In this interesting study, the authors replicated the use of the paradigm testing-the-limits paradigm (Digit Symbol Substitution Test- DSST) in a group of 136 healthy elderly subjects (age range: 60–75 years) with high level of education (≥13 years of schooling). This mentally challenging task is considered to assess Cognitive Reserve (CR) in a dynamic way. Additionally, the authors assessed several cognitive and non cognitive variables that contribute to, or interact with CR, as cognitive multitasking, verbal learning and long-term memory, verbal short-term and working memory, information processing speed/attention and cognitive flexibility, interference, verbal fluency, visual problem-solving, and reading performance and psychological variables as mood, social and physical activities, sleep quality, and well-being.
The authors in their introduction consider that it is important to demonstrate that the DSST is a valid and reliable measure of the CR, and should be reproduced. While I strongly agree with this assertion, the authors had already applied this instrument in a similar population (N=140, age 57–74 yrs, M = 67.27, SD = 4.16, >13 years education, see Zihl J et al. PLoS One. 2014;9: e84590). The use of a high selected healthy elderly group (with high educational level) was possibly related with the observed lack of association between CR and the non-cognitive variables assessed with this study. While the authors reflect this in the discussion, these should also be justified in the introduction section.
Regarding the methods and results sections, some minor comments/suggestions:
1) The authors acknowledge that “All corresponding p-values reported are Bonferroni-corrected due to multiple comparisons, with significance level set at α = .05”. These Bonferroni corrections are not clear in the data presented (eg., report significant data with p=.05 – see table 2).
2) Considering the relations between psychological variables and cognitive variables, between cognitive variables and CR, as well as between CR and both cognitive and psychological variables, a moderation/mediation analysis should be also investigated.
3) As the authors report that there was no evidence that psychological measures were influenced by aging, the same should be reported for cognitive measures.
4) The authors could provide the data showing that “comparison of high and low performers in this study revealed higher performance in measures of cognitive architecture in subjects with higher mood scores and higher engagement in leisure activities, supporting the view that mood and cognitive lifestyle are key determinants of cognitive performance in healthy older ” page 6, if we consider that no significant differences were found in psychological measures obtained from low and high performers (results section) – page 5.