PONE-D-20-08899

Prefrontal activation while listening to a letter of gratitude read aloud by a coworker face-to-face: A NIRS study

PLOS ONE

Dear Dr. Hori,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

1) The effectiveness of the present paradigm to truly target ‘gratitude’ is in question. By comparing the so-called gratitude condition with a baseline condition, the results could be explained by factors such as general arousal, increased attention, increased interpersonal interaction, etc. In addition, there are several types of gender pair and the sample size is very limited, preventing further interpretation of the results.

2) The background science does not match the study design. Much of the introduction describes research on gratitude - studies that evaluate the origins, biological underpinnings, and mental and physical implications of possessing gratitude, either as a trait characteristic or through training. The study, then, measures the experience of being thanked - that is - the target of another person's gratitude. Is this gratitude, or is the experience of being thanked a variety of pleasure, affection, or in some cases embarrassment or guilt or humility? This issue is a central problem - the authors never mention this distinction nor acknowledge their focus, again, on the experience of being a recipient of gratitude, not feeling gratitude oneself.

3) The writing and language are unclear and in many places, linguistically and/or grammatically unclear or incorrect. The authors should invite a native English speaker to review and advise on the writing. A few examples: a) line 43 describes gratitude as a "temporary condition" - which is unclear. A better phrase would be "specific emotional experience or feeling state", b) line 44 should say "appreciated", not "appreciates", c) line 61 should say "have", not "has", d) line 69 should say "few studies have", not "few study has", e) line 80: the authors should explain the pain-money exchange task; readers are not necessarily familiar with what that entails, f) line 82: using the word "unnatural" is misleading - the authors should say something like "laboratory tasks designed to simulate naturalistic experiences", etc... In general, using phrases like "real-life" and "natural" are misleading - in part because the authors emphasize the extent to which their procedures are more "real life" than a scenario based simulation process (not true, both are equally real-life). What distinguishes the procedures used in this study is that the experimental condition is in-person, interpersonal, and in a physically and visually ordinary environment (minus the NIRS rig).

4) The description of the methods is unclear - particularly the "activation task" that involves murmuring vowels. Why was this task included? Was NIRS data gathered during this task? Was the "activation task" data analyzed in connection with the weather description and gratitude letter tasks? Some of the descriptions of the procedures were overly simple, while other parts were overly detailed.

5) The subjects were notified ahead of time as to whether they would receive words of gratitude or words about the weather (control).  Do the authors believe this expectation could have altered brain activity as recorded in the results?  Perhaps there are other ways to control for this, by using a dedicated control group that only received words about the weather (control), instead of a predictable back-and-forth exchange.

6) Why was NIRS chosen instead of EEG, or both simultaneously?

7) The fNIRS analysis was not very solid. First, the definition of the three regions needs further explanation and the readers might speculate this operation as a way to simplify the statistical analysis towards more positive results. Second, what is Hb and why the authors did not report HbO, HbR and HbT? All three should be reported. Third, the correction for multiple comparison was not clearly stated.

8) Based on the position of optical poles, it is doubtful whether the existing channel covers bilateral DLPFC. Stronger evidence of this should be presented in the manuscript.

9) The fNIRS data consists of 22 channels and 3 ROIs. Was FDR or Bonferoni correction used for the separate statistical tests? In addition to the 3 ROI statistical test results, please provide the statistical test results of each 22 channels with FDR or Bonferoni correction.

10) It may be helpful to see the NIRS moving-average plot across the entire session.  There was no discussion about the baseline results (during the vowel recitation and resting periods).  In order for the reader to fully interpret the results independently, seeing this moving-average with 95% Confidence Intervals, perhaps as a Supplemental figure, would provide a more complete picture of the data and how it changed over time.  It would allow the reader to see if there were any anticipatory effects during the resting period, as well.

11) Is the sample large enough? Has the time power analysis been done to prove that all powers of the main tests are over 0.8?

