‘Not only faces’: specialized visual representation of human hands revealed by adaptation

Classical neurophysiological studies demonstrated that the monkey brain is equipped with neurons selectively representing the visual shape of the primate hand. Neuroimaging in humans provided data suggesting that a similar representation can be found in humans. Here, we investigated the selectivity of hand representation in humans by means of the visual adaptation technique. Results showed that participants' judgement of human-likeness of a visual probe representing a human hand was specifically reduced by a visual adaptation procedure when using a human hand adaptor but not when using an anthropoid robotic hand or a non-primate animal paw adaptor. Instead, human-likeness of the anthropoid robotic hand was affected by both human and robotic adaptors. No effect was found when using a non-primate animal paw as adaptor or probe. These results support the existence of specific neural mechanisms encoding human hand in the human's visual system.

I'm also a little concerned about the stimulus selection. Using only two exemplars for each category seems problematic, especially when they appear to be quite different in terms of lowlevel features (e.g. the paw stimuli appear to be much darker than the human hand stimuli). I appreciate that you changed the sizes of the adaptors in order to try to counteract this, but it seems to me that if the adaptors were in the same position (just larger) you might still expect low level adaptation that could transfer to the probe stimuli? I am not totally convinced that your results couldn't be explained by the differences in luminance between your stimuli and would appreciate further clarification from the authors on why they think that higher level mechanisms should be evoked.
If I understand the results on P.12 correctly, the only significant interaction effects seem to be for a small range of morphing conditions (20-35%). Some discussion of this pattern would be usefulis it what would be expected based on your stimulus design?
It would also be nice to see individual level data on Figures 3A, 4A and 5A (e.g. in the form of dots superimposed on the bar graphs) in order to get a more intuitive understanding of the variability between participants (which appears quite large, though there currently isn't any indication of what the error bars represent on these graphs).
There are a number of minor typographical errors (e.g. "we showed that participants' judgement of human-likeness of a visual probe representing (a) human hand" in the abstract on P3, incorrect font in P2 on P11, "with no differences between the four three conditions from 0-to-15% of morphing" on P12) and thus I would recommend the authors carry out a full proofread of the manuscript.
Decision letter (RSOS-200948.R0) We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.

Dear Professor Conson
The Editors assigned to your paper RSOS-200948 ""NOT ONLY FACES": SPECIALIZED VISUAL REPRESENTATION OF HUMAN HANDS REVEALED BY ADAPTATION" have now received comments from reviewers and would like you to revise the paper in accordance with the reviewer comments and any comments from the Editors. Please note this decision does not guarantee eventual acceptance.
We invite you to respond to the comments supplied below and revise your manuscript. Below the referees' and Editors' comments (where applicable) we provide additional requirements. Final acceptance of your manuscript is dependent on these requirements being met. We provide guidance below to help you prepare your revision.
We do not generally allow multiple rounds of revision so we urge you to make every effort to fully address all of the comments at this stage. If deemed necessary by the Editors, your manuscript will be sent back to one or more of the original reviewers for assessment. If the original reviewers are not available, we may invite new reviewers.
Please submit your revised manuscript and required files (see below) no later than 21 days from today's (ie 03-Sep-2020) date. Note: the ScholarOne system will 'lock' if submission of the revision is attempted 21 or more days after the deadline. If you do not think you will be able to meet this deadline please contact the editorial office immediately.
Please note article processing charges apply to papers accepted for publication in Royal Society Open Science (https://royalsocietypublishing.org/rsos/charges). Charges will also apply to papers transferred to the journal from other Royal Society Publishing journals, as well as papers submitted as part of our collaboration with the Royal Society of Chemistry (https://royalsocietypublishing.org/rsos/chemistry). Fee waivers are available but must be requested when you submit your revision (https://royalsocietypublishing.org/rsos/waivers). Comments to the Author(s) The authors used a visual adaptation behavioural paradigm to test selective mechanisms to the human hand. The results are interesting, but confirm previous evidence, rather than provide novel evidence as, instead, suggested by the authors.
The experiment is sound. The main issue to me is how results are presented. Throughout the paper, the authors repeatedly suggest that their results provide novel evidence for specialized neural mechanisms selective to process the human hand. However, this evidence was long ago reported with neurophysiology in non-human primates (Desimone et al., 1984;Gross et al., 1969) and extensively with neuroimaging in humans (Bracci et al., 2010;2013;Op de Beeck et al., 2010;Orlov et al., 2010 etc). Interestingly, an fMRI study published 10 years ago (Bracci et al., 2010) already ruled out the role of shape similarity including robotic hands among the control categories.
I would suggest toning down the writing and better acknowledge the already existing extensive evidence on this matter. For instance, the following sentences are not acceptable.
"It is still far from clear, instead, whether humans and monkeys also share specialized brain systems encoding a visual representation of hands".
"Whether similar representation is found in humans is not known".
The authors report some of the evidence in the literature, but then go on saying "However, no straightforward data are available on existence in humans of neural populations selectively encoding hand shape." The above sentences are not true and also misleading, since the authors are well aware of the existing literature. They cite the evidence but they seem to believe that "good" evidence for neural mechanisms encoding hands can be provided with behavioural data, not neuroimaging data: "Such evidence can be provided by psychophysical studies using visual adaptation". I would suggest presenting the results as confirmation of existing literature rather than novel data providing evidence from neural mechanisms not shown before.
Other misleading sentences that point to novelty rather than confirmatory evidence are the following: "Our study provides evidence that the human visual system is equipped with selective neural mechanisms encoding the visual representation of human hand".
"These results support the view that neural mechanisms representing faces can be differentiated from those representing hands, but more importantly allow to demonstrate that a shape-selective mechanism specifically coding human hand can be identified".

