Academic journal article Learning Disability Quarterly

Development and Disorders of Neurocognitive Systems for Oral Language and Reading

Academic journal article Learning Disability Quarterly

Development and Disorders of Neurocognitive Systems for Oral Language and Reading

Article excerpt

Abstract. There are four goals of this article. First, a tentative neurocognitive model of oral language and reading is outlined. Second, our recent functional magnetic resonance imaging studies (fMRI) on the development of oral language and reading are briefly reviewed with reference to this neurocognitive model. Third, brain-imaging research on dyslexia is discussed in light of the neurocognitive model. Fourth, research on the plasticity of neural systems and the implication of this plasticity for studying normative development and disorders is presented.

Reading disorder is the most common form of learning problem in the United States (estimates range from 5-15%) and, therefore, represents a biomedical issue of major importance (Shaywitz, Shaywitz, Fletcher, & Escobar, 1990). Before discussing the neural basis of reading disorders, we need to review the neural representation and development of oral language and reading.

Neurocognitive Model of Oral Language and Reading

Oral language and reading differ at the most basic level in terms of the nature of perceptual input (see Figure 1). In oral language, auditory input is transmitted from the medial geniculate nucleus in the thalamus to both primary auditory areas (PA) and auditory association areas (AA), the latter of which may contain phonologic representations of words (Binder et al., 1994). In written language, visual input is transmitted from the lateral geniculate nucleus in the thalamus to primary visual areas (PV) in the striate cortex and to secondary visual areas in the extrastriate cortex (EX). From the primary visual area, information has a ventral "what" projection and a dorsal "where" projection (Mishkin, Ungerleider, & Macko, 1983). The ventral projection includes the unimodal visual area of the fusiform gyrus (FG), which may contain orthographic representations of words (Fujimaki et al., 1999; Herbster, Mintun, Nebes, & Becker, 1997; Nobre, Allison, & McCarthy, 1994). The dorsal projection includes the superior parietal lobule, which may be important in aspects of reading that involve spatial attention.

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Wernicke's area (W) and surrounding areas, including angular gyrus (AG) and supramarginal gyrus (SG), are heteromodal areas that may be responsible for the integration of spoken and written word forms with arbitrary associations that give rise to meaning or semantics (Mesulam, 1998; Pugh et al., 1996). Consistent with this role, Wernicke's area is interconnected to category-specific areas in the inferior temporal lobe that appear to contain representations of faces, animals and tools (Damasio, Grabowski, Tranel, Hichwa, & Damasio, 1996; Di Virgilio & Clarke, 1997).

Wernicke's area also has massive connections through the arcuate fasciculus with Broca's area (B), which tends to show later peaks of activation in language tasks (Thierry, Boulanouar, Kherif, Ranjeva, & Demonet, 1999). Broca's area includes brain regions for overt (Hagoort et al., 1999) and covert speech production of articulatory word forms involving segmented phonology (Friedman et al., 1998; Paulesu et al., 1996) as well as syntactic processing (Booth et al., 200lb; Caplan, Alpert, & Waters, 1998). The anterior temporal region (AT) is strongly interconnected with Broca's area through the uncinate fiber tract and is also implicated in syntactic processing (Mazoyer et al., 1993; Vorob'ev et al., 1998).

Finally, the prefrontal cortex (PF) may be responsible for modulation of processing in posterior regions such as Wernicke's area (Frith, Friston, Liddle, & Frackowiak, 1991; Raichle et al., 1994) and/or for maintaining and manipulating verbal information in memory (Gabrieli, Desmond, Demb, Wagner et al., 1996; Jonides et al., 1997). For further details on the functional anatomy of language processing, we refer the reader to chapters in a recent edited book, which presents a critical evaluation of research that is largely consistent with the model outlined above (Hagoort, Brown, & Osterhout, 1999; Price, Indefrey, & van Turennout, 1999). …

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