12) The conclusion, that any of the patterns of activation are specifically related to gratitude vs. statements about the weather, is not warranted. There is little evidence to support the claim that hearing the gratitude letter involved feelings of gratitude in the listener. Second, the experimental condition differed on too many other levels that also could explain the observed pattern of increased frontal activation during the gratitude letter listening experiences. First, hearing another person describe one's own past actions engages memory retrieval, self-referential appraisals and attributions, and value-assessment. The weather statements, being impersonal and irrelevant to autobiographical knowledge and lacking any social salience, offers only those distinctions. This project does not measure experiences of gratitude in any direct way - nor does it measure activation during a behavior that is known to evoke gratitude - and thus, cannot claim that the data reveal anything intrinsic to gratitude. It IS possible that the 2nd listener in the experimental sequence, having already read their letter of gratitude, was feeling continued and/or residual gratitude - but the researchers do not explore this.

13) The study would strongly benefit from 1) adding several more participants pairs (doubling the sample) 1) refraining from claiming that the NIRS data indicate anything specific about gratitude (they can only suggest that the pleasant interpersonal experience was associated with greater prefrontal activation than the neutral weather-statement experience), and 3) major rewriting of all sections to situate the article as a methodological advance rather than a scientific insight into the neural correlates of gratitude.

Please submit your revised manuscript by Jul 16 2020 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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We look forward to receiving your revised manuscript.

Kind regards,

Linda Chao

Academic Editor

PLOS ONE

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Additional Editor Comments (if provided):

1) The effectiveness of the present paradigm to truly target ‘gratitude’ is in question. By comparing the so-called gratitude condition with a baseline condition, the results could be explained by factors such as general arousal, increased attention, increased interpersonal interaction, etc. In addition, there are several types of gender pair and the sample size is very limited, preventing further interpretation of the results.

2) The background science does not match the study design. Much of the introduction describes research on gratitude - studies that evaluate the origins, biological underpinnings, and mental and physical implications of possessing gratitude, either as a trait characteristic or through training. The study, then, measures the experience of being thanked - that is - the target of another person's gratitude. Is this gratitude, or is the experience of being thanked a variety of pleasure, affection, or in some cases embarrassment or guilt or humility? This issue is a central problem - the authors never mention this distinction nor acknowledge their focus, again, on the experience of being a recipient of gratitude, not feeling gratitude oneself.

3) The writing and language are unclear and in many places, linguistically and/or grammatically unclear or incorrect. The authors should invite a native English speaker to review and advise on the writing. A few examples: a) line 43 describes gratitude as a "temporary condition" - which is unclear. A better phrase would be "specific emotional experience or feeling state", b) line 44 should say "appreciated", not "appreciates", c) line 61 should say "have", not "has", d) line 69 should say "few studies have", not "few study has", e) line 80: the authors should explain the pain-money exchange task; readers are not necessarily familiar with what that entails, f) line 82: using the word "unnatural" is misleading - the authors should say something like "laboratory tasks designed to simulate naturalistic experiences", etc... In general, using phrases like "real-life" and "natural" are misleading - in part because the authors emphasize the extent to which their procedures are more "real life" than a scenario based simulation process (not true, both are equally real-life). What distinguishes the procedures used in this study is that the experimental condition is in-person, interpersonal, and in a physically and visually ordinary environment (minus the NIRS rig).

4) The description of the methods is unclear - particularly the "activation task" that involves murmuring vowels. Why was this task included? Was NIRS data gathered during this task? Was the "activation task" data analyzed in connection with the weather description and gratitude letter tasks? Some of the descriptions of the procedures were overly simple, while other parts were overly detailed.

5) The subjects were notified ahead of time as to whether they would receive words of gratitude or words about the weather (control). Do the authors believe this expectation could have altered brain activity as recorded in the results? Perhaps there are other ways to control for this, by using a dedicated control group that only received words about the weather (control), instead of a predictable back-and-forth exchange.

