Dear Dr Tashjian,

Thank you for submitting your manuscript entitled "Adaptive Safety Coding in the Prefrontal Cortex" for consideration as a Research Article by PLOS Biology.

Your manuscript has now been evaluated by the PLOS Biology editorial staff and I am writing to let you know that we would like to send your submission out for external peer review.

Please note that we unfortunately have not been able to receive advice from one of our academic editors on your study and have, therefore, not yet made a firm decision on whether the conceptual advance is sufficient for PLOS Biology. We will discuss this after review with one of our editorial board members and will be looking for strong reviewer support.

However, before we can send your manuscript to reviewers, we need you to complete your submission by providing the metadata that is required for full assessment. To this end, please login to Editorial Manager where you will find the paper in the 'Submissions Needing Revisions' folder on your homepage. Please click 'Revise Submission' from the Action Links and complete all additional questions in the submission questionnaire.

Once your full submission is complete, your paper will undergo a series of checks in preparation for peer review. After your manuscript has passed the checks it will be sent out for review. To provide the metadata for your submission, please Login to Editorial Manager (https://www.editorialmanager.com/pbiology) within two working days, i.e. by Aug 29 2024 11:59PM.

If your manuscript has been previously peer-reviewed at another journal, PLOS Biology is willing to work with those reviews in order to avoid re-starting the process. Submission of the previous reviews is entirely optional and our ability to use them effectively will depend on the willingness of the previous journal to confirm the content of the reports and share the reviewer identities. Please note that we reserve the right to invite additional reviewers if we consider that additional/independent reviewers are needed, although we aim to avoid this as far as possible. In our experience, working with previous reviews does save time.

If you would like us to consider previous reviewer reports, please edit your cover letter to let us know and include the name of the journal where the work was previously considered and the manuscript ID it was given. In addition, please upload a response to the reviews as a 'Prior Peer Review' file type, which should include the reports in full and a point-by-point reply detailing how you have or plan to address the reviewers' concerns.

During the process of completing your manuscript submission, you will be invited to opt-in to posting your pre-review manuscript as a bioRxiv preprint. Visit http://journals.plos.org/plosbiology/s/preprints for full details. If you consent to posting your current manuscript as a preprint, please upload a single Preprint PDF.

Feel free to email us at plosbiology@plos.org if you have any queries relating to your submission.

Kind regards,

Christian

Christian Schnell, PhD

Senior Editor

PLOS Biology

cschnell@plos.org

Dear Dr Tashjian,

Thank you for your patience while your manuscript "Adaptive Safety Coding in the Prefrontal Cortex" was peer-reviewed at PLOS Biology. It has now been evaluated by the PLOS Biology editors, an Academic Editor with relevant expertise, and by several independent reviewers.

In light of the reviews, which you will find at the end of this email, we would like to invite you to revise the work to thoroughly address the reviewers' reports.

As you will see below, the reviewers find the topic of your study important and are overall supportive of publication of your study. However, they also raise a few concerns that need to be addressed. Among the concerns raised, we would like to highlight the need to (i) address Reviewer 1's comment to map the different sub-regions of vmPFC to any specific anatomical boundaries and speculate on why these different portions were involved in the different processes, (ii) more carefully define and introduce the concepts and how they relate to your study, and (iii) provide evidence that the task did engage the constructs and processes it is supposed to engage and that the fMRI sequence is suitable.

Given the extent of revision needed, we cannot make a decision about publication until we have seen the revised manuscript and your response to the reviewers' comments. Your revised manuscript is likely to be sent for further evaluation by all or a subset of the reviewers.

We expect to receive your revised manuscript within 3 months. Please email us (plosbiology@plos.org) if you have any questions or concerns, or would like to request an extension.

At this stage, your manuscript remains formally under active consideration at our journal; please notify us by email if you do not intend to submit a revision so that we may withdraw it.

**IMPORTANT - SUBMITTING YOUR REVISION**

Your revisions should address the specific points made by each reviewer. Please submit the following files along with your revised manuscript:

1. A 'Response to Reviewers' file - this should detail your responses to the editorial requests, present a point-by-point response to all of the reviewers' comments, and indicate the changes made to the manuscript.

