Is there a brain basis of recursion?

Abstract

The processing of recursive structures realized as embedded sentences or as sequences of nonsense syllables in which embeddedness is indicated by the relation of long-distance dependencies between certain categories has been shown to involve Broca's area, in particular the pars opercularis (BA 44), in the inferior frontal gyrus. Since the processing of embedded sentences has been claimed to depend on working memory, we varied the factors embeddedness and working memory systematically in order to clarify the interplay between these factors and their neural bases. Results indicate that the left pars opercularis activates as a function of embeddedness, whereas the left inferior frontal sulcus activates as a function of working memory, thus pointing to a segregation of the two factors. Functional connectivity analyses, furthermore, reveal that these two regions interact highly during sentence comprehension. Additional brain imaging experiments on the processing of sentences involving scrambling and movement see Broca's area (BA 44) to increase its activation parametrically with increased complexity. Thus Broca's area, in particular the pars opercularis, should be viewed as a brain region supporting the processing of hierarchically complex structures. The finding that this holds for complex hierarchically structured language sequences, both in natural and in artificial grammars, suggests that the recruitment of this brain region is crucial for the processing of complex hierarchies.