Several studies have examined effects of tDCS as a noninvasive technique on balance and postural control in the elderly [55]. Since these effects vary depending on a number of factors including where stimulation is applied in the brain [56], number of training sessions [57], and intensity of the current applied [58], it is difficult to make a general conclusion about the extent to which tDCS can influence balance and postural control, and thus further studies are needed in this area.
To extend the previous studies and shed more light on their findings, the present study examined the effects of balance exercise combined with tDCS on balance performance and working memory in the elderly. We hypothesized that participation in balance exercise + tDCS sessions can positively influence working memory and balance performance in the elderly compared to a sham group. Our findings showed that 16 sessions of balance exercise combined with anodal tDCS on M1 in the left hemisphere of the brain of the physically unimpaired elderly can improve balance performance immediately following the intervention and even one month after the intervention is finished. However, the findings failed to demonstrate positive effects of tDCS + balance exercise on working memory in the elderly.
Since the intervention group outperformed the sham group in terms of balance performance, our findings are largely consistent with major previous studies in this area [5, 18, 33, 34, 36, 37, 59, 60]. For example, Costa et al. [18] showed that an exercise program combined with tDCS can positively influence performance capacity of the elderly participants 24 hours following the intervention or even 30 days after the intervention is complete. In another study, Fujiyama et al. [60] observed significant improvements in motor skills associated with the upper limbs in the elderly who carried out isometric strength training and received tDCS compared to the control group.
In the present study, we examined balance using two common methods: Berg Balance Scale (BBS) and timed up-and-go test (TUG). Similar results were reported in the literature using BBS. For example, Ehasni et al. [59] showed that a tDCS-based training session applied to the cerebral cortex can improve balance performance in the elderly. A major difference between our study and Ehsani et al. [59] consist in the stimulated region in addition to the number of sessions. Here, we examined how 16 tDCS sessions combined with balance exercise influenced balance while Ehsani et al. [59] only used one session. Therefore, our findings present a set of more reliable results.
Anodal tDCS-based training can cause shifts in neurotransmitters of this region by increasing local concentration of glutamate and glutamine where tDCS is applied, thereby enhancing brain activity which, in turn, can eventually improve motor performance [2]. In addition, experimental evidence has shown that anodal tDCS can improve motor performance and learning by increasing excitability of the motor cortex, leading to amplified stimulation and engaging a greater number of motor units [61].
In other words, when excitability of M1 region in the primary motor cortex is increased through tDCS interventions, it is possible that supraspinal fatigue is delayed due to increased M1 output and downward shifts, and this in turn could enhance motor performance [19]. Furthermore, tDCS can influence the activity of the insular cortex, thereby reducing rate of perceived exertion (RPE) in participants. This can also improve motor performance in individuals [62]. Moreover, anodal tDCS can also amplify active muscle outputs by increasing M1 excitability, facilitating supraspinal stimulation, and reducing inhibitory feedback in M1 [63, 64]. Although we did not measure the motor cortex excitability in the current research, it can be said presumably that following 16 sessions of balance exercise combined with 20 minutes of unilateral a-tDCS on M1, the participants in the present study experienced higher motor excitability and engaged in a greater number of motor units in balance tests to outperform the sham group. This improved balance performance may also be caused by reduced number of inhibitory feedbacks in M1. Based on the studies described above, improved balance performance in the tDCS group can be attributed to increased activity of the insular cortex and lower levels of perceived exertion during training sessions. However, one limitation of our study that future studies should address is that we did not record this perceived exertion using standard checklists during training sessions. Another limitation of the current research was the use of clinical tests such as the Berg balance test to measure balance performance. Some researchers have shown that Berg balance test is not a suitable test for predicting falls in the older adults [65, 66]. Therefore, it is suggested that future researches use more accurate and reliable tools such as the force plate to measure balance performance.
In line with some previous research [67] our findings failed to indicate tDCS effects on working memory, although there are reports of these effects in the existing literature. For example, Berryhill & Jones [68] showed that three a-tDCS training sessions on the left and right premotor cortices (F3 and F4) can improve working memory in the elderly with higher levels of academic educations although they found no improvement in the elderly with lower education levels. Ineffectiveness of tDCS in improving our participants’ working memory may be associated with the fact that they had low levels of academic education.
On the other hand, studies that demonstrated positive effects of tDCS on working memory [31, 69] assumed that tDCS enhances excitability in the outer anterior prefrontal cortex, probably due to increased levels of glutamate, an amino acid associated with working memory, recognition, and learning how to respond to a stimulus [70]. Another probable reason behind ineffectiveness of tDCS in improving working memory of our participants is the fact that the outer anterior prefrontal cortex was not stimulated here. Thus, future studies can re-examine these effects by including the factors noted above.
In our study, the active electrode was placed on M1 in the left hemisphere of the brain to examine effects of unilateral tDCS. The technique was similar to those reported in the literature [71, 72]. The left hemisphere appears to play a more important role in motor control and balance performance [73]. However, it is recommended that future studies should examine the other hemisphere as well or examine how bilateral tDCS influences balance performance and working memory. We applied 2mA anodal tDCS, but since the outcome may vary depending on current and type of stimulation (anodal vs. cathodal) [55], future studies may examine tDCS effects by manipulating these two variables. Of the different cognitive functions associated with motor performance, we only examined working memory using the n-back task. Therefore, we recommend future studies to assess working memory using other available standard tests while noting other cognitive functions such as selective attention and cognitive flexibility. As we only examined the elderly, our findings cannot be generalized to other age groups including adults, children, or the middle-aged. Thus, further studies can be conducted to examine the effects of unilateral a-tDCS in M1 on balance performance and other cognitive functions in adults, children, or the middle-aged. Nevertheless, our findings indicated that the effect of the within-group was significant, it can be concluded that sixteen sessions of balance exercises, regardless of the presence or absence of tDCS, had a positive effect on improving working memory. These results are in line with the findings in this field that have shown physical exercise, especially balance practice improve elderly’s working memory [74, 75]. For example, Azhdar et al [74] showed that a balance exercises intervention can increase cognitive performance and especially working memory in the elderly. In the present research, both the intervention and sham groups were able to increase the working memory of the elderly, since the balance exercise was applied to the same extent in both groups, so it can be concluded that the balance exercises has been able to improve the cognitive function and especially the working memory of the elderly. Of course, since in the current study, we did not have a control group (a group that does not have balance exercise), so this conclusion should be used with caution and it is suggested that future studies add a control group to the research for more clarification. In addition, the present study used balance exercises, where the authors suppressed a single sensory pathway, such as the visual. However, a specific balance program should mix multiple sensory pathways such as the visual, vestibular, and somatosensory systems. Further studies are warranted to investigate which type of balance training is most effective to improve balance when combined with anodal tDCS.