A growing number of studies on the acquisition of lexical tone by adult learners have revealed that factors such as language background, musical experience, cognitive abilities, and neuroanatomy all play a role in determining tone learning success. On the basis of these findings, it has been argued that the effectiveness of tone learning in adulthood depends on individual differences in these factors. However, it is not clear whether similar individual differences play an analogous role in tone learning in childhood. Indeed, relatively few studies have made comparisons between how adults and children learn lexical tones. Here, we review recent developments for tone learning in both adults and children. The review covers tone training in a range of contexts, including in naive listeners, in native speakers of other tone languages, in listeners with varying levels of musical experience, and in individuals with speech and hearing disorders. Finally, we discuss the parallels between adult and child tone learning, and provide recommendations concerning how findings in adult tone training can provide insights into tone learning for children by accommodating the needs of individual learners.

Bilingualism was once thought to result in cognitive disadvantages, but research in recent decades has demonstrated that experience with two (or more) languages confers a bilingual advantage in executive functions and may delay the incidence of Alzheimer's disease. However, conflicting evidence has emerged leading to questions concerning the robustness of the bilingual advantage for both executive functions and dementia incidence. Some investigators have failed to find evidence of a bilingual advantage; others have suggested that bilingual advantages may be entirely spurious, while proponents of the advantage case have continued to defend it. A heated debate has ensued, and the field has now reached an impasse. This review critically examines evidence for and against the bilingual advantage in executive functions, cognitive aging, and brain plasticity, before outlining how future research could shed light on this debate and advance knowledge of how experience with multiple languages affects cognition and the brain.

Speech production research has demonstrated that the first language (L1) often interferes with production in bilinguals’ second language (L2), but it has been suggested that bilinguals who are L2-dominant are the most likely to suppress this L1-interference. While prolonged contextual changes in bilinguals’ language use (e.g., stays overseas) are known to result in L1 and L2 phonetic shifts, code-switching provides the unique opportunity of observing the immediate phonetic effects of L1-L2 interaction. We measured the voice onset times (VOTs) of Greek–English bilinguals’ productions of /b, d, p, t/ in initial and medial contexts, first in either a Greek or English unilingual mode, and in a later session when they produced the same target pseudowords as a code-switch from the opposing language. Compared to a unilingual mode, all English stops produced as code-switches from Greek, regardless of context, had more Greek-like VOTs. In contrast, Greek stops showed no shift toward English VOTs, with the exception of medial voiced stops. Under the specifically interlanguage condition of code-switching we have demonstrated a pervasive influence of the L1 even in L2-dominant individuals.

Over the next fifty years, the number of older adults is set to reach record levels. Protecting older adults from the age-related effects of cognitive decline is one of the greatest challenges of the next few decades as it places increasing pressure on families, health systems, and economies on a global scale. The disease-state of age-related cognitive decline—Alzheimer's disease and other dementias—hijacks our consciousness and intellectual autonomy. However, there is evidence that cognitively stimulating activities protect against the adverse effects of cognitive decline. Similarly, bilingualism is also considered to be a safeguard. We propose that foreign language learning programs aimed at older populations are an optimal solution for building cognitive reserve because language learning engages an extensive brain network that is known to overlap with the regions negatively affected by the aging process. It is recommended that future research should test this potentially fruitful hypothesis.

The way that bilinguals produce phones in each of their languages provides a window into the nature of the bilingual phonological space. For stop consonants, if early sequential bilinguals, whose languages differ in voice onset time (VOT) distinctions, produce native-like VOTs in each of their languages, it would imply that they have developed separate first and second language phones, that is, language-specific phonetic realisations for stop-voicing distinctions. Given the ambiguous phonological status of Greek voiced stops, which has been debated but not investigated experimentally, Greek-English bilinguals can offer a unique perspective on this issue. We first recorded the speech of Greek and Australian-English monolinguals to observe native VOTs in each language for /p, t, b, d/ in word-initial and word-medial (post-vocalic and post-nasal) positions. We then recorded fluent, early Greek–Australian-English bilinguals in either a Greek or English language context; all communication occurred in only one language. The bilinguals in the Greek context were indistinguishable from the Greek monolinguals, whereas the bilinguals in the English context matched the VOTs of the Australian-English monolinguals in initial position, but showed some modest differences from them in the phonetically more complex medial positions. We interpret these results as evidence that bilingual speakers possess phonetic categories for voiced versus voiceless stops that are specific to each language, but are influenced by positional context differently in their second than in their first language.