•Specific language impairment is a heterogeneous family of genetic developmental disorders that affects the acquisition of language in 7% of children.
•We have identified a subtype, Grammatical-SLI, which affects the children's syntax, morphology, and phonology in similar ways.
•Grammatical abilities are not impaired across the board: the children handle forms that are local, linear, semantic, and holistic, while stumbling on those that are nonlocal, hierarchical, abstract, and composed.
•The mosaic of impaired and spared abilities is consistent with new models of the neural bases of syntax, morphology, and phonology which distinguish several dorsal and ventral language pathways in the brain.
•We foresee substantial progress in the biology of language – evolution, genetics, neurobiology, computation, behavior – if language and language impairments are differentiated into underlying pathways and components.
Specific language impairment (SLI), a genetic developmental disorder, offers insights into the neurobiological and computational organization of language. A subtype, Grammatical-SLI (G-SLI), involves greater impairments in ‘extended’ grammatical representations, which are nonlocal, hierarchical, abstract, and composed, than in ‘basic’ ones, which are local, linear, semantic, and holistic. This distinction is seen in syntax, morphology, and phonology, and may be tied to abnormalities in the left hemisphere and basal ganglia, consistent with new models of the neurobiology of language which distinguish dorsal and ventral processing streams. Delineating neurolinguistic phenotypes promises a better understanding of the effects of genes on the brain circuitry underlying normal and impaired language abilities.The article contains some very useful summary graphics describing language areas and their interactions (click to enlarge):
Legend - Neural correlates of Extended and Basic syntax. Syntactic processing in the brain is implemented in distinct dorsal and ventral circuits which may correspond to Extended and Basic syntax. The dorsal route (unbroken red arrow) links Brodmann Area 44 (BA44, a part of Broca's area) via the arcuate fasciculus to the posterior superior temporal gyrus (STG, a part of Wernicke's area); this pathway has been implicated in complex syntactic processing, including hierarchical phrase structure and movement, that is, Extended syntax. The caudate nucleus of the basal ganglia (not shown), a subcortical structure, is interconnected with frontal cortex, and it has also been found to affect Extended syntax. The first of the two ventral circuits (blue arrow) links the frontal operculum (FO, the cortex inferior and medial to BA 44 and 45, mostly hidden) via the uncinate fasciculus to the anterior STG; it supports local phrase structure. The second (purple arrow) links Brodmann Areas 45 (BA45, another part of Broca's Area) and 47 via the extreme capsule fiber system to the middle portion of the superior and middle temporal lobe; it supports retrieval of stored words and associated semantic processing. The two ventral pathways, therefore, may correspond to Basic syntactic processing. Abbreviations: MTG, middle temporal gyrus; ITG, inferior temporal gyrus.
Legend - Neural correlates of Extended and Basic morphology. Regular inflectional forms (walked, played) are computed by Extended processes that closely overlap with those underlying Extended syntax, namely BA 45 extending into BA 44 and BA47, the arcuate fasciculus, and the superior and middle temporal cortex. The frontal regions are part of a circuit that also includes the caudate nucleus (not shown). In contrast, the storage and retrieval of irregular forms (‘brought’, ‘went’) appears to be mediated bilaterally (blue outline) in a more diffuse set of posterior and middle temporal lobe structures. Derived morphological forms, both regular (‘bravely’) and irregular (‘archer’), activate a third, bilateral network, which we tentatively associate with the ventral pathway, specifically, BA47 extending into BA45, and the anterior superior temporal gyrus (STG) and middle temporal gyrus (MTG) (purple lines). This network may support a network of related but whole word forms. Individuals with G-SLI are impaired in productive regular inflection, an Extended process that engages the dorsal route, but their performance is less impaired with the retrieval of irregular and derived forms, a Basic process which is more tried to lexical memory, and which engages bilateral ventral and posterior routes.
Legend - Neural correlates of Extended and Basic phonology. Phonological processing begins with spectrotemporal and segmental processing in bilateral auditory cortex (superior temporal gyrus, STG, and superior temporal sulcus, STS; right hemisphere not shown). From there it splits into two streams. A left-hemisphere dorsal stream runs to a sensorimotor integration area in the Sylvian portion of the parietal–temporal junction (SPT), and from there further bifurcates into a pathway along the superior longitudinal fasciculus to premotor areas (PM; pink arrow) and a pathway along the arcuate fasciculus to Broca's area (BA44; red arrow). These pathways connect acoustic speech representations to articulatory ones, the former perhaps to basic articulatory phonetic skills, the latter to complex syllables and words, self-monitoring speech, and verbal working memory. A bilateral ventral stream (right hemisphere portion not shown) runs from auditory cortex to the middle and inferior temporal gyri (MTG and ITG), and from there to the anterior temporal lobe, and also to a conceptual network widely distributed through the temporal and other lobes. This pathway connects the sounds of words to their meanings. We suggest that the Extended phonology which challenges G-SLI is associated with the part of the dorsal pathway that runs to Broca's area (red), but perhaps not the part that runs to premotor areas (pink), as articulation in the syndrome is relatively unimpaired. Basic phonology is associated with acoustic and phonological analysis in auditory cortex and with the ventral pathway.