PONE-D-23-20708Frequencies and functions of vocalizations and gestures in the second year of lifePLOS ONE

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Reviewers' comments:

Reviewer 1

The manuscript consists of a timely study that challenges gesture-first theories of language. The paper begins with a review of the literature, suggesting that gesture-first theories are in the ascendency. The authors then argue that on the contrary, it is more plausible that humans had selection pressure to use vocalisations more (compared at least to other primates). The main hypothesis of the paper is that if earliest language was gestural – we should expect extensive gestural use early in development, whereas if vocal – we should expect extensive vocal use in development. Comparisons between gestures and vocalisation (on any metric) are difficult, so I appreciate the authors descriptive framework (universal, non-social and conventional vocalisations/gestures). The sample size is quite small, but the observation time is densely sampled (60 mins total per participant across three sessions), so this is still very informative. The results suggest that (a) non-social vocalisations are far more frequent than non-social gestures (which does not change across the second year of life), (b) neither universal vocalisations and gestures were more frequent across the second year of life, however the tendency was that vocalisations decreased across the year and gestures increased, (c) conventional vocalisations were more frequent and increased across the second year of life, while conventional gestures were infrequent and did not increase/decrease over time, and finally (d) all types of gestures were more likely to be directed to caregivers (as indexed by gaze) than vocalisations across the second year. The paper is well written and important. I agree with the authors that clearly the claim that early gesture use exceeds vocalisation use in human infancy is unfounded in light of this (and other data). I especially enjoyed the discussion of action gestures – and share the authors concern that “true gestures” are often confounded with actions (that are not gestures) simply because they could one day be ritualised into gestures, but that does not mean that they are true gestures when first used by infants! The same concern might also be applied to chimpanzee gesture coding schemes (which often include manual actions, e.g., splashing, as gestures…). I do however have a number of concerns about the manuscript in its current form, detailed below. I think that all the issues I raise are surmountable and very much look forward to reading a revised manuscript. Introduction and literature review There is a missing step in the introduction. I agree that there are proponents of gesture-first theories (and there is a sense at which many in the field simply uncritically appear to accept it as a premise), but there also vocal detractors (excuse the pun). It would be of benefit to the introduction to include an extra section detailing the body of opinion and evidence already amassed that disputes gesture-first theories (in the Vocalization and Gesture in Communicative Evolution section) before launching into the evolutionary argument (Natural Selection of Communicative Signals). For example, there was a review paper suggesting that great ape data simply doesn’t support a gesture-first or vocal-first origin story (Frohlich et al., 2019 https://doi.org/10.1111/brv.12535). On the developmental side of things, there are other papers that provide evidence that would already count against gesture-first theories (e.g., McGillion et al., https//doi.org/10.1111/cdev.12671 ; Donnellan et al., 2020 https://doi.org/10.1111/desc.12843; McCune et al., 2020 https://doi.org/10.1177/014272372096682; Werwach et al., 2021 https://doi.org/10.1016/j.infbeh.2021.10158). Inclusion of a review of the literature that already supports the authors main contention would strengthen the manuscript. Universal vocalisations I am unconvinced by the justification in the current manuscript that crying, whimpering and laughter should be included as universal vocalisations. Universal gestures are defined with respect to communicative intent, however it’s not clear that laughter, crying and whimpering necessarily are produced with communicative intent (though it’s possible – and of course whether deictic gestures are always produced with communicative intent is debatable too [examples of children pointing for themselves, see Delgado, Gómez, & Sarriá, 2011: https://doi.org/10.1016/j.jecp.2011.04.010]). I am much more convinced that non-word conversational protophones should be included as universal vocalisations – both these and deictic gestures promote engagement with caregivers (and more richly, could be intended for this purpose on the part of the infant – though this is speculative [but often a speculation accorded to gestures only in the literature…]). I don’t quite see the parallels between deictic gestures with affective vocalisations – I do take the authors point that they have a modality-specific function (expressing affect -which gestures can’t do without a lexicon), but that doesn’t seem a good enough reason to group them together, especially because they are also grouped with vocalisations that might not have a modality-specific function (e.g., the conversational protophones – whose function [promoting engagement] could also be achieved gesturally [e.g., through showing: holding up objects to caregivers face, or even pointing at caregivers]). I think as it stands, universal vocalisations should consist either of crying, whimpering and laughter (to maintain the modality-specific function element) or conversational protophones (to capture the engagement element). As is stands, the justification