Significance, future directions and limitations

Chronic pain conditions may advance with the age-related cognitive decline. Besides, pain is associated with changes in the brain that may worsen the cognitive decline observed in older adults. For instance, patients with chronic neuropathic or radicular pain show decreased cognitive performance and this decline is particularly observed in older patients (Moriarty et al., 2011, 2017). This suggests that clinical pain can decrease cognitive function and that this effect is moderated by age. Conversely, studies in patients with dementia indicate that cognitive decline is associated with greater amplitude and duration of pain-related activity in regions associated with sensory, affective and cognitive processes (Summers et al., 2016). Based on these interactions between cognition, pain and age, anodal tDCS may be especially useful in older persons affected by cognitive decline, chronic pain or both (Hsu et al., 2015). Besides, It remains to be determined whether anodal tDCS of the DLPFC may be effective at improving pain inhibition by WM in different clinical populations in which an attentional bias to pain was reported (Eccleston et al., 1999b; Torta et al., 2017a). Another limitation that should be mentioned is that tDCS is not a focal method and other regions and their networks may be stimulated in addition to the DLPFC, over which the anode was placed. There- fore, like in other tDCS studies, the effects reported here cannot be attributed to the DLPFC exclusively.

5. Conclusion

The present study shows that WM and pain inhibition is enhanced by anodal tDCS in older persons. This warrants future studies to examine whether multiple tDCS sessions with cognitive training may produce long-lasting changes in pain regulation and pain symptoms in healthy older persons and patients with cognitive decline or chronic pain.

Author’s contributions to the manuscript

1- Zoha Deldar contributed to all aspects of the research.

2- Nabi Rustamov contributed to data acquisition and interpretation.

3- Isabelle Blanchette contributed to experimental design, data interpretation and manuscript preparation.

4- Mathieu Piché contributed to all aspects of the research and obtained funding for the study.

All authors read and approved the final version of the manuscript.

Conflicts of interest

The authors report no financial or other relationship that may lead to any conflict of interest.

Acknowledgements

This study was supported by a grant from the Natural Sciences and Engineering Research Council of Canada (MP - grant number 402176). The contribution of MP was supported by the research Chair in Pain Neurophysiology from Université du Québec à Trois- Rivières and the

Fonds de Recherche du Québec en Santé (FRQS). The contribution of Zoha Deldar was supported by the department of Anatomy from UQTR and the Centre de Recherche en Neuropsychologie et Cognition (CERNEC) from Université de Montréal.

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