Subcortical mechanisms in language: Lexical-semantic mechanisms and the thalamus

Four previously published cases of dominant thalamic lesion in which the au thor has participated are reviewed to gain a better understanding of thalam ic participation in lexical-semantic functions. Naming deficits in two case s support Nadeau and Crosson's (1997) hypothesis of a selective engagement mechanism involving the frontal Lobes, inferior thalamic peduncle, nucleus reticularis, and other thalamic nuclei, possibly the centromedian nucleus. This mechanism selectively engages those cortical areas required to perform a cognitive task, while maintaining other areas in a state of relative dis engagement. Deficits in selective engagement disproportionately affect lexi cal retrieval based on semantic input, as opposed to lexical and sublexical processes, because the former is more dependent upon this attentional syst em. The concept of selective engagement is also useful in understanding tha lamic participation in working memory, as supported by data from one recent functional neuroimaging study. Other processes also may be compromised in more posterior thalamic lesions which damage the pulvinar but not other com ponents of this selective engagement system. A third case with aphasia afte r a more superior and posterior thalamic lesion also had oral reading error s similar to those in neglect dyslexia. The pattern of deficits suggested a visual processing problem in the early stages of reading. The fourth case had a category-specific naming deficit after posterior thalamic lesion. Tak en together, the latter two cases indicate that the nature of language func tions in more posterior regions of the dominant thalamus depends upon the c ortical connectivity of the thalamic region. Together, findings from the fo ur cases suggest that thalamic nuclei and systems are involved in multiple processes which directly or indirectly support cortical language functions, (C) 1999 Academic Press.