6) Why was NIRS chosen instead of EEG, or both simultaneously?

7) The fNIRS analysis was not very solid. First, the definition of the three regions needs further explanation and the readers might speculate this operation as a way to simplify the statistical analysis towards more positive results. Second, what is Hb and why the authors did not report HbO, HbR and HbT? All three should be reported. Third, the correction for multiple comparison was not clearly stated.

8) Based on the position of optical poles, it is doubtful whether the existing channel covers bilateral DLPFC. Stronger evidence of this should be presented in the manuscript.

9) The fNIRS data consists of 22 channels and 3 ROIs. Was FDR or Bonferoni correction used for the separate statistical tests? In addition to the 3 ROI statistical test results, please provide the statistical test results of each 22 channels with FDR or Bonferoni correction.

10) It may be helpful to see the NIRS moving-average plot across the entire session. There was no discussion about the baseline results (during the vowel recitation and resting periods). In order for the reader to fully interpret the results independently, seeing this moving-average with 95% Confidence Intervals, perhaps as a Supplemental figure, would provide a more complete picture of the data and how it changed over time. It would allow the reader to see if there were any anticipatory effects during the resting period, as well.

11) Is the sample large enough? Has the time power analysis been done to prove that all powers of the main tests are over 0.8?

12) The conclusion, that any of the patterns of activation are specifically related to gratitude vs. statements about the weather, is not warranted. There is little evidence to support the claim that hearing the gratitude letter involved feelings of gratitude in the listener. Second, the experimental condition differed on too many other levels that also could explain the observed pattern of increased frontal activation during the gratitude letter listening experiences. First, hearing another person describe one's own past actions engages memory retrieval, self-referential appraisals and attributions, and value-assessment. The weather statements, being impersonal and irrelevant to autobiographical knowledge and lacking any social salience, offers only those distinctions. This project does not measure experiences of gratitude in any direct way - nor does it measure activation during a behavior that is known to evoke gratitude - and thus, cannot claim that the data reveal anything intrinsic to gratitude. It IS possible that the 2nd listener in the experimental sequence, having already read their letter of gratitude, was feeling continued and/or residual gratitude - but the researchers do not explore this.

13) The study would strongly benefit from 1) adding several more participants pairs (doubling the sample) 1) refraining from claiming that the NIRS data indicate anything specific about gratitude (they can only suggest that the pleasant interpersonal experience was associated with greater prefrontal activation than the neutral weather-statement experience), and 3) major rewriting of all sections to situate the article as a methodological advance rather than a scientific insight into the neural correlates of gratitude.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Partly

Reviewer #3: No

Reviewer #4: No

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2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: No

Reviewer #3: No

Reviewer #4: I Don't Know

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3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: No

Reviewer #3: No

Reviewer #4: Yes

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4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: No

Reviewer #3: No

Reviewer #4: No

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Overall an excellent and tightly controlled experiment with compelling statistical results. The discussion includes a great discussion of the limitations of the study, including aims for future studies. I have some thoughts for further improving your manuscript, but these are not necessary as your Conclusions are appropriate and scientifically drawn from the data you have presented. I see no need for changes to methodology, study design, or reporting prior to acceptance.

It is interesting to note that the subjects were notified ahead of time as to whether they would receive words of gratitude or words about the weather (control). Do you believe this expectation could have altered brain activity as recorded in your results? Perhaps there are other ways to control for this, by using a dedicated control group that only received words about the weather (control), instead of a predictable back-and-forth exchange.

Why was NIRS chosen instead of EEG, or both simultaneously? Is the timescale of the EEG data not suitable for the hypothesis you were testing?

It would be interesting to see the NIRS moving-average plot across the entire session. There was no discussion about the baseline results (during the vowel recitation and resting periods). In order for the reader to fully interpret your results independently, seeing this moving-average with 95% Confidence Intervals, perhaps as a Supplemental figure, would provide a more complete picture of the data and how it changed over time. It would allow the reader to see if there were any anticipatory effects during the resting period, as well.

