## 4 Discussion
### Corticospinal Excitability Effects  
We did not observe significant CSE changes among the three interventions at any time of the assessment. However, a time of assessment effect was highlighted, and changes were observed when individual interventions were then considered. Indeed, higher MEP values were measured postintervention, revealing the potential of a single 15 min stimulation intervention to induce significant CSE changes poststroke. Additionally, larger, gradual, and less variable CSE increases were observed after ShamPAS/MI (Figure 2). Only few studies have investigated CSE changes related to mental training [42]. A CSE increase has been described in the literature after MI training, alone or in combination with other rehabilitation interventions, in healthy [25; 36] and in stroke participants [43]. Better recovery of hand function and higher amplitude of the APB MEP were observed in stroke patients who underwent mental training associated with traditional rehabilitation compared to patient groups with traditional rehabilitation alone [43]. In a recent study, CSE changes persisted beyond the imagination period and were still measurable 30 min postintervention [44], which is in keeping with our findings in a stroke population. The observed CSE increase (in \(\approx\) 60% of the participants) might be due to the progressive decrease in the corticospinal threshold and/or the increase in motor neuron recruitment during the course of MI training [45]. This CSE modulation induced by the ShamPAS/MI seems less subject to variability, as compared to the sessions where real PAS was also administered. The observed effects might, however, not be due to the effects of MI alone, given the design of the ShamPAS intervention in _MIPAS_ and the presence of the muscle ES.  
The absence of a significant effect of PAS alone on CSE may be explained by several factors. Firstly, a recent paper [46] showed that the ability to induce plastic changes in motor cortical output after stimulation requires the integration of sensory input, as in the PAS paradigm, which follows a certain gradient in the human brain. Potentially, the participants included in _MIPAS_ had additional sensory impairments, preventing the PAS-induced LTP-like plasticity. Additionally, intrinsic hand flexor muscles seem to have a higher potential to show plasticity than extensor muscles in response to PAS, and the induced plastic changes in extensors are among the most difficult to highlight [46]. Moreover, the observed results confirm once again that the intra- and interindividual variability in response to PAS is significant and is extensively described in the literature [47]. This variability is thought to be due to numerous factors among which are age, gender, level of attention, baseline CSE, and stimulation frequency [9; 10; 18; 19; 48]. Even though we tried to control most of these factors, we observed a variability in CSE changes in our population ranging from \(-\)57.1 to \(+\)258.8%. Only 45% of the patients could be considered as responders, and only 4% had facilitation greater than 30%, which is a low rate compared to similar studies [10]. This might be due to the stimulation frequency used during PAS, which was higher in _MIPAS_ (0.2 Hz vs. 0.1 Hz in other protocols [40; 49]) in an effort to reduce the experimental time and participants' mental fatigue.