Executive control in speech comprehension: bilingual dichotic listening studies
In this dissertation, the traditional dichotic listening paradigm was integrated with the notion of working memory capacity (WMC) to explore the cognitive mechanism of bilingual speech comprehension at the passage level. A bilingual dichotic listening (BDL) task was developed and administered to investigate characteristics of bilingual listening comprehension, which include semantic relatedness, unattended language, ear preference, auditory attentional control, executive control, voluntary note-taking, and language switching. The central concept of the BDL paradigm is that the auditory stimuli are presented in the bilinguals’ two languages and their attention is directed to one of their ears while they have to overcome cognitive and linguistic conflicts caused by information in the other ear. Different experimental manipulations were employed in the BDL task to examine the characteristics of bilingual listening comprehension. The bilingual population examined was Japanese- English bilinguals with relatively high second language (L2) proficiency and WMC. Seven experiments and seven cross-experimental comparisons are reported. Experiment 1 employed the BDL task with pairs of passages that had different semantic relationships (i.e., related or unrelated) and were heard in different languages (i.e., L1 or L2). The semantically related passages were found to interfere with comprehension of the attended passage more than the semantically unrelated passages, whether the attended and unattended languages were the same or different. Contrary to the theories of bilingual language control, unattended L1 was found to enhance comprehension of the attended passage, regardless of semantic relationships and language it was heard in. L2 proficiency and WMC served as good predictors of resolution of the cognitive and linguistic conflicts. The BDL task is suggested to serve as an experimental paradigm to explore executive control and language control in bilingual speech comprehension. Experiment 2 was conducted to investigate language lateralisation (i.e., ear preference) on bilingual speech comprehension, hence, the participants in Experiment 1 used their preferred ear, whereas participants in Experiment 2 used their non-preferred ear, whether it was left or right, in the BDL task. Comprehension was better through the preferred ear, indicating that there is a favourable ear-to-hemisphere route for understanding bilinguals’ two languages. Most of the participants were found to be left-lateralised (i.e., right-eared) and some to be right-lateralised (i.e., left-eared) presumably depending on their L2 proficiency and WMC. Experiment 3 was concerned with auditory attentional control, and explored whether there would be a right-ear advantage (REA). The participants indicated an REA whether the attended and unattended languages were L1 or L2. When they listened to Japanese in the left ear, they found it more difficult to suppress Japanese in the right ear than English. WMC was not required as much as expected for auditory attentional control probably because the passages in Experiment 3 did not yield as much semantic competition as those in Experiment 1. L2 proficiency was crucial for resolving within- and between-language competition in each ear. Experiments 4, 5, and 6 were replications of Experiments 1, 2 and 3, but these latter experiments considered the effect of note-taking that is commonly performed in everyday listening situations. Note-taking contributed to better performance and clearer understanding of the role of WMC in bilingual speech comprehension. A cross-experimental analysis between Experiments 1, 2, 4, and 5 revealed not only a facilitatory role of note-taking in bilingual listening comprehension in general, but also a hampering role when listening through the preferred ear. Experiment 7 addressed the effect of predictability of language switching by presenting L1 and L2 in a systematic order while switching attention between ears and comparing the result with that of Experiment 6 where language switching was unpredictable. The effect of predictability of language switching was different between ears. When language switches were predictable, higher comprehension was observed in the left ear than the right ear, and when language switches were unpredictable, higher comprehension was observed in the right ear than the left ear, thereby suggesting a mechanism of asymmetrical language control. WMC was more related to processing of predictable language switches than that of unpredictable language switches. The dissertation ends with discussions of the implications from the seven BDL experiments and possible applications, along with experimental techniques from other relevant disciplines that might be used in future research to yield additional insight into how bilingual listeners sustain their listening performance in their two languages in the real-life situations.