Mr Wong Yuk Cheung, Tommy, a PhD student, reflected on a lecture delivered by Professor Jason Rothman, titled “Individual differences are revealing, relevant and not random in multilingual language acquisition/processing and related adaptations in neurocognition.” The lecture was held in May 2026 at the Hong Kong Polytechnic University.

Rothman argued that the individual differences exhibited across multilingual speakers in language processing and brain structure are systematic. These individual differences are governed by experience-based factors:

(1)       Experience-based factors

            a.         Age of second language (L2) acquisition

            b.         Length of L2 immersion

            c. Frequency of L2 use in community settings

            d.         Years actively using the L2

            Rothman presents fMRI findings from DeLuca et al. (2019), which suggests that the different factors in (1) independently predict neural plasticity among bilingual speakers. That is, when and how a bilingual engages with an additional language are shown to have different impact on the structure of the brain. For example, a later (1a) age of L2 acquisition predicts a more structurally organized corpus callosum. This reflects a more efficient communication between the two hemispheres. A longer (1b) L2 immersion predicts contraction in the bilateral thalamus, bilateral nucleus accumbens, and right caudate. Rothman interprets this as increased automation in language control. These structures are recruited less because language selection has become more efficient with prolonged exposure. By contrast, a higher (1c) frequency of L2 use in community settings predicts expansion in the same structures. This is because more frequent language switching in social context requires heavier recruitment of brain structures, which results in their expansion. Greater number of (1d) years actively using the L2 predicts expansion in the left nucleus accumbens. Rothman interprets this expansion as the language faculty becoming more finely tuned to selecting competing linguistic options across sustained L2 usage.

            Importantly, each experience-based factors in (1) independently influences the brain structure of each bilingual speaker. This leads Rothman to critique the categorical view of bilingualism that ignores these nuances. Under the categorical view, a speaker either is or is not a bilingual. This collapses important differences between bilinguals as they may differ on every factor in (1), which predicts different neural plasticity in the brain. Rothman proposes that bilingualism should instead be understood along a multidimensional spectrum, defined by speakers’ individual combination of experience-based factors.

            While the neuroanatomical differences across bilingual speakers are real in the physiological sense, does it correspond to different grammatical competence? For instance, a speaker with less automatized language control may still command the same grammar. What differs is how consistently that grammar is deployed under real-time processing pressure. Whether the spectrum of experience-based factors maps onto a corresponding spectrum of grammatical competence remains an open empirical question.

Reference

DeLuca, Vincent, Jason Rothman, Ellen Bialystok, and Christos Pliatsikas (2019) Redefining Bilingualism as a Spectrum of Experience that Differentially Affects Brain Structure and Function. PNAS, vol.116(15): 7565-7574.