Reviewer #2: The present study aimed to study the neural correlates of gratitude by using a novel naturalistic paradigm. While the authors’ efforts in this direction is much appreciated, I have several critical concerns:

1) The effectiveness of the present paradigm to truly target ‘gratitude’ is in question. By comparing the so-called gratitude condition with a baseline condition, the results could be explained by factors such as general arousal, increased attention, increased interpersonal interaction, etc. In addition, there are several types of gender pair and the sample size is very limited, preventing further interpretation of the results.

2) The fNIRS analysis was not very solid. First, the definition of the three regions needs further explanation and the readers might speculate this operation as a way to simplify the statistical analysis towards more positive results. Second, what is Hb and why the authors did not report HbO, HbR and HbT? All three should be reported. Third, the correction for multiple comparison was not clearly stated.

Reviewer #3: The topic is interesting. However, some concerns should be addressed.

1.Based on your position of optical poles, I doubt whether the existing channel covers bilateral DLPFC. Please provide a stronger evidence.

2.The fNIRS data consists of 22 channels and 3 ROIs. Did you use FDR or Bonferoni correction for the separate statistical tests? In addition to the 3 ROI statistical test results, please provide the statistical test results of each 22 channels with FDR or Bonferoni correction.

3.Is the sample large enough? Has the time power analysis been done to prove that all powers of the main tests are over 0.8?

4. The language needs to be further polished by native editors.

Reviewer #4: This manuscript describes a very innovative and ambitious study that uses a pioneering methodology, NIRS to examine changes in oxygenated and deoxygenated hemoglobin in select cortical regions related to several face-to-face experiences. The researchers recruited pairs of co-workers, either current or past, and administered a series of tasks and processes and gathered both self-report and NIRS data. First, study participants were asked to write a letter of gratitude to their coworker, following a codified approach described in proceedings from a business presentation delivered by Kelly McGonigal. Then, participants were brought into the laboratory in pairs, invited to fill out a mood state survey, then guided through a dyadic procedure that involved rewriting their gratitude letter, performing a vowel murmuring "activation task", listening to one another talk about the weather/seasons from a script (i.e. control task), listening to one another read aloud the gratitude letter (i.e. experimental task), and finally filling out the mood state questionnaire once again. For capturing the NIRS data, the researchers alternated the measurement device from one person in the pair to the other, gathering data from the listeners (hearing either about the weather or the letter of gratitude).

The paper reports that the experience was associated with decreases in self-reported negative mood characteristics and increases in positive mood characteristics from before to after the procedure. Also, the results suggest that listening to a gratitude letter being read aloud was associated with greater NIRS responses from left, right, and polar areas of the prefrontal cortex. The authors conclude that hearing a letter of gratitude engages prefrontal cortex more than hearing about the weather, and that this report provides a unique level of ecological validity, related to the more "natural", "face-to-face" experimental setup (contrasted with other neuroimaging studies of gratitude that have employed simulation or scenario-based strategies for evoking feelings of gratitude).

While once again, this is an impressive demonstration of using a newer technology to explore a less-frequently focused-upon construct (gratitude) during a live interpersonal interaction, this study feels more like a pilot, or methodological contribution than a generalizable contribution to science. Here are the key challenges that this project faces:

1) The background science does not match the study design. Much of the introduction describes research on gratitude - studies that evaluate the origins, biological underpinnings, and mental and physical implications of possessing gratitude, either as a trait characteristic or through training. The study, then, measures the experience of being thanked - that is - the target of another person's gratitude. Is this gratitude, or is the experience of being thanked a variety of pleasure, affection, or in some cases embarrassment or guilt or humility? This issue is a central problem - the authors never mention this distinction nor acknowledge their focus, again, on the experience of being a recipient of gratitude, not feeling gratitude oneself.

