Cognition
The Temporal Lobe and Language Development
Presence of the temporal lobe may be needed for frontal language development.
Posted July 6, 2021 Reviewed by Devon Frye
The human language system appears to recruit a broad network of frontal and temporal brain regions. Even though different aspects of language processing have by now been confidently apportioned to very narrow portions of the frontal and temporal lobes, how this network emerges during childhood development remains much less clear.
One major obstacle to addressing this issue is the obvious difficulty in directly probing the functional organization of the brain between the ages of 1-4 years of age. By the age of 4-6 years, the language network seems to be organized in a highly similar manner to the adult brain, despite more bilateral activation in children. The frontal cortex exhibits protracted development, which some have taken to suggest that frontal language areas reach maturity later than temporal areas.
Other researchers have documented certain developing properties of brain dynamics in children and through adolescence, but more fundamental questions, like whether temporal areas are necessary for the development of language-relevant frontal areas, remain unanswered.
A team at MIT recently tried to address these fundamental questions. In a new manuscript written by Greta Tuckute and colleagues (not yet certified by peer review), the team reports a unique case study of an individual (EG) born without a left temporal lobe, likely as a result of pre-/perinatal stroke.
EG appears to have a fully functioning language network in her right hemisphere, performing normally on standardized language assessments. Yet no reliable response to language (probed via contrasting fMRI responses to sentences vs. pronounceable pseudowords) is detected on the lateral surface of EG’s left frontal lobe. The team also tested brain responses to domain-general multiple demand network engagement (probed via an arithmetic addition task), finding reliable activation across right and left frontal lobes.
The team concluded from this that the presence of temporal language areas is a prerequisite for the emergence of the language areas in the frontal cortex. More specifically, they argue that frontal areas emerge through intra-hemispheric fronto-temporal pathways (potentially via the earlier-developing ventral pathway), such that temporal areas “set up” frontal language areas.
Since EG exhibits right-hemispheric language activation, it seems that frontal inter-hemispheric connectivity (i.e., between left and right frontal cortex) is insufficient to map higher-order language computations onto the left frontal cortex. Interestingly, EG also self-reported that she is a “terrible speller,” potentially reinforcing the role of traditional ventral temporal areas, and temporo-parietal pathways, in a range of orthographic computations.
In principle, this research may validate the importance of temporal areas as the seat of core syntactic computations, and frontal sites as providing a higher-order memory, attention, and control center. The team plans to further investigate EG’s left frontal lobe to develop a more specific computational profile of her unique cortical architecture.