Two recent studies conducted by the University of Haifa reveal that bilingual speakers demonstrate more brain plasticity – the ability of the brain to modify its connections or rewire itself - than those who speak only one language.
In two studies of 59 and 60 normal hearing adults from the ages 19–35 years, researcher Dr. Hanin Karawani Khoury and her PhD student Dana Bsharat-Maalouf documented the differences in perception and physiological reactions in native Arabic speakers who are fluent in Hebrew as a second language and native Hebrew speakers. The studies are an attempt to solve the puzzling question as to why bilinguals take much longer than monolinguals to react to audible cues, yet still demonstrate more subcortical brain activity than those who only speak one language.
“The synchrony between neural and cognitive-perceptual measures makes the research unique and reveals direct brain-behavior links, serving as the basis for a fuller understanding of bilingual speech perception in challenging listening conditions,” Dr. Karawani said.
“Whereas the effects of bilingualism on speech perception in noise are widely studied, few studies to date have compared bilingual-monolingual performance when all participants are operating in their dominant language. This important aspect of the research will inform us of whether bilinguals perform poorer in challenging listening conditions, such as noisy environments due to reduced secondary proficiency or whether increased competition due to language co-activation contributes to bilingual performance in noise.”
One of their studies published in PLOS ONE, an inclusive journal community, titled, "Bilinguals’ speech perception in noise: Perceptual and neural associations," marks the first attempt to examine both perceptual and brain activity in bilingual populations.
As such, researchers tested auditory brainstem responses and the perception of words and sentences to assess perceptual performance and to evaluate the relationship between what people hear and how the brain reacts when exposed to that stimuli. All testing was done in both quiet and noisy conditions.
As expected, both groups - bilinguals and native Hebrew (in this case monolingual) speakers - performed better in quiet conditions as opposed to noisy ones. However, mixed results were observed among bilinguals in perceptual and physiological outcomes within noisy conditions. With regards to speech perception, bilinguals were significantly less accurate than their monolingual counterparts when attempting to decipher their second language. However, in neural responses, bilinguals demonstrated earlier auditory neural timing compared to monolinguals.
Researchers theorize that bilinguals’ superior performance brain activity may result in the enriched language environment bilinguals cultivate for themselves to manage two linguistic systems. This may explain the earlier neural timing observed, as these listeners become faster in detecting the characteristics of speech stimuli.
These correlations further the understanding of the neural processes underlying perception of speech among bilinguals, especially given that these correlations were not significant in the monolingual group. It also suggests that subcortical processes could be one source that explains variability across bilingual individuals in daily challenging listening conditions.
Thus, it can be argued that bilinguals who tend to use more cortical resources in background noise may have more efficient activation and top-down processes, and consequently, their brainstem responses were found to be less susceptible to the effect of noise.
A second study, published in the prestigious Journal Cognition and titled, "Learning and bilingualism in challenging listening conditions: How challenging can it be?" follows a similar line of thought and examines during degraded (speech in noise, vocoded — or scrambled — speech) and quiet listening conditions.
Like the previous study, this one also demonstrated more sophisticated brain activity in the bilingual group, as they fared much better in deciphering vocoded speech. Moreover, these findings suggest that bilinguals use a shared mechanism for speech processing under challenging listening conditions.
This coincides with the former study’s results, which demonstrate that noise had a relatively greater effect on bilinguals’ performance compared to the monolingual group, even when tested in their dominant language. This is an innovative finding, since in previous studies, comparisons were limited to examining differences between the perceptual performance of bilinguals in their second language and monolingual speakers, without discussing what happens to bilinguals in their native language.
Dr. Karawani and her team in her AudioNeuro Lab are currently expanding these studies by investigating perceptual and neural processing of Arabic- Hebrew-English multilinguals and Hebrew-English bilinguals with the collaboration of Dr. Tamar Degani.