Review Reports

Round 1

Reviewer 1 Report

This is an interesting study on a timely topic that is the relationship between cognitive functions, specifically cognitive failures, and vagally-mediated heart rate variability. Results showed that 5-min resting vagally-mediated heart rate variability predicted distractibility, a subscale of the cognitive failure questionnaire. Despite the fact that the target of the study is up-to-date and attractive, the study suffers from some main methodological and statistical flaws. 

The introduction is well written, nonetheless, I see a major flaw in this paper that is the fact that the relationship between perceived cognitive failure and vmHRV. The lack of an objective measure of cognitive performance and cognitive failure is a limit. In fact, cognitive failure was measured through a retrospective questionnaire that asks participants to think about the last 6 months. This is relevant considering that subjective and objective measures are often little or not at all correlated. Therefore I suggest the authors modify the introduction and discussion referring to vmHRV as a predictor of perceived cognitive failures.  

 

Methods: 

• Participants were mainly male (22 female and 47 male). Some studies have found significant gender differences in vmHRV (Antelmi, I., De Paula, R. S., Shinzato, A. R., Peres, C. A., Mansur, A. J., & Grupi, C. J. (2004). Influence of age, gender, body mass index, and functional capacity on heart rate variability in a cohort of subjects without heart disease. The American journal of cardiology, 93(3), 381-385.; Estévez-Báez, M., Carricarte-Naranjo, C., Jas-García, J. D., Rodríguez-Ríos, E., Machado, C., Montes-Brown, J., ... & Pié, E. A. (2018). Influence of heart rate, age, and gender on heart rate variability in adolescents and young adults. In Advances in Medicine and Medical Research (pp. 19-33). Springer, Cham.). Despite the fact that gender was included in the regression model, age should be taken into account or listed in the limit section. 

• How was the sample size determined? Please report the power analysis. 
• I appreciated the fact that the authors controlled for some confounding factors such as alcohol consumption, physical activity, food, and beverage intake. Did the authors also ask participants to refrain from drinking caffeinated beverages before the study? Did they collect participants' body mass index, which has been shown to influence vmHRV? 

 

• Regarding the statistical analysis did the authors control for multiple comparisons in the correlation matrix? Analyzing both correlations and regression analysis seems redundant. Given the very specific hypothesis stated by the authors, I would have expected only regressions. Also, in the correlation matrix, it is included the variable sex, which is a categorical variable and should not be included in a correlation analysis.  
• Similar to the previous comment did the authors include sex in the regression model as a categorical variable? It is not clear from table 3 

• Please report the measure of the dimension of the effect size. This would help the readers to understand the strength of this relationship. 
• I would like to see a figure of the significant results in the regression including data points for all the subjects. 
• The mean and SD of the score of each of the subscales in the CFQ was very low. I expect that such a young sample did not experience many cognitive failure symptoms. This should be recognized by the authors in the limit section. 

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The authors have submitted a well-written paper that establishes a link between cognitive failures and cardiac vagal activity. I have a few recommendations and questions, in the spirit of improving the manuscript:

Introduction: The section is well-written overall.

1a. However, I wonder if the authors could present a simpler narrative that is also better connected to the extant lab studies of vmHRV and cognition. Many studies have reported correlations between resting vmHRV and attentional performance (often in the presence of distractors). Most pertinent to your present findings and distractibility, the following papers have negatively linked resting vmHRV and response time(RT)-based indices of attentional lapses in the lab. Here, greater attentional lapses [i.e., greater proportion of rare but very long RTs and greater RT variability] indicate greater distractibility.

Williams, D. P., Thayer, J. F., & Koenig, J. (2016). Resting cardiac vagal tone predicts intraindividual reaction time variability during an attention task in a sample of young and healthy adults. Psychophysiology, 53(12), 1843-1851.

Spangler, D. P., Williams, D. P., Speller, L. F., Brooks, J. R., & Thayer, J. F. (2018). Resting heart rate variability is associated with ex-Gaussian metrics of intra-individual reaction time variability. International Journal of Psychophysiology, 125, 10-16.

Based on the aforementioned papers, it makes me wonder how the current results are novel. It could be more palatable to suggest in the Intro that others -- including the studies above--have linked resting vmHRV with laboratory performance-based metrics of cognitive failures (distractibility, poor attention, lapsed memory function), but the present study is novel because it assesses self-reported cognitive failures outside of the lab in everyday life. Focusing on the real-world aspect of your self-report cognitive failure metric better accentuates the novelty of the present report, in my view.

Method & Results:

2a. I ask that the authors provide more technical detail about the RR interval pre-processing and HRV/respiration rate quantification, for the purposes of replication. For example, how was an artifact defined, how was it replaced? What were the parameters of the spectral analysis (window type, size, overlap)? Were RR time courses detrended? Also, what is the Kubios-derived respiration rate? This is likely a rough surrogate of respiration rate based either on the median or maximum power value of the power spectral density plot (PSD by Hz), correct? This should be better explained and justified.

2b. The authors compute spectral and time-domain indices of HRV, but only report in RMSSD in the main correlations. Why is that so? Ideally, correlations with the other HRV measures (HF-HRV, etc.) should be mentioned.

2c. I encourage the use of confidence intervals in all the primary correlation and regression analyses. This will help your readers focus on the effect sizes of your results, as opposed to just p-values.

2d. The hierarchical regression approach seems like the proper choice. Yet, it is unclear in the text or Table 3 exactly what variables are being entered at step 1 vs. step 2, etc. If emotion variables are significant but are no longer so when RMSSD is entered in the model [or vice versa], this could be conceptually important (e.g., reflective of potential mediation).

Discussion:

3a. As noted in my earlier comment, I suggest better discussing the vmHRV-distractibility finding in relation to prior lab studies on the same topic- while accentuating how prior studies focus on controlled lab cognitive metrics. In other words, the current study provides novel support for a link between vmHRV and “real-world distractibility.”

3b. What is the significance of resting VmHRV only being related to distractibility but not the other cognitive failure facets? Please provide more explanation in the Discussion, at least. Also, was there a priori justification for testing the facets separately? Were there hypotheses about vmHRV being differentially associated with one facet versus another?

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The manuscript has greatly improved. 
In the results section, I would like to see the tables with the results of the two non-significant regressions. I think it is important to report the results of all the analyses, not only the significant ones.

Author Response

Thank you for your general positive appreciation and for taking the time to check the revised version of our manuscript. We now added Table 3 and Table 4, to display the results of the two non-significant regressions.