The neural basis of language development: Changes in lateralization over age

Abstract

We have long known that language is lateralized to the left hemisphere (LH) in most neurologically healthy adults. In contrast, findings on lateralization of function during development are more complex. As in adults, anatomical, electrophysiological, and neuroimaging studies in infants and children indicate LH lateralization for language. However, in very young children, lesions to either hemisphere are equally likely to result in language deficits, suggesting that language is distributed symmetrically early in life. We address this apparent contradiction by examining patterns of functional MRI (fMRI) language activation in children (ages 4 through 13) and adults (ages 18 through 29). In contrast to previous studies, we focus not on lateralization per se but rather on patterns of left-hemisphere (LH) and right-hemisphere (RH) activation across individual participants over age. Our analyses show significant activation not only in the LH language network but also in their RH homologs in all of the youngest children (ages 4 through 6). The proportion of participants showing significant RH activation decreases over age, with over 60% of adults lacking any significant RH activation. A whole-brain correlation analysis revealed an age-related decrease in language activation only in the RH homolog of Broca’s area. This correlation was independent of task difficulty. We conclude that, while language is left-lateralized throughout life, the RH contribution to language processing is also strong early in life and decreases through childhood. Importantly, this early RH language activation may represent a developmental mechanism for recovery following early LH injury.

Functional organisation for verb generation in children with developmental language disorder

“…The whole-brain averages of task-evoked activity discussed above may not fully reflect inter-subject variability (see Olulade et al., 2020 for a discussion of similar issues), which we expected might be greater in those with DLD. To visualise consistency in patterns of activation across TD and DLD, we generated probabilistic overlap maps.…”

Lateralized functional responses in the cortex arise from the dynamic interactions in the structural connectome

“…Finally, by simulating auditory nodes as externally driven functional units in a whole-brain network of Kuramoto oscillators (Kuramoto, 1984), we could generate the functional lateralization of ASSR. Second, when exchanging identities of areas that receive direct environmental input, e.g., replacing auditory areas that receive tonal cues with Broca's area receiving speech signals, we could predict the left hemispheric dominance of functional brain responses as reported by several language studies (Szaflarski et al, 2006;Riès et al, 2016;Olulade et al, 2020). Interestingly, our model also propose time-delays in coupling can affect the lateralization, e.g., allow transitions of left to right lateralization (Figure 4), which can provide important insights on why lateralization shifts are observed during ageing (Olulade et al, 2020) that is often associated with neurophysiological time-delays.…”

“…Second, when exchanging identities of areas that receive direct environmental input, e.g., replacing auditory areas that receive tonal cues with Broca's area receiving speech signals, we could predict the left hemispheric dominance of functional brain responses as reported by several language studies (Szaflarski et al, 2006;Riès et al, 2016;Olulade et al, 2020). Interestingly, our model also propose time-delays in coupling can affect the lateralization, e.g., allow transitions of left to right lateralization (Figure 4), which can provide important insights on why lateralization shifts are observed during ageing (Olulade et al, 2020) that is often associated with neurophysiological time-delays. Thus, two levels of validationprediction of right hemispheric dominance during generation of ASSR and left hemispheric dominance of language qualifies this model as a canonical model for brain lateralization, which remains elusive to date to the best of our knowledge.…”

“…While left hemispheric dominance for language processing is considered sacrosanct (Olulade et al, 2020), compensating right-hemispheric analog to Broca's area is associated with recovery from stroke (Xing et al, 2016) can be understood with our model by making appropriate changes in the parameter space.…”

Spoken language comprehension activates the primary visual cortex

“…Aside from a fixation cross that participants were asked to rest their eyes on throughout the scan, no visual stimulation was provided. The Forward>Reverse activation differences evoked by this task are highly robust and reproducible, making them suitable for localizing language-associated brain areas across development (Olulade et al, 2020) and even in cases of atypical functional organization, such as participants with a history of chronic epilepsy (Berl et al, 2014) or perinatal stroke (Newport et al, 2017). Imaging data were acquired on Georgetown’s research-dedicated 3T Siemens Trio Tim scanner with a 12-channel birdcage head coil.…”