Paired-pulse transcranial magnetic stimulation over the dorsolateral prefrontal cortex interferes with episodic encoding and retrieval for both verbal and non-verbal materials

Neuroimaging studies report that encoding and retrieval processes of episodic memory constantly activate the left and right prefrontal cortex, even in the simplest memory tasks. Conversely, individuals with frontal lesions displayed impaired memory performance mainly in the most complex memory tasks. This discrepancy regarding the involvement of dorsolateral prefrontal cortex (DLPFC) may be explained by methodological divergences. Our goal was to study the critical role of the DLPFC in episodic encoding and retrieval processes according to the nature of the material (verbal or non-verbal) by using transcranial magnetic stimulation (TMS). Eighteen healthy adults performed a recognition task with words or random shapes. Paired-pulse TMS was applied (3-ms inter-pulse interval, at 90% of motor threshold) for inducing transient and safe interferences over the left or right DLPFCs during encoding or retrieval. Data analysis showed a reduced discrimination rate following TMS over the left DLPFC during encoding, as compared to a sham condition. Both hit and discrimination rates were also reduced after TMS over the right DLPFC, compared to TMS over left DLPFC, during retrieval. There was no difference between verbal and non-verbal material. Our results showed that the left and the right DLPFC play a critical role respectively in episodic encoding and retrieval processes for both verbal and non-verbal materials. Our findings are discussed in light of the findings obtained from other complementary approaches.