*NOTE: In your point-by-point response to the reviewers, please provide the full context of each review. Do not selectively quote paragraphs or sentences to reply to. The entire set of reviewer comments should be present in full and each specific point should be responded to individually, point by point.

You should also cite any additional relevant literature that has been published since the original submission and mention any additional citations in your response.

2. In addition to a clean copy of the manuscript, please also upload a 'track-changes' version of your manuscript that specifies the edits made. This should be uploaded as a "Revised Article with Changes Highlighted" file type.

*Re-submission Checklist*

When you are ready to resubmit your revised manuscript, please refer to this re-submission checklist: https://plos.io/Biology_Checklist

To submit a revised version of your manuscript, please go to https://www.editorialmanager.com/pbiology/ and log in as an Author. Click the link labelled 'Submissions Needing Revision' where you will find your submission record.

Please make sure to read the following important policies and guidelines while preparing your revision:

*Published Peer Review*

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out. Please see here for more details:

https://blogs.plos.org/plos/2019/05/plos-journals-now-open-for-published-peer-review/

*PLOS Data Policy*

Please note that as a condition of publication PLOS' data policy (http://journals.plos.org/plosbiology/s/data-availability) requires that you make available all data used to draw the conclusions arrived at in your manuscript. If you have not already done so, you must include any data used in your manuscript either in appropriate repositories, within the body of the manuscript, or as supporting information (N.B. this includes any numerical values that were used to generate graphs, histograms etc.). For an example see here: http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1001908#s5

*Blot and Gel Data Policy*

We require the original, uncropped and minimally adjusted images supporting all blot and gel results reported in an article's figures or Supporting Information files. We will require these files before a manuscript can be accepted so please prepare them now, if you have not already uploaded them. Please carefully read our guidelines for how to prepare and upload this data: https://journals.plos.org/plosbiology/s/figures#loc-blot-and-gel-reporting-requirements

*Protocols deposition*

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols

Thank you again for your submission to our journal. We hope that our editorial process has been constructive thus far, and we welcome your feedback at any time. Please don't hesitate to contact us if you have any questions or comments.

Sincerely,

Christian

Christian Schnell, PhD

Senior Editor

PLOS Biology

cschnell@plos.org

------------------------------------

REVIEWS:

Reviewer #1 (Patricia Lockwood): This is an exciting, novel and rigorously conducted study that I enjoyed reading. The behavioural and neural mechanisms of safety and protection remain poorly understood but are crucial drivers of human valuation and decision-making. The combination of a behavioural study with an fmri study is impressive, and studies were preregistered. The use of real shock stimuli is challenging to implement but adds ecological validity to the task. The analyses are sophisticated including univariate and multivariate approaches as well as connectivity analyses. The conclusions are balanced and appropriate. I have just a few comments the authors may wish to consider.

1. It is very interesting to find different sub-regions of vmPFC involved in different aspects of the task. I wondered if the authors could map these to any specific anatomical boundaries and speculate on why these different portions were involved in the different processes. In light of this I would also hesitate to suggest that these portions represent a gradient but rather different portions of vmPFC.

2. I wonder how the authors see their results fitting with the work on self prioritisation and self relevance in vmPFC? (e.g. Sui & Humphreys, 2015; Sui & Gu., 2017; Lockwood et al., 2018). As several studies have shown that self relevant, as opposed to stranger and friend relevant information, is preferentially encoded in vmPFC.

Minor

It could be helpful for the figure legends to start with a more informative subtitle than 'neural activation' and 'behavioural results'. A brief summary of the main finding of each figure would be helpful for readers to grasp the main take home from the figure as an anchor for the longer legends.