is clearer to me that it should just be the protophones. Measures Sessions are approximately 20 mins (not exactly 20 mins). This makes it hard to interpret individual differences across participants. How close to 20 minutes are the sessions? If within a few seconds, this is fine. However, if discrepancies are significant, behaviours should be standardized as rates per minute (or rate per 20 mins if preferred) instead of as a raw frequency. The analysis should then be re-run with these standardized measures. Reliability Coding The first paragraph of Coder Agreement and Outcomes section is unnecessary – this is simply reporting reliability statistics from a previous study – not the current study – and it’s not clear why this is relevant. I suggest simply omitting this. The final paragraph concerning categorisation of vocalization and gesture into the three functional categories requires more quantitative reliability metrics. I agree (by eyeballing the plot) that this probably shows pretty decent agreement, however, given the way in which the data was coded, it should be possible to calculate kappa reliability (if I have understood the coding procedure correctly). I.e., for the instances in which both coders agreed there was a vocalisation or a gesture – did they agree on the function (i.e., did they both select an illocution code that would be categorized as the same function?). Overall though, it should be noted that reliability correlations currently reported are solid (and my impression is that the kappas will equally demonstrate this) – I have no doubt that this coding has been conducted meticulously, but there do need to be missing metrics included to confirm this. Analysis GEE does seem an appropriate method for the data, and likewise Mann-Whitney tests are appropriate in my understanding. The GEE models obtained need to be included in the paper in full (if even simply in tables in an Appendix). Data visualisation Given that the authors rely heavily on the visualisation of the data to do much of the work of inference (and correctly so I think!), the visualisations need work. I have a few suggestions: Distributions of data: Ideally the distribution of the datapoints themselves needs to be included (not just 95% confidence intervals and a mean). Numerous ways exist to do this, for example violin plots, half violin plots, and/or plotting the individual data points. E.g., in figures 3, 4 and 5, individual points (representing a participant) could be plotted in red at each time point for vocalisations and in blue for gestures, with summary statistics (mean/CIs) overlaid, perhaps in tandem with violin plots or similar. Model predictions. Equally, if GEE was implemented using an R package, it may be possible to plot model predictions (perhaps overlaid) on the figures automatically – which would be very informative. Of course it is possible to do manually, but this does require expertise. This is becoming much more of a standard approach to visualising data (i.e., raw scores – as you have here, overlaid with model estimates). This would really support the authors inferences from the data and I would strongly encourage that they attempt this. This would also remove the need for caveats to be included in the figure captions about what the model shows compared to what the figure seems to show (i.e., the confidence intervals relating to the the raw data not the SE/confidence intervals of the model predictions). Figures 2 and 5. For hypothesis 2 a pie chart is not informative – as the data comes from individual participants. Please plot the data so that the range of proportions of gestures/vocalisations combined with eye gaze across participants is clear (i.e., in the same way as figures relating to hypothesis 1 would be fine, y axis being proportion). As currently plotted, the discrepancy between the data and the model results (especially with regard to 20m timepoints) is hard to interpret (i.e., why there is a main effect of modality, not an interaction with age). Likewise for figure 2, though there are error bars, perhaps consider plotting the individual data points with summary statistics overlaid as suggested above (i.e., not as a bar chart), so the distribution of individual participant data can be observed. Data availability Currently the data for the study is not available. I agree fully that videos cannot be shared. It is perfectly possible however to have the data worksheet mentioned in the availability statement already uploaded into the OSF for example (it can be a private submission), with an anonymized link provided only for the purpose of review. Minor comments Fig 3. - Typo on y axis label (“Frequancy”). Also figure 3 error bars do not look correct and it’s a screenshot containing “plot area” in a box over the middle. I have suggested more substantial changes to figures above. Typo – in the Results paragraph headed “Hypothesis 1a”, the text refers to Figure 5 – this should be figure 3? Please include in the limitations section that this is still a relatively small sample size (in terms of unique participants). Please include line numbers in the next draft to facilitate navigating the text and providing feedback. When discussing possibility of vocalization to serve function of communication very early in development (e.g., page 2) consider citing Papaeliou et al., 2002 (10.1044/1092-4388(2002/024)) and/or Esteve-Gibert & Prieto 2012 (https://doi.org/10.1017/S0305000912000359 ) as these seem relevant too. Note, I could not see tables in my copy of the manuscript. Whether this is an editorial mistake or submission mistake, I’m unsure (I contacted the journal to ask, but didn’t get a response before submitting my review to the deadline). I look forward to reviewing the content of the tables in the next round of review.

