TMS-task synchronization
To select the synchronization time between the spTMS and the phase of the task, we relied on Hebbian principles that define the change in synaptic force as a function of co-activation (Hebb, 1949). When the items were presented to the subjects, they had to memorize them in order and space; thus, we assumed that the application of a TMS pulse at that time would be paired with the endogenous activation of the neurons responsible for that codification and retention. To confirm or refuse this hypothesis, further research with respect to the optimal Inter Stimulus Interval (ISI) between TMS and endogenous activity during WM tasks is needed.The difference between the presented combined-stimuli methodology that pairs TMS (an exogenous stimulus) with hippocampal activity when performing a memory task (endogenous stimulus), with the original PAS methodology is the nature of the second stimulus. The memory task that evokes activity in the hippocampus provides a way to associate its activity to TMS, which otherwise would not be possible. However, only when the effect of the ISI factor will be researched, could be confirmed that the repeated combination of activities (exogenous and endogenous) induces potentiation by STDP induction. If this was not the case, then the mechanisms induced by this combined-stimulation should be investigated. In addition, our results showed that this combination is effective depending on the basal level of memorization capacity. As previously discussed, the baseline score reflects the level of functionality of the subject’s neuronal pathways. Hence, the synaptic strength of the neuronal pathway at a specific level of functionality would be sensitive to this combined-stimuli intervention and would dictate which areas would be the best targets for TMS synchronization.