Reviewer #2: Tashjian and colleagues submit a manuscript describing highly interesting and timely results form a study that combined a novel behavioral paradigm with fMRI. The study specifically examined whether the level of safety (e.g. having a weapon) influences the perception of a threat (e.g. dangerous animal) and further segregates the behavioral and neural subprocesses involved. The study is well designed, the manuscript is well organized, the results address an important gap in the literature and are cautiously interpreted. I have mainly minor points that may help to improve the manuscript further:

- Authors discuss the background of the study and the results in the context of fear models. I wonder whether the design of the paradigm and the interpretation may also link with the Theory of Emotion from Lazarus, a cognitive emotion theory which underscorese the role of (re)appraisal (primary, secondary) in emotion processes. The level of evaluation that the authors operationalize in their paradigm is on a rather high cognitive evaluation level.

- The vmPFC has been engaged in several functions related to fear and safety, but also associated phsyiological reactions such as regulation of cardiac reactivity etc. In their account the authors relate this region to meta-representation and updating. Authors may want to discuss or provide further analyses to differentiate the cognitive computations from the associated phsyiological reactivity or regulation.

- For the multivariate connectivity authors report in the text that results are presented on the level of "all connections p<0.05". Given the number of multivariate connections please clarify if multiple comparisons were applied.

- Authors report that (line 337) 'subjects fixated more on the initial safety value if protection stimuli were presented first…. ' however, it appears that the analyses in the behavioral and MRI sample were not consistent (p = .004, and p = .75). Please clarify and interpret the potential differences.

- Authors observed that activation corresponding to decreases in safety was observed in the occipital pole and postcentral gyrus (in Fig. S1a-b). How could this be explained? Do these regions reflect preparatory visual and motor engagement to facilitate threat detection?

Reviewer #3: Tashjian and colleagues provide a novel approach to unpacking the neural dynamics of safety processes in the human vmPFC, combining behavioral and fMRI experiments to target understudied domains of safety encoding. Their findings appear to fit well with the known role of vmPFC in safety processing but also highlight more precise mechanistic contributions and differentiate discrete roles of vmPFC subregions (anterior vs posterior), and wider multivariate brain connectivity. I am impressed by the sophistication of the task - and the results are compelling. For these to be convincing, additional details on the study design, experiment methods, and analyses that should be provided. Conceptually, there ought to be clearer explanation of certain key concepts that are central to the work (e.g., meta-representation), which may not be familiar to other researchers who work on safety processing and the vmPFC and require greater contextualizing. I hope my comments below are helpful in outlining these points.

Major Comments:

1. The main strength of the work also presents some potential areas of concern. Authors provide some novel and interesting angles to the safety processing literature, which I agree do well to complement existing models (e.g., avoidance/Pavlovian paradigms). However, the high novelty means that these concepts need to be well supported, defined, and justified. For example, phrases like 'meta-representation' may seem obscure to some. First, it is necessary (where applicable) to clarify the extent to which these kinds of constructs are entirely new - introduced here by the authors for the first time - or if they are using concepts that already exist in other research areas and leveraging these to gain a better model of safety-related neural processes.

2. Overall, the figures are useful, but the legends could be improved to clarify pertinent details. For example, Figure 1's legend should be able to explain each part of the figure - leaving no nomenclature unturned. Terms 'R1', R2, etc., are used, as is 'naïve', in parentheses, for certain areas of the figure. Please ensure there is no room for confusion, as understanding the experiment design in full helps understand the results. Most of the other figure legends have very comprehensive descriptions - which are appreciated.

3. More detailed reporting is required when outlining experiment task parameters, such as the length of the inter-trial/inter-stimulus intervals, and so forth, for all parts of the experiments. It should not be too onerous to add seconds/milliseconds in parentheses wherever applicable in-text for the methods sections. I hasten to note that the authors ought not to place too much reliance in figures or in-text descriptions exclusively, the methods should provide all relevant details, and these can be illustrated in figures as a useful adjunct, but ultimately, critical parameters ought not be missing in-text. Other examples of critical information include stimulus durations, how these varied in sync with behavioral responses (if they did), and the time gaps/breaks between task phases (even if approximate). It is a very novel and idiosyncratic set of tasks, and if one sought to replicate/translate the tasks, these details are critical.

4. As per above, please reconcile/explain using terms ITI and ISI (intertrial and interstimulus intervals) to describe a similar thing in the same paragraph (page 25).