Reviewer 2

INTRODUCTION

The literature review is relevant. The paper is clearly written with bibliographic references on par with latest published research. On page 14 the authors provide a clear description of the different types of gestures. I was glad to this this section because it provided a clear pathway to the information following the introduction.

I suggest that the authors provide the term typically developing (experimental group) earlier in the paper.  

METHODOLOGY

I am not clear on how toddlers were examined to exclude developmental/language challenges.  Dis children have any episodes of otitis media? Where they excluded? I mention this because under such circumstances even children and toddlers with neurotypical development use more communicative gestures to compensate for reduced vocal output and compromised hearing sensitivity.

Is there a rater-to-rater reliability score?  

RESULTS

No comments. This is a qualitative examination and data are clearly presented in a via descriptive non-param stats.

DISCUSSION

The authors describe the important performance and provide adequate explanation of the findings.  The writing is clear and the manuscript is well-organized.

The authors provide adequate description of findings which are combined with plausible interpretations.

The novelty of the study rests on the fact that the findings contribute and enrich the  database related to vocalizations versus gestures.

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Reviewer's Responses to Questions

Comments to the Author

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

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Reviewer #1: Yes

Reviewer #2: Yes

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

Reviewer #1: Yes

Reviewer #2: Yes

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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: No

Reviewer #2: Yes

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

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Reviewer #2: Yes

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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: Please see attached comments (in pdf form). Overall, my review is positive, but there are revisions that need to be made in order to get the manuscript into publishable form. I look forward to the next round of review.

Reviewer #2: I would like to thank you for giving me the opportunity to review this interesting and well-written piece of work. As per your request, here is the review of PONE-D-23-20708 entitled Frequencies and functions of vocalizations and gestures in the second year of life”. My recommendation is “Acceptance with minor changes”. My comments are mostly related to methodological and stylistic issues. The paper focuses on the critical theme related to the importance of gestures vis a vis vocalization as early indications of the origins of language. It follows a previously conducted study, with the current paper adding and supporting evidence of their theoretical EVO-DEVO framework. The theoretical framework of observation highlights the fact that more than ¾ of gestures across these second-year data were deictics (e.g., pointing and reaching), acts that while significant in supporting the establishment of referential vocabulary in both spoken and signed languages, are not signs, but have single universal deictic functions in the here and now. In contrast, words and signs are functionally flexible, making possible reference to abstractions that are not bound to any “particular illocutionary force nor to the here and now”. This topic is long overdue given the scarcity of data available from such topics related to the origins of language.

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Reviewer #1: No

Reviewer #2: No

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Attachments

Attachment

Submitted filename: PLoS One Sep 2023.pdf

Attachment

Submitted filename: PLOS VOCALIZATIONS AND GESTURES.doc

PONE-D-23-20708R1Frequencies and functions of vocalizations and gestures in the second year of lifePLOS ONE

Dear Dr. Burkhardt-Reed,

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.

Please submit your revised manuscript by Jul 12 2024 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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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

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Reviewer #1: (No Response)

Reviewer #3: (No Response)

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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 #3: Partly

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

Reviewer #1: Yes

Reviewer #3: Yes

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

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Reviewer #1: Yes

Reviewer #3: Yes

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

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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: Please see my attached comments for one final minor revision. Overall, again my review is very positive.

Reviewer #3: The paper in general is very well written. I do have a few considerations though, regarding methodology and the conclusions. My comments are shown below:

• Line 96: it has been reasoned that in the massive amounts of vocal explorations …

METHODOLOGY/RESULTS

• I have an issue with the inclusion of the category “social plauful” in Universal vocalizations. Although animal sounds or object sounds are echopoietic, they are still conventional across languages, so they are the product of learning/associations. I would have liked to know what the percentage of this category is in Universal Vocalizations across ages, and also I suspect that the fact that there is no significant difference between gestures and vocalizations in the Universal category is due to the fact that this subcategory was wrongly included in Universal instead of conventional. When this happens the difference should become evident.

• The fact that the two coders agree only at .8 and .65 regarding social/non-social interactions creates a problem. The percentage is very low. (line 523/524). This suggests highly ambiguous data and problematic categories. Can you please explain how you handled this problem?

• I cannot see any figures in the Revision 1 attachment. Seeing them in the initial submission, please add descriptions of the axes in all figures

DISCUSSION

• I also have an issue with the discussion about what constitutes a gesture and a general utilitarian movement. Also I would like to bring into this discussion the issue of development of fine motor skills which takes time for infants, even in the first year of life. In other words, gestures involve fine motor movements, which take longer to develop than gross motor movements and sounds (a bit like having protophones and language sounds). So although there are movements with the hands, they are not considered gestures, as the latter take long to be produced in a controlled manner.

• A final issue concerns a point acknowledged in the limitations section – there are no data from deaf children and this suggests that we cannot know whether the patterns observed are due to an evolutionary reason or whether a special emphasis on vocalizations is the result of communicative reinforcement from parent interactions throughout the first year. So if we are going to talk about an evolutionary perspective emerging from the current data, we need to account for the fact that parents respond to vocalizations from day 1 but not to gestures. So how do we know that the current data, and preference for vocalizations over gestures, are the result of a genetic/evolutionary basis of language and not just conditioning/reinforcement for sounds instead of gestures?

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Reviewer #1: No

Reviewer #3: Yes: Loukia Taxitari

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Attachments

Attachment

Submitted filename: PLoS One Re-review 21_03_2024.docx

Frequencies and functions of vocalizations and gestures in the second year of life

PONE-D-23-20708R2

Dear Dr. Burkhardt-Reed

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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 #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 #3: Yes

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

Reviewer #1: Yes

Reviewer #3: Yes

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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 #3: Yes

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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 #3: Yes

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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: Thanks again for your positive response to my (and the others reviewers') comments. I am fully satisfied that my only outstanding comment about the figures from the previous review has been addressed.

Reviewer #3: Thank you for addressing all my comments. I really like your paper! I would still like the conditioning discussion to be part of the text, as I'm sure other people will have the same question. It's a pity to leave it out. Thank you for a quick and fruitful correspondence.

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Reviewer #1: No

Reviewer #3: Yes: Loukia Taxitari

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