate female talkers.

Figure 2

Classification performance by perceived native language background across all four conditions (Hi-NonNatOnly, Lo-NonNatOnly, Hi-NativeIncl, Lo-NativeIncl), in difference scores. “Hi/Lo” indicate the condition’s overall intelligibility level; “NativeIncl” conditions are where native talkers were included, and “NonNatOnly” conditions are when the stimuli set consisted only of nonnative talkers. Error bars indicate standard error.

Figure 3

Classification matrices indicating rates of correct pairings (cells on the diagonal) and of incorrect pairings (cells off of the diagonal) between each of the native language backgrounds presented to listeners in each condition. Each cell indicates the rate at which talkers with the two language backgrounds of the corresponding row and column were classified together. Each cell contains a score between 0 and 100, where 0 indicates that no talker of a given native language background was paired with any talker from the other native language background. The diagonal indicates rates of correct native language grouping (i.e., “hits”); the off-diagonal cells are the rates of incorrect pairings (i.e., “false alarms”). An off-diagonal score of 100 would indicate that all listeners grouped all four talkers from one native language background with all four talkers from the other native language background. Numbers in bold indicate native language pairings that were numerically above chance (i.e., greater than 16.7 for Hi-NonNatOnly and Lo-NonNatOnly; greater than 14.3 for Hi-NativeIncl and Lo-NativeIncl).

Figure 4

Two-dimensional MDS solutions for four conditions in Experiment 2, in which listeners grouped talkers by perceived native language background. Each point on the MDS solution represents a talker and is labeled with a unique talker ID as indicated in Appendix A. Each talker ID includes information about the talker’s native language background (indicated with the first letter of the language name). Male talkers are indicated by open triangles; filled circles indicate female talkers.

Figure 5

ADDTREE clustering solutions across all four conditions.