2) The writing and language unclear and in many places, linguistically and/or grammatically unclear or incorrect. The authors should invite a native english speaker to review and advise on the writing. A few examples: a) line 43 describes gratitude as a "temporary condition" - which is unclear. A better phrase would be "specific emotional experience or feeling state", b) line 44 should say "appreciated", not "appreciates", c) line 61 should say "have", not "has", d) line 69 should say "few studies have", not "few study has", e) line 80: the authors should explain the pain-money exchange task; readers are not necessarily familiar with what that entails, f) line 82: using the word "unnatural" is misleading - the authors should say something like "laboratory tasks designed to simulate naturalistic experiences", etc... In general, using phrases like "real-life" and "natural" are misleading - in part because the authors emphasize the extent to which their procedures are more "real life" than a scenario based simulation process (not true, both are equally real-life). What distinguishes the procedures used in this study is that the experimental condition is in-person, interpersonal, and in a physically and visually ordinary environment (minus the NIRS rig).

3) The description of the methods is unclear - particularly the "activation task" that involves murmuring vowels. Why was this task included? Was NIRS data gathered during this task? Was the "activation task" data analyzed in connection with the weather description and gratitude letter tasks? Some of the descriptions of the procedures were overly simple, while other parts were overly detailed.

4) The conclusion, that any of the patterns of activation are specifically related to gratitude vs. statements about the weather, is not warranted. As suggested in point #1 there is little evidence to support the claim that hearing the gratitude letter involved feelings of gratitude in the listener. Second, the experimental condition differed on too many other levels that also could explain the observed pattern of increased frontal activation during the gratitude letter listening experiences. First, hearing another person describe one's own past actions engages memory retrieval, self-referential appraisals and attributions, and value-assessment. The weather statements, being impersonal and irrelevant to autobiographical knowledge and lacking any social salience, offers only those distinctions. This project does not measure experiences of gratitude in any direct way - nor does it measure activation during a behavior that is known to evoke gratitude - and thus, cannot claim that the data reveal anything intrinsic to gratitude. It IS possible that the 2nd listener in the experimental sequence, having already read their letter of gratitude, was feeling continued and/or residual gratitude - but the researchers do not explore this.

In sum, I have enthusiasm about this project, but do not feel like it is analyzed or written in a way that is publishable. The study would strongly benefit from 1) adding several more participants pairs (doubling the sample) 1) refraining from claiming that the NIRS data indicate anything specific about gratitude (they can only suggest that the pleasant interpersonal experience was associated with greater prefrontal activation than the neutral weather-statement experience), and 3) major rewriting of all sections to situate the article as a methodological advance rather than a scientific insight into the neural correlates of gratitude.

**********

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Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: James Sonne

Reviewer #2: No

Reviewer #3: No

Reviewer #4: No

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Prefrontal activation while listening to a letter of gratitude read aloud by a coworker face-to-face: A NIRS study

PONE-D-20-08899R1

Dear Dr. Hori,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

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Kind regards,

Linda Chao

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

Reviewer #3: All comments have been addressed

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2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: I am satisfied with the rigor and conclusions, based on the limitations generally associated with complex behavioral tasks and the interpretation of neural functionality. Ideally, future experiments would perform NIRS with simultaneous or repeated EEG to fully validate the methodology. This kind of validation paper would be an important publication which could be cited repeatedly by future works. However, the current paper, while stating appropriate limitations in the interpretation, does produce anticipated results of increased activation within the ventromedial prefrontal cortex during a task involving reward and moral decision-making (receiving gratitude). Statistical analyses appear to have been chosen and performed appropriately, and the conclusions drawn are scientifically sound.

Reviewer #2: All my concerns properly addressed. No further comments.

I think the manuscript is now ready for publication.

Reviewer #3: The authors have adequately addressed my comments raised in a previous round of review and I feel that this manuscript is now acceptable for publication.

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7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: Yes: Dan Zhang

Reviewer #3: No