Hemispheric asymmetry in processing low- and high-pass filtered Cantonese speech in tonal and non-tonal language speakers

In auditory perception, a right hemisphere (RH)/left ear advantage(LEA) for low-pass filtered stimuli and a left hemisphere (LH)/right ear advantage(REA) for high-pass filtered stimuli have been reported. Here we investigated how tonal language experience modulates this hemispheric asymmetry. We recruited Cantonese, Mandarin (tonal languages), and English (non-tonal language) speakers, and asked them to recognise dichotically presented Cantonese speech pairs in either high- or low-pass filtered conditions. The results showed that in perception accuracy, whereas English speakers demonstrated the typical RH/LEA for low-pass filtered stimuli and LH/REA for high-pass filtered stimuli, for both high- and low-pass filtered stimuli, Cantonese speakers had similarly high accuracy in the two ears, and Mandarin speakers had higher right ear accuracy. In addition, Cantonese speakers had a preference to report the stimulus presented to the right ear; Mandarin speakers showed a similar, insignificant trend of preference, whereas English speakers showed no preference. This result is consistent with the hypothesis of language-experience-dependent specialisation of the LH in auditory perception, in contrast to an experience-independent general auditory or linguistic mechanism. While English speakers showed the typical hemispheric asymmetry in auditory perception, the automaticity of LH language processing pathways in Cantonese speakers resulted in no accuracy difference between the two ears, and a right ear preference regardless of the frequency condition. In contrast, although Mandarin speakers did not understand Cantonese, they generalised their tonal language experience to Cantonese speech perception and had a REA even in the perception of low-pass filtered Cantonese speech.