非ネイティブおよび地域アクセントにおけるリスニング努力：瞳孔測定研究

Previous work has shown that L2-accented speech incurs a processing cost even when accurately understood. It remains unknown, however, whether an online processing cost is found when listeners process speech produced in L1 accents that are not their own. In this study, we examine this question by using comparative pupil dilation as a measure of cognitive load. Participants from the South of England heard sentences produced in four different accents: Southern British English (the listeners' own familiar accent), American English (a standard L1 accent widely used in media), Glaswegian English (a less-familiar regional L1 accent), and Mandarin Chinese-accented English (an L2 English accent). Results show that Chinese-accented speech elicited significantly larger pupil dilation responses compared with Southern British English. Speech from less-familiar L1 accents elicited pupil dilation responses of different shapes and trajectories, suggesting differences in processing of these accents. Furthermore, participants showed larger mean pupil dilation when they heard relatively less-familiar L1 American-accented speech than when hearing Glaswegian English. Interestingly, this effect was found despite participants self-reporting that they were less familiar with the Glaswegian accent and found it more effortful to comprehend compared with American English. These results suggest that accurately perceived and highly intelligible L1 accents such as American English also incur a cognitive cost in processing, but to a smaller extent compared with L2-accented speech. We discuss the implications of our findings for the relationship between exposure, subjective effortfulness measures, and pupil dilation responses.