The role of the striatum in sentence processing: evidence from a priming study in early stages of Huntington's disease.

The role of sub-cortical structures such as the striatum in language remains a controversial issue. Based on linguistic claims that language processing implies both recovery of lexical information and application of combinatorial rules it has been shown that striatal damaged patients have difficulties applying conjugation rules while lexical recovery of irregular forms is broadly spared (e.g., Ullman, M. T., Corkin, S., Coppola, M., Hickok, G., Growdon, J. H., Koroshetz, W. J., et al. (1997). A neural dissociation within language: Evidence that the mental dictionary is part of declarative memory, and that grammatical rules are processed by the procedural system. Journal of Cognitive Neuroscience, 9(2), 266-276). Here we bolstered the striatum-rule hypothesis by investigating lexical abilities and rule application at the phrasal level. Both processing aspects were assessed in a model of striatal dysfunction, namely Huntington's disease (HD). Using a semantic priming task we compared idiomatic prime sentences involving lexical access to whole phrases (e.g., "Paul has kicked the bucket") with idiom-derived sentences that contained passivation changes involving syntactic movement rules (e.g., "Paul was kicked by the bucket"), word changes (e.g., "Paul has crushed the bucket") or either. Target words that were either idiom-related (e.g., "death") reflecting lexical access to idiom meanings, word-related (e.g., "bail") reflecting lexical access to single words, or unrelated (e.g., "table"). HD patients displayed selective abnormalities with passivated sentences whereas priming was normal with idioms and sentences containing only word changes. We argue that the role of the striatum in sentence processing specifically pertains to the application of syntactic movement rules whereas it is not involved in canonical rules required for active structures or in lexical processing aspects. Our findings support the striatum-rule hypothesis but suggest that it should be refined by tracking the particular kind of language rules depending on striatal computations.