Pupil dilation reflects English /l//r/ discrimination ability for Japanese learners of English: a pilot study

The importance of the English language has been increasing as various fields have become more globalized. When Japanese people try to acquire foreign language such as English, learners find it difficult to perceive speech-sounds such as the phonemes /l/ and /r/ that are absent in their native language (e.g., "light"/lάIt/ and "right"/rάIt/). Recent studies report that a unique sound that deviates from a repetitive background sound induces pupillary dilation response (PDR) regardless of whether attention is directed to the sound or not. In this study, we investigated whether deviation in higher-order processing such as language processing induces PDR, and the possibility of determining implicit subjective English proficiency. A behavioural auditory distinguishing ability test was performed prior to the main experiment to quantitatively evaluate participants' ability to distinguish English words. Then, by conducting an oddball paradigm-employing stimulus including the phonemes /l/ and /r/ with simultaneous pupil diameter recording, a significant dilation was evoked by /l/-/r/ speech sounds presented as deviant stimuli. Moreover, a strong correlation between the PDR amplitude and participants' ability to distinguish English words was found; that is, individuals with higher ability to distinguish such words displayed a prominent PDR. Also, the PDR difference between the two groups classified by discrimination ability suggests that PDR might be sensitive to higher-order characteristics involved in language processing, which is independent from the aspects of physical sound and cognitive load.