Linking ordering in Broca's area to storage in left temporo-parietal regions: the case of sentence processing

In sentence processing, storage and ordering of the verb and its arguments (subject and object) are core tasks. Their cortical representation is a matter of ongoing debate, and it is unclear whether prefrontal activations in neuroimaging studies on sentence processing reflect the storage of arguments or their ordering. Moreover, it is unclear how storage during sentence processing relates to the neuroanatomy of storage outside the sentence processing domain. To tackle these questions, we crossed the factor "ordering" (subject-first vs. object-first German sentences) with the factor "storage" (one vs. four phrases intervene between the critical argument and the verb) in an auditory fMRI study. Ordering focally activated the left pars opercularis in Broca's area, while storage activated deep left temporo-parietal (TP) regions. Notably, left TP activation correlated with listener's digit span, while Broca's area activation did not. Furthermore, fractional anisotropy of listeners' left arcuate fasciculus/superior longitudinal fasciculus (AF/SLF) is shown to covary with the functional effect of increased storage demands at sites along the tract. Functionally, the results suggest that storage during sentence processing relies on TP regions, likely shared between sentence processing and other working memory-related tasks, while Broca's area appears as a distinct neural correlate of ordering. We conclude that the abstract notion of sentence processing can be captured by the interplay of concrete cognitive concepts such as ordering and storage.