5. The methods for designing stimuli for proper validity appear highly commendable and the detail in reporting is appreciated. Firstly, if there is any available data as to the choice of stimulus design, this could be helpful in supplementary material. Secondly, concerning post-task checks and balances, I would be interested to know if authors assured construct validity of the tasks, in terms of participants having the intended experiences. For instance, on what basis do authors feel confident that they successfully evoked and manipulated thoughts/feelings/behaviors that were selective to the construct of 'safety' versus other affective states (or merely nonaffective processing)? At a basic level, can we be confident in assuming that this study effectively evoked active safety-specific processes, rather than non-emotional/natural stimulus processing/estimations? Finally, the animals and defensive items are likely to have varied in terms of inherent pleasant/unpleasant/anxiety-arousing connotations (e.g., domestic cats compared to charismatic megafauna, or sticks compared to guns/knives). To be clear, I am strongly convinced by behavioral data (Fig2) that the stimuli and task worked well for computing the probabilities of safety/threat. Though not directly central, I wonder if the affective value (prior to any learning) may be an interesting component to prediction/estimation/meta-representation.

6. Related to the above comment, do authors confidently assert that there was/wasn't some element of associative learning occurring/interacting with their intended effects? It is hard to avoid the possibility that partial aversive reinforcement (20% shocked 'lost battle' trials) may have induced learning effects. Since the task seems to emphasize inherent properties of stimuli (e.g., assumed knowledge about the efficacy of handguns vs sticks, or danger of house cats vs lions), is there a potential influence of shock-induced associative fear/safety learning? If this is a large factor, then how much can it truly be said the task differs from 'external' threat/safety tasks (e.g., fear conditioning/extinction).

7. The 'objective' underlying probabilities of threat, concerning the different combinations of animals + defense strategies (e.g., Fig.1C) is an appealing means for ecological validity in the task (combining a bear and gun = higher % of safety versus combination of bear and stick). Since these are 'objective' and represented in discrete numerical values (64.29%, 78.57%, as examples), can authors please clarify their methods for deriving said percentages? This was a little unclear to me on first reading. The term 'objective' may also raise eyebrows. How do we deliberate on the issue of whether a bear + gun situation is objectively more/less safe than a bear + hand grenade scenario? Is the room for variability here a feature or a bug?

8. Details are provide concerning what was performed for each group, but I would appreciate clarification on why. For the task features present/absent in behavioral/fMRI groups, please explain reasons for there being a difference, the purpose of said differences.

9. Briefly, given the high spatial and temporal resolution of the fMRI acquisition (for a 3Tesla scanner, settings seem to have a combination of relatively small voxels + fast TR), please explain confidence in the quality of imaging data. What head coil (e.g., number of channels) was used? Did fmriprep provide useful quality checks that validated basic levels of data quality? Additionally, do the authors consider accelerated resolution of this kind to be a generally applicable approach or are these settings tuned to be sensitive to their specific study needs?

10. I became a little lost looking between the results, methods, and figures concerning how many ROIs were included and which analyses these were each applied to. Based on its current form, the in-text methods suggest 1 vmPFC ROI, but it also there are at least 2 as per the multivariate connectivity results (Fig.4, for example). I believe the use of multivariate searchlight analysis may obscure things. For readers grounded in more conventional fMRI approaches, clarification is needed on which results were whole-brain, ROI, or where regions were defined based on searchlight search within an ROI (or indeed across whole brain if this was done). To be specific, there needs to be clarity on how authors identified one area of voxels as being region X, and another as region Y. There are phrases in some parts of the paper that suggest whole brain searchlight identified the vmPFC, others that indicate that searchlight was more targeted. Wording must remain consistent - as to whether activations in certain areas were discovered incidentally or sought out explicitly.

11. Some areas use mathematical notation or symbols in ways that needlessly complicate simple things. Specifically, conjunction analyses. Figure 3's legend includes "Conjunction analyses for the safety prediction increasing safety > increasing danger ∩ safety meta-representation increasing safety > increasing danger for all stimuli, with overlapping activation in the vmPFC; Z=2.3, p<.05." which may be missing something. It would doubtless be easier to say there was a conjunction in activation/a shared activation in vmPFC between the two contrasts?