Reviewer: 2
Comments to the Author(s) The manuscript addresses the question of whether humans have neurons that selectively represent the visual shape of the primate hand, by conducting adaptation experiments using human hands, robot hands and non-human animal paws. The authors conclude that the fact that judgements of 'human-likeness' of a visual probe representing a human hand were reduced when using a human hand adaptor but not a robot or a non-human animal one offers support for the hypothesis that there are specific neural mechanisms encoding human hands.
The stimuli section is lacking in detail: e.g. there is no information about how many different stimuli were created, and it's not clear to me what was actually morphed (I presume items from different stimulus categories were morphed e.g. in Figure 1B, the human hand looks like it's been morphed with an animal paw to me?) I'm also a little concerned about the stimulus selection. Using only two exemplars for each category seems problematic, especially when they appear to be quite different in terms of lowlevel features (e.g. the paw stimuli appear to be much darker than the human hand stimuli). I appreciate that you changed the sizes of the adaptors in order to try to counteract this, but it seems to me that if the adaptors were in the same position (just larger) you might still expect low level adaptation that could transfer to the probe stimuli? I am not totally convinced that your results couldn't be explained by the differences in luminance between your stimuli and would appreciate further clarification from the authors on why they think that higher level mechanisms should be evoked.
If I understand the results on P.12 correctly, the only significant interaction effects seem to be for a small range of morphing conditions (20-35%). Some discussion of this pattern would be usefulis it what would be expected based on your stimulus design?
It would also be nice to see individual level data on Figures 3A, 4A and 5A (e.g. in the form of dots superimposed on the bar graphs) in order to get a more intuitive understanding of the variability between participants (which appears quite large, though there currently isn't any indication of what the error bars represent on these graphs).
There are a number of minor typographical errors (e.g. "we showed that participants' judgement of human-likeness of a visual probe representing (a) human hand" in the abstract on P3, incorrect font in P2 on P11, "with no differences between the four three conditions from 0-to-15% of morphing" on P12) and thus I would recommend the authors carry out a full proofread of the manuscript.

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Author's Response to Decision Letter for (RSOS-200948.R0)
See Appendix A.

Comments to the Author(s)
The authors addressed my comments.

