Unravelling subcortical linguistic substrates: Comparison of thalamic versus cerebellar cognitive-linguistic regulation mechanisms

Background: The basal ganglia and cerebellum traditionally constitute motor-dedicated neural systems that facilitate movement via basal ganglia-thalamo-cortical and cerebellar-ponto-thalamo-cortical pathways. Lesions of the basal ganglia typically result in poverty of movement (e.g., akinesia) or excessive movement (e.g., hyperkinesia), and lesions of the cerebellum result in incoordination of movement (e.g., ataxia). However, recent research has revealed that in addition to the primary motor cortex, the basal ganglia and cerebellum also demonstrate projection zones to the premotor cortex, frontal eye field, and prefrontal, inferotemporal, and posterior parietal cortices, suggesting a supplementary role for these structures (i.e., basal ganglia and cerebellum) in the regulation of cognitive processes. Subcortical cognitive deficits paralleling motor impairments such as difficulty in shifting attentional set and difficulty in coordinating mental activity have been described in the neuropsychological literature. However, correlates relative to "hypolinguistic" or dysmetric language processes are yet to be adequately defined. Aims: The aim of the current research, therefore, was to compare and contrast the language profiles of individuals with thalamic versus cerebellar lesions. Methods & procedures: Two cases with surgically induced lesions of the left ventral intermediate thalamus, and two cases with spontaneous vascular infarcts in the left cerebellar hemisphere, served as experimental subjects in this research. General and high-level language profiles were compiled for each subject and performance compared to a group of 16 non-neurologically impaired controls (NC). The criterion for anomalous performance was established as ≥ 2 SD below the NC group mean. Outcomes & results: Evident deficits were largely restricted to complex language functions, irrespective of lesion type. Of note, however, was that thalamic lesions were associated with a higher overall proportion of significantly reduced test scores, as well as greater magnitudes of decline from normal when compared to cerebellar lesions. Conclusions: These results lend support to working theories of subcortical participation in language that promote a superordinate role for the thalamus in the regulation of higher-level lexical-semantic processes. Furthermore, the present findings also highlight the need to consider a novel role for the left cerebellar hemisphere in coordinating cognitive-linguistic systems.