12. In Discussion section, the following sentence presents some obstacles to easy interpretation: "This study identifies neural systems involved in safety coding, provides evidence that Safety Prediction evokes dissociable circuits depending on whether the stimulus has self-relevance, and supports the hypothesis that the brain integrates threat and protective information to Meta-represent safety.". As with some other parts of the paper, there is perhaps too much assumed knowledge on the part of the reader as to knowing (for example) what it means 'to meta-represent' something, or what the authors are referring to by 'self-relevance' in the context of their current task. The latter is particularly important since the vmPFC is so closely linked with self-related processes - which are often very different from the authors' task. This comment is not specific to this sentence however, it may only require one area of the manuscript where such concepts or framings are precisely defined.

Minor Comments:

13. Page 3, line 57 "reducing stress, and initiating in other survival" - typo - the 'in' is probably not meant to be there

14. Page 3 line 67: "…relevant information changes so can the safety estimate even if the…" - a comma needs to separate 'estimate' and 'even' I think.

15. In some figures the brain activation images that are surrounded by small grey borders that appear to be unintended, perhaps an issue with converting file types. It should be fixed in next submission. Likewise, Figure 3 panel J seems to be showing some errors, half the axial slice is cut off (assuming this is not intentional).

Dear Dr Tashjian,

Thank you for your patience while we considered your revised manuscript "Adaptive Safety Coding in the Prefrontal Cortex" for publication as a Research Article at PLOS Biology. This revised version of your manuscript has been evaluated by the PLOS Biology editors and the Academic Editor.

Based on our Academic Editor's assessment of your revision, we are likely to accept this manuscript for publication, provided you satisfactorily address the address the following data and other policy-related requests.

* We would like to suggest a different title to improve its accessibility for our broad audience: "Subregions in the ventromedial prefrontal cortex integrate threat and protective information to meta-represent safety"

* Please include the approval/licences number of the ethical approval from the institutional review board.

* Please include information in the Methods section whether the study has been conducted according to the principles expressed in the Declaration of Helsinki.

* Please specify whether the participants provided written or oral consent.

* DATA POLICY:

You may be aware of the PLOS Data Policy, which requires that all data be made available without restriction: http://journals.plos.org/plosbiology/s/data-availability. For more information, please also see this editorial: http://dx.doi.org/10.1371/journal.pbio.1001797

Note that we do not require all raw data. Rather, we ask that all individual quantitative observations that underlie the data summarized in the figures and results of your paper be made available in one of the following forms:

1) Supplementary files (e.g., excel). Please ensure that all data files are uploaded as 'Supporting Information' and are invariably referred to (in the manuscript, figure legends, and the Description field when uploading your files) using the following format verbatim: S1 Data, S2 Data, etc. Multiple panels of a single or even several figures can be included as multiple sheets in one excel file that is saved using exactly the following convention: S1_Data.xlsx (using an underscore).

2) Deposition in a publicly available repository. Please also provide the accession code or a reviewer link so that we may view your data before publication.

Regardless of the method selected, please ensure that you provide the individual numerical values that underlie the summary data displayed in the following figure panels as they are essential for readers to assess your analysis and to reproduce it: 2A, 4BC and S2.

NOTE: the numerical data provided should include all replicates AND the way in which the plotted mean and errors were derived (it should not present only the mean/average values).

Please also ensure that figure legends in your manuscript include information on where the underlying data can be found, and ensure your supplemental data file/s has a legend.

Please ensure that your Data Statement in the submission system accurately describes where your data can be found.

* CODE POLICY

Per journal policy, if you have generated any custom code during the course of this investigation, please make it available without restrictions. Please ensure that the code is sufficiently well documented and reusable, and that your Data Statement in the Editorial Manager submission system accurately describes where your code can be found.

