Chapter 4. General Discussion and conclusion
3. Limitations and future considerations
There were some limitations in the presented studies. The first concerns differences in the sensations perceived during anodal versus sham tDCS, and the potential involvement of a placebo effect. It is important to describe how we confirmed the success of our double-blind procedure. While the sensations of anodal and sham tDCS stimulation are different, participants were unaware that tDCS stimulation could vary between sessions or that we were testing two different types of stimulation (i.e. they wouldn’t know that the sham is sham, they would only know it felt different). They were informed that they might feel itching or burning, but they were not told that the presence or intensity of itching or burning was in any way correlated with the effects of the stimulation on task performance or pain perceived. Therefore, participants did not expect a placebo condition or a condition with stimulation that was more or less effective. In fact, the tDCS stimulator that was used is specifically designed to limit the placebo effect by employing the same ramp-up at the beginning of the stimulation (Deldar et al., 2019; Deldar et al., 2018). Future studies need to measure the sensation in order to confirm whether participants have similar or different sensations during anodal and sham tDCS. For example, Saruco and colleagues asked participants after sham and anodal tDCS sessions whether they received a real stimulation. Participants could answer by choosing “Yes,” “No,” or “I do not know.”
Conformity Chi-squared test was performed on the proportions of participants’ “Yes” and “No”
answers for each brain stimulation session against proportions corresponding to the chance level (50%). The conformity Chi-squared test was not statistically significant (all p > 0.05), indicating that Participants were blind about the effect of sham and anodal tDCS sessions (Saruco et al., 2017).One possibility that should be considered to explain the improvement of WM by anodal
tDCS is that the sensation produced by anodal tDCS on the scalp may increase alertness, thus leading to improvement of reaction times. However, this possibility is unlikely since the same effect should apply to the performance in the 0-back task, which was not observed.
Another limitation that should be cited is that tDCS is not a focal method; in addition to the targeted region – the DLPFC, other regions may be affected by anodal stimulation.
Therefore, as in other tDCS studies, the effects cannot be attributed definitely to the DLPFC (Woods et al., 2016a; Woods et al., 2015). However, based on several brain simulation studies, the left DLPFC is stimulated with montages similar to that used in our studies (Andrews et al., 2011a; Fregni et al., 2005; Mariano et al., 2016; Wolkenstein et al., 2013).
There are also a few limitations related to research on pain inhibition via WM engagement. First, we asked participants to rate their pain after each painful condition (i.e. 0-back with pain, 2-0-back with pain and pain alone conditions). The act of rating pain can make the painful stimuli relevant for the participant’s behavioral goals (Torta et al., 2017b). In addition, it has been shown that rating pain directly after painful stimulation more reliably reflects the pain experience compared to pain ratings were provided at the end of the painful conditions, meaning they might reflect pain memory rather than the direct pain experience.
However, the goal of our studies was to assess pain modulation; therefore, we used the pain rating method. Moreover, we asked participants to rate their pain after each painful condition in order to avoid any interruption while performing the cognitive task.
Having discussed these limitations, I will now propose some suggestions regarding future work.
First, regarding the effect of WM engagement on pain inhibition, there are still some specific questions to address. For example, individuals with low WM capacity are less able to use attentional control over WM processes (Fukuda et al., 2016). This may result in greater attentional shifts towards distractors, and thus the storage of more distracting information with a limited-capacity compared to individuals with high WM capacity. Although these individual differences have been highlighted through a critical link between WM capacity and attentional control (Fukuda et al., 2016), it is unclear how attentional control ability can modulate pain perception and the impact of individual differences on this process. By identifying individuals with low WM capacity, it may be possible to train people to encode and store non-painful information in WM more selectively. Therefore, future work should evaluate the effect of WM performance on pain inhibition in people with both high and low WM capacities by assessing WM capacity of participants even before starting experimental protocol. It was not possible to do this in our studies because of our small sample size also; we did not separate participants before running the main experimental protocol. Moreover, it might be interesting to examine the effect of tDCS over DLPFC on pain inhibition in people with high WM capacity compared to people with lower WM capacity (see Figure 19).
Second, previous studies have shown that motivation affects performance on competing tasks (Inzlicht et al., 2012) and pain perception (Van Damme et al., 2010b; Verhoeven et al., 2010). Therefore, it might be interesting to evaluate the effects of motivation in the current paradigm or to reward participants for better task performance and observe the effects of rewards on pain perception. Moreover, as we previously mentioned the impact of cognitive effort on the
performance of a cognitive task in this chapter, future studies might need to evaluate the effects of cognitive effort and cognitive fatigue according to a NRS scale (i.e. 0 = no effort/fatigue; 100
= extreme effort/fatigue) at the end of each condition (see Figure 19) (Silvestrini et al., 2013).
Future studies might need to consider some updates in the neurocognitive model of attention to pain. We suggest that the balance between top-down, bottom-up selections and WM process might be affected by some factors. 1) individual differences in working memory capacity (high vs. low capacities) (Fukuda et al., 2016); 2) in attentional control ability (Fukuda et al., 2016); 3) in the cognitive effort: the intensity or amplitude of mental and/or physical actions in order to achieve a goal. Some people enjoy performing tasks, which need to think deeply. They invest more cognitive effort to perform them for their own sake. Whereas, others might avoid mental exertion whenever they can. People who consider the higher value to the mental effort, require fewer motivations to perform it. They seek it out instead of avoiding it (Inzlicht et al., 2018; Sandra et al., 2018; Westbrook et al., 2015). 4) in motivation (Van Damme et al., 2010b); 5) in their learning histories (Eccleston et al., 1999a); 5) in pain coping strategies (Erpelding et al., 2013; Kucyi et al., 2013); 6) in psychological factors (Torta et al., 2017a); 7) in emotional status of the person (Sussman et al., 2016; Vanlessen et al., 2014); 8) in mental health (e.g., anxiety and depression disorders) (Bagnato et al., 2018; Torta et al., 2017a) (see Figure 19).
Figure 19. Future consideration in the neurocognitive model of attention to pain.
Third, the interaction between pain inhibition, WM engagement, the effect of tDCS, and brain activity is unclear. A future study might examine this interaction by adding EEG and using high-definition tDCS with small multi-electrode arrays (Edwards et al., 2013; Saturnino et al., 2015).
Fourth, further research is needed to examine the effects of repeated tDCS sessions (Jones et al., 2015a), and the long-term effects of tDCS on pain inhibition and WM engagement, and the brain mechanisms behind this process. Another topic to explore is how various new paradigms and pain conditions can be applied to extend the benefits of tDCS.
Working memory
Stimuli in the environment
Central executive component:
Attentional Set
Working memory capacity:
Attentional load
• Working memory capacity
• Attentional control ability
• Cognitive effort
• Motivation
• Leaning histories
• Pain coping strategies
• Psychosocial factors
• Emotional status of person
• Mental health