The neural bases of word encoding and retrieval: a fMRI-guided transcranial magnetic stimulation study

There is evidence that the human prefrontal cortex is asymmetrically involved in long-term episodic memory processing. Moreover, abstract and concrete words processing has been reported to differentially involve prefrontal and parietal areas. We implemented a two-stages functional magnetic resonance imaging (fMRI)-repetitive transcranial magnetic stimulation (rTMS) paradigm to investigate the role of the dorsolateral prefrontal cortices (DLPFCs) and parietal cortices (PARCs) in encoding and retrieval of abstract and concrete words. Using this paradigm we could select areas to be stimulated on the basis of single-subject (SS) anatomical and functional data, investigating the usefulness of this integration approach. With respect to fMRI, abstract and concrete words differed only for a greater left fusiform gyrus activation for concrete words. In turn, significant rTMS effects were found, but only for the retrieval of abstract words. Consistent with previous findings, repetitive stimulation of the right DLPFC had a specific impact on episodic retrieval. Memory retrieval performance was also disrupted when rTMS was applied to the left PARC. Finally, we found a significant positive correlation between the effect sizes of SS right PARC activations for abstract word retrieval and the consequent rTMS interference effects. Taken together these data provide for the first time evidence that also the PARC has a necessary role in episodic retrieval of abstract words. Importantly, from a methodological perspective, our data demonstrate that fMRI-guided rTMS with a SS approach provides a powerful tool to investigate the neural underpinnings of cognitive functions.