Dissociating neural mechanisms of temporal sequencing and processing phonemes.
Using fMRI, we sought to determine whether the posterior, superior portion of Broca's area performs operations on phoneme segments specifically or implements processes general to sequencing discrete units. Twelve healthy volunteers performed two sequence manipulation tasks and one matching task, using strings of syllables and hummed notes. The posterior portion of Broca's area responded specifically to the sequence manipulation tasks, independent of whether the stimuli were composed of phonemes or hummed notes. In contrast, the left supramarginal gyrus was somewhat more specific to sequencing phoneme segments. These results suggest a functional dissociation of the canonical left hemisphere language regions encompassing the "phonological loop," with the left posterior inferior frontal gyrus responding not to the sound structure of language but rather to sequential operations that may underlie the ability to form words out of dissociable elements.