Classical attempts to capture the nature of aphasia have been “corticocentric” in identifying language processes. Recent conceptual and technical advances force us to reconsider the neural bases of language. In this work we aim to review several studies of damaged subcortical structures (thalamus and basal ganglia) that show large effects in language performance.
Different sources of evidence have shown the close relationship between damage to the thalamus, and the basal ganglia, and language deficits. The observation of acquired lesions such as cerebro-vascular accidents and traumatic brain injuries, degenerative processes such as dementia, and treatment studies for aphasia are starting to bring some insight into the contribution of these subcortical structures to the language network (Damasio et al., 1982; Weiller et al., 1993; Luzzatti et al., 2006). However, so far, scattered information and different methodological approaches make it difficult to build up a clear picture.
After our exploration of the relationship between aphasia and lesion site when this affects the thalamus and the basal ganglia and the comparison of language profile of both damaged structures we found that the incidence of aphasia as a consequence of lesions affecting subcortical portions is similar to that of cortical insult. We feel that this has important consequences for both assessment and recovery. The review also exposes the need to add association tracts to the discussion.
The literature review conducted on PubMed and Medline included the key words ‘aphasia’ AND ‘lesion’ AND (‘thalamus’ OR ‘basal ganglia’). After a first search, which retrieved 159 results (80 on thalamus, 79 on basal ganglia), we applied the following inclusion criteria: articles referring to adult acquired aphasia (rather than congenital deficits) written in English. A total of 43 studies were finally included (18 referring to the thalamus, 25 to the basal ganglia). Two were literature reviews and 41 reported the results of either case or group studies in typologically different languages, including among others Chinese, Dutch, English, German, Japanese, Portuguese, Serbian, and Turkish. A final set of data of 682 individuals (288 with thalamus lesion and 394 with basal ganglia lesion) was classified according to the presence/absence of assessable language deficits. When possible, subcortical damage was further specified.
Taken together, the results indicate that aphasia is a common outcome after a lesion to subcortical structures. Findings show that 110 out of 394 aphasic patients with lesion in the basal ganglia exhibited comprehension deficits, while 31 participants out of 288 with thalamic aphasia. Likewise, 129 aphasics of affected basal ganglia out of 394 had impaired naming, whereas 12 participants had impaired naming out of 288 individuals with thalamic aphasia. See figure 1.
Figure 1: The percentage of language impairment in two sets of aphasic patients (the thalamus and the basal ganglia).
Despite contradictory results and even cases of double dissociation (for an example of absence of language deficits in the event of thalamic lesions see Cappa et al., 1986), our literature review confirms the major role of subcortical structures in language processing.