Bilingualism has been shown to modulate the neural mechanisms of selective attention, with differences between monolinguals and bilinguals observed even when they display equivalent behavioural performance in a selective attention task. This suggests that the crucial consequence of learning and using multiple languages might be that it triggers neuroplastic adaptation that allows bilinguals to achieve optimal performance under increased processing demands. This functional plasticity yielding equivalent outcomes (also known as degeneracy) is a common feature in biological systems, allowing flexible adaptation to changing environments.

Yet the exact mechanism by which bilingualism affects selective attention is still not entirely clear. While the currently dominant view suggests that the need for constant management of competing languages in bilinguals increases attentional capacity; another possibility is that this language control may be drawing on the available attentional resources such that they need to be economised to support optimal task performance. Another question concerns the development of this adaptation over time, where the demands of competition and inhibition between co-activated languages might be reconfiguring the patterns of attentional processes right from the onset, such that the effects can be seen by the time children can respond to selective attention tasks. Alternatively, these modifications might have a protracted maturation dependent on the length and intensity of exposure to the demands of bilingualism, in which case they would manifest differently in adults and in children, as well as in speakers with different levels of exposure to L2. Finally, another aspect is to establish the extent to which these modifications might affect attentional processing beyond the language domain, extending to auditory processing more generally. Here I present a series of behavioural and neuroimaging experiments that address these questions.

To investigate whether bilingualism enhances attentional processing or triggers redistribution of the existing capacity, I used EEG to track the neural encoding of attended continuous speech in monolingual and bilingual children aged 7-12, in the context of different types of acoustic and linguistic interference. Participants attended to a narrative in English while four different types of interference were presented to the unattended ear. The neural encoding of attended and unattended streams was assessed by reconstructing their speech envelopes from the EEG data in each condition, using the mTRF toolbox. Results showed more accurate reconstruction of the attended envelopes than ignored ones across all conditions for both bilinguals and monolinguals. Critically however, there was no evidence of enhanced attentional processing in bilinguals; instead data showed a pattern consistent with redistribution of the available capacity, economised to achieve optimal performance on the selective attention task. The follow up behavioural experiments tested the limits of this adaptation by using a dual task (dichotic listening + visual attention) to further increase processing load. The results over three experiments (on children, and adults with different levels of exposure to L2) showed consistently comparable performance on both tasks for monolingual and bilingual adults, suggesting that bilingual adaptation can accommodate high processing loads. However there were also subtle differences in performance on the secondary (visual) task between the monolingual and bilingual children, and across the two groups of bilingual adults, suggesting that maturation and exposure do exert influence on this functional adaptation. The findings of the final EEG study on auditory processing beyond language domain indicate comparable but attenuated modification of attentional processing in bilinguals, compared to the first EEG study using linguistic interference.

Findings from all experiments are explored in the context of theories of selective attention and bilingualism.