Abstract:

Previous research has investigated the bottom-up effect of stimulus property (e.g., linguistic information) and the top-down effect of attention control on patterns of speech processing using the forced-attention dichotic listening (FADL) task. However, the processing patterns of lexical tones, a key component of tonal languages like Cantonese, have been relatively understudied. The thesis investigates the bottom-up effect of two Cantonese tone types (contour vs. level) differing in linguistic information and the top-down effect of attention manipulation on the tone processing patterns at a group level. Sixty Cantonese-dominant bilingual speakers participated in the FADL task, identifying Cantonese tones according to the attention instructions in the non-forced (NF), forced-left (FL), and forced-right (FR) conditions. Accuracy and reaction time in each ear were analyzed. Results showed a left ear advantage in the NF condition without being influenced by the bottom-up effect of the tone type, suggesting a dominant right hemisphere processing for Cantonese tones. Importantly, the instructed ear preference patterns in the forced conditions were in line with the expectations of the attention instructions, suggesting a top-down effect on Cantonese tone processing. These results support the role of linguistic information on the brain lateralization pattern and the role of higher-level attention on regulating lower-level processing patterns.

Based on the group-level results of the top-down effect, the thesis further focuses on how individual differences in bilingual experiences contribute to varied attention control abilities. Previous research has suggested better attention performance of the bilingual group than the monolingual group, which neglects the individual variances in each group and cannot identify which dimension of bilingual experiences is the most crucial one. To address these issues, we treated bilingualism as a continuous variable and collected the language profile (e.g., proficiency and multilingual language diversity) of the same sixty Cantonese-dominant bilingual speakers, whose second dominant language (L2) was English. The relationship between bilingual experiences and attention control performance was analyzed. Results showed that participants’ L2 proficiency predicted a larger change in the instructed (right) ear preference patterns between the NF and FR conditions for level tone processing, suggesting a positive relationship between L2 proficiency and attention control ability. These findings underscore the bilingual effect on attention control and highlight L2 proficiency as a critical dimension for enhancing attention abilities. Taken together, this thesis provides implications for tone processing and the relationship between bilingualism and attention control.