Future considerations and study limitations
Cognitive Impairment, MCI
The participants in this study were between 19 and 43 years of age, which can be considered a population of young adults. We propose that future studies of this new combined-stimuli protocol for the improvement of memorization seek the feasibility of its application in elderly or Mild Cognitive Impairment (MCI) populations. WM is a fundamental ability for daily living and tends to decline significantly with age [The limitations of this combined-stimuli intervention study are the following. First, direct probing of TMS-triggered activation at the hippocampal level is not yet possible due to the lack of directly measurable TMS effects, such as MEPs for M1. Although we assumed that using a TMS intensity level capable of stimulating the foot M1 would stimulate the hippocampus (at least the most proximal portion), TMS stimulation in deeper locations may be more diffuse. Therefore, further research on TMS activation of the hippocampus and surrounding areas in the temporal lobe is necessary to ascertain whether specific hippocampal activation or stimulation of other temporal lobe areas leads to memory potentiation.Second, the effect of ISI on TMS and endogenous memory activity in our design has not been systematically analyzed. In our experiments, the beginning of the coding phase in the WM task and the TMS pulse were synchronized. Unveiling whether our results on WM task outcome facilitation correspond to a Hebbian mechanism, and therefore likely triggering STDP-like plasticity, would require a systematic investigation of the ISI variable.Regarding methodological limitations, although the hippocampal-oriented stimulation in the HIP group has been supported by neuronavigation, the MRI of the 96 subjects who participated in the study were not available. Thus, as explained in Material and methods section, the neuronavigation in HIP group was conducted with the aim of a universal brain map of 152 MRIs (MNI152). In the future, this stimulation protocol could be improved with neuronavigation with individual MRIs to avoid variability. Although the hippocampus is a relatively large structure, it is also deep, which makes its stimulation less precise. Therefore, it is necessary to use all possible techniques or adjustments to ensure that stimulation is as precise as possible.