Five-year-old children engage temporal but not frontal mechanisms during morphosyntactic processing of sentences.

The processing of morphemes, the smallest linguistic units that create differences in meaning, can have cascading effects on language and literacy development. The neural underpinnings of morphological processing have mostly been examined in school-aged children. We examined if phonological and semantic mechanisms play a role during morphosyntactic processing in auditory sentences in five-year-old children using functional MRI. We focused on morphemes expressing finiteness in verbs, which are morphemes used to express subject-verb agreement. We employed in-scanner sound and meaning tasks and combined these with behavioral data to create functional localizers in the dorsal (inferior frontal gyrus-pars opercularis and posterior superior temporal gyrus) and ventral (inferior frontal gyrus-pars triangular and posterior middle temporal gyrus) pathway for phonological and semantic processing respectively. Both temporal regions of interest were significantly activated while the frontal regions of interest were not. The cluster in the superior temporal gyrus was relatively larger in size than that in the middle temporal gyrus. We did not find relationships with language skill. We conclude that five-year-old children rely on temporal but not frontal mechanisms during morphosyntactic processing of finiteness-related morphemes in auditory sentences. The relatively stronger involvement of the posterior superior temporal gyrus suggests that processing of these morphosyntactic markers may rely more on phonological-form based mechanisms than semantic mechanisms at this age. These results are in line with previous work showing that language specialization is present in the temporal lobe in five-year-old children while a frontal language network has not yet been established at this age.