The main finding of this double-blind sham-controlled cross-over study was that a 20 min period of noninvasive anodal tDCS applied to M1affected hemisphere resulted in transient improvements in maximal pinch force and reaction times of the paretic hand relative to sham in a group of chronic stroke patients in the absence of measurable nonspecific changes in attention, discomfort or fatigue. These effects were documented in every single patient tested with RT and in all but one with PF. All patients were blind to the intervention type as were investigators testing the endpoint measures.
Previous reports using tDCS and rTMS showed improvements in performance of tasks that mimic activities of daily living performed with the paretic hand after noninvasive cortical stimulation of M1affected hemisphere
]. Performance of these tasks, like the Jebsen-Taylor test, require complex visuomotor integration and skilled coordination of force production as well as proper management of attentional and motivational resources [8
]. This study, designed to determine if anodal tDCS over M1affected hemisphere
exerted comparable effects over simpler motor tasks, identified improvements in pinch force and reaction times in the paretic hand with this intervention but not with Sham or No stimulation. These effects suggest that anodal tDCS to M1affected hemisphere
enhanced activity within the primary motor cortex [8
], actively engaged in control of force production and simple visuomotor integration processes [17
The lack of effects on attention, fatigue and discomfort evidenced by the analysis of visual analog scales, as well as the inability of patients to identify the stimulation and Sham sessions are consistent with results from previous investigations [8
]. Furthermore, tDCS was applied over the primary motor cortex, distant from areas involved in modulating attentional and motivational processes (e.g., cingulate cortex, prefrontal cortex, limbic system) [34
]. Interestingly, repetition of both tasks after Sham resulted in a trend towards longer reaction times and weaker pinch force. The finding of this effect in the control experiment with both Sham (30 sec tDCS) and No stimulation (no tDCS) indicates that it was unrelated with the application of 30 sec tDCS during Sham. Most likely, it reflects mild experimental fatigue, subthreshold for detection by the VAS questionnaires [8
Recovery of motor function after stroke starts first with improvements in performance of repetitive, relatively simple motor activities like force production, evolving later to relearning of more complex motor synergies and skillful tasks. It is a common finding that while most chronic stroke patients are able to generate various levels of force, only a fraction of them are able to perform skilled motor tasks like those involved in activities of daily living tested with the Jebsen-Taylor test [36
]. Since the ability to control properly force production and visuomotor integration represents a prerequisite to meaningful skilled complex motor activity [37
], it can be speculated that a first rehabilitative step to promote functional recovery after stroke might focus on reacquisition of these motor primaries. Subsequent training could then utilize these basic skills to train more complex actions, such as proposed in sports and musical practice programs [40
]. In this way, motor training of these primaries could be utilized to orchestrate more complex skillful motor tasks [41
]. The finding that anodal tDCS applied to M1affected hemisphere
improved force production and a simple visuomotor integration task relative to Sham suggests that these mechanisms could contribute to the behavioral gains reported by anodal tDCS of M1affected hemisphere
on more complex activities of daily living [8
]. Of note is that these improvements were present in patients with higher impairment levels unable to perform skilled ADL-like motor tasks (Fig ).
In summary, the present study provides novel evidence that anodal tDCS of M1affected hemisphere may enhance performance of a wider range of motor tasks than previously thought, some of them relatively simple and mediated predominantly by M1 function.