Please note that we cannot accept sole deposition of code in GitHub, as this could be changed after publication. However, you can archive this version of your publicly available GitHub code to Zenodo. Once you do this, it will generate a DOI number, which you will need to provide in the Data Accessibility Statement (you are welcome to also provide the GitHub access information). See the process for doing this here: https://docs.github.com/en/repositories/archiving-a-github-repository/referencing-and-citing-content

* Please note that per journal policy, the model system/species studied should be clearly stated in the abstract of your manuscript.

As you address these items, please take this last chance to review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the cover letter that accompanies your revised manuscript.

We expect to receive your revised manuscript within two weeks.

To submit your revision, please go to https://www.editorialmanager.com/pbiology/ and log in as an Author. Click the link labelled 'Submissions Needing Revision' to find your submission record. Your revised submission must include the following:

- a cover letter that should detail your responses to any editorial requests, if applicable, and whether changes have been made to the reference list

- a Response to Reviewers file that provides a detailed response to the reviewers' comments (if applicable, if not applicable please do not delete your existing 'Response to Reviewers' file.)

- a track-changes file indicating any changes that you have made to the manuscript.

NOTE: If Supporting Information files are included with your article, note that these are not copyedited and will be published as they are submitted. Please ensure that these files are legible and of high quality (at least 300 dpi) in an easily accessible file format. For this reason, please be aware that any references listed in an SI file will not be indexed. For more information, see our Supporting Information guidelines:

https://journals.plos.org/plosbiology/s/supporting-information

*Published Peer Review History*

Please note that you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out. Please see here for more details:

https://plos.org/published-peer-review-history/

*Press*

Should you, your institution's press office or the journal office choose to press release your paper, please ensure you have opted out of Early Article Posting on the submission form. We ask that you notify us as soon as possible if you or your institution is planning to press release the article.

*Protocols deposition*

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. Additionally, PLOS ONE offers an option for publishing peer-reviewed Lab Protocol articles, which describe protocols hosted on protocols.io. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols

Please do not hesitate to contact me should you have any questions.

Sincerely,

Christian

Christian Schnell, PhD

Senior Editor

cschnell@plos.org

PLOS Biology

Dear Dr Tashjian,

Thank you for the submission of your revised Research Article "Subregions in the ventromedial prefrontal cortex integrate threat and protective information to meta-represent safety" for publication in PLOS Biology. On behalf of my colleagues and the Academic Editor, Benjamin Becker, I am pleased to say that we can in principle accept your manuscript for publication, provided you address any remaining formatting and reporting issues. These will be detailed in an email you should receive within 2-3 business days from our colleagues in the journal operations team; no action is required from you until then. Please note that we will not be able to formally accept your manuscript and schedule it for publication until you have completed any requested changes.

While you attend to the requests to come, please also add a statement to the corresponding figure legends where the source data can be found. For example: "Source data can be found https://osf.io/8qg7y/." Please also specify the folder, so it's easier for readers to find the corresponding data.

Please take a minute to log into Editorial Manager at http://www.editorialmanager.com/pbiology/, click the "Update My Information" link at the top of the page, and update your user information to ensure an efficient production process.

PRESS

We frequently collaborate with press offices. If your institution or institutions have a press office, please notify them about your upcoming paper at this point, to enable them to help maximise its impact. If the press office is planning to promote your findings, we would be grateful if they could coordinate with biologypress@plos.org. If you have previously opted in to the early version process, we ask that you notify us immediately of any press plans so that we may opt out on your behalf.

We also ask that you take this opportunity to read our Embargo Policy regarding the discussion, promotion and media coverage of work that is yet to be published by PLOS. As your manuscript is not yet published, it is bound by the conditions of our Embargo Policy. Please be aware that this policy is in place both to ensure that any press coverage of your article is fully substantiated and to provide a direct link between such coverage and the published work. For full details of our Embargo Policy, please visit http://www.plos.org/about/media-inquiries/embargo-policy/.

Thank you again for choosing PLOS Biology for publication and supporting Open Access publishing. We look forward to publishing your study. 

Sincerely, 

Christian

Christian Schnell, PhD

Senior Editor

PLOS Biology

cschnell@plos.org