rTMS evidence of different delay and decision processes in a fronto-parietal neuronal network activated during spatial working memory

The existence of a specific and widely distributed network for spatial working memory (WM) in humans, involving the posterior parietal cortex and the prefrontal cortex, is supported by a number of neuroimaging studies. We used a repetitive transcranial magnetic stimulation (rTMS) approach to investigate the temporal dynamics and the reciprocal interactions of the different areas of the parieto-frontal network in normal subjects performing a spatial WM task, with the aim to compare neural activity of the different areas in the delay and decision phases of the task. Trains of rTMS at 25 Hz were delivered over the posterior parietal cortex (PPC), the premotor cortex (SFG) and the dorsolateral prefrontal cortex (DLPFC) of the right hemisphere alternatively during the two phases. We found a pattern of interference of TMS during the delay phase for both parietal and DLPFC sites of stimulation, with no effect observed for the SFG site. When rTMS trains were applied during the decision phase, an interference was observed selectively for DLPFC. The present study shows the existence of a parallel processing in the parieto-frontal network of spatial WM during the delay phase. Furthermore, it provides new evidence of the critical role of the DLPFC during both the delay and the decision phases. We suggest that in DLPFC, two different networks coexist: A local neural network subserving the decisional processes and a second neural population functionally interconnected with the PPC and activated when a certain spatial information has to be kept in memory, available to use.