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Recommendation? Accept with minor revision (please list in comments)
Comments to the Author(s) I'd like to thank the reviewers for their responses.
I have just one further question: the authors state: "As for the morphing procedure, we specify here that we did not morph together items from different categories (e.g., human hand morphed with paw) but we morphed the configural shape of each single item." I'm more familiar with adaptation paradigms where the 'morphs' are intermediate between the two categories of interest e.g. male and female face adaptation stimuli, thus creating ambiguity when a participant is asked to judge these morphs as belonging to one or the other category. However, in this case, the shape of an individual item is being morphed. Perhaps this is simply my unfamiliarity with the Photoshop terminology used, but it would be useful to explain more clearly exactly what the manipulation was designed to do -it seems that it was aimed to reduce the 'human-likeness' of the stimulus in order to create ambiguity (judging by your additional discussion of the results for the human probe/human adaptor condition). However, the paw morph stimuli in the baseline condition always seemed to have very low 'human-like' judgements -perhaps suggesting that these were fairly unambiguous stimuli and would be therefore less prone to adaptation effects? I don't think this necessarily affects the main conclusions of the paper (which predominantly seem to be based on the hand probe condition) but it would be useful to have greater clarification about these manipulations and how they might have affected the results.

Decision letter (RSOS-200948.R1)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.

Dear Professor Conson
On behalf of the Editors, we are pleased to inform you that your Manuscript RSOS-200948.R1 ""NOT ONLY FACES": SPECIALIZED VISUAL REPRESENTATION OF HUMAN HANDS REVEALED BY ADAPTATION" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referees' reports. Please find the referees' comments along with any feedback from the Editors below my signature.
We invite you to respond to the comments and revise your manuscript. Below the referees' and Editors' comments (where applicable) we provide additional requirements. Final acceptance of your manuscript is dependent on these requirements being met. We provide guidance below to help you prepare your revision.
Please submit your revised manuscript and required files (see below) no later than 7 days from today's (ie 05-Nov-2020) date. Note: the ScholarOne system will 'lock' if submission of the revision is attempted 7 or more days after the deadline. If you do not think you will be able to meet this deadline please contact the editorial office immediately.
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Thank you for submitting your manuscript to Royal Society Open Science and we look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch.

Kind regards, Anita Kristiansen Editorial Coordinator
Royal Society Open Science openscience@royalsociety.org on behalf of Dr Isabelle Mareschal (Associate Editor) and Essi Viding (Subject Editor) openscience@royalsociety.org Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) The authors addressed my comments.
Reviewer: 2 Comments to the Author(s) I'd like to thank the reviewers for their responses.
I have just one further question: the authors state: "As for the morphing procedure, we specify here that we did not morph together items from different categories (e.g., human hand morphed with paw) but we morphed the configural shape of each single item." I'm more familiar with adaptation paradigms where the 'morphs' are intermediate between the two categories of interest e.g. male and female face adaptation stimuli, thus creating ambiguity when a participant is asked to judge these morphs as belonging to one or the other category. However, in this case, the shape of an individual item is being morphed. Perhaps this is simply my unfamiliarity with the Photoshop terminology used, but it would be useful to explain more clearly exactly what the manipulation was designed to do -it seems that it was aimed to reduce the 'human-likeness' of the stimulus in order to create ambiguity (judging by your additional discussion of the results for the human probe/human adaptor condition). However, the paw morph stimuli in the baseline condition always seemed to have very low 'human-like' judgements -perhaps suggesting that these were fairly unambiguous stimuli and would be therefore less prone to adaptation effects? I don't think this necessarily affects the main conclusions of the paper (which predominantly seem to be based on the hand probe condition) but it would be useful to have greater clarification about these manipulations and how they might have affected the results.

===PREPARING YOUR MANUSCRIPT===
Your revised paper should include the changes requested by the referees and Editors of your manuscript. You should provide two versions of this manuscript and both versions must be provided in an editable format: one version identifying all the changes that have been made (for instance, in coloured highlight, in bold text, or tracked changes); a 'clean' version of the new manuscript that incorporates the changes made, but does not highlight them. This version will be used for typesetting.
Please ensure that any equations included in the paper are editable text and not embedded images.
Please ensure that you include an acknowledgements' section before your reference list/bibliography. This should acknowledge anyone who assisted with your work, but does not qualify as an author per the guidelines at https://royalsociety.org/journals/ethicspolicies/openness/.
While not essential, it will speed up the preparation of your manuscript proof if you format your references/bibliography in Vancouver style (please see https://royalsociety.org/journals/authors/author-guidelines/#formatting). You should include DOIs for as many of the references as possible.
If you have been asked to revise the written English in your submission as a condition of publication, you must do so, and you are expected to provide evidence that you have received language editing support. The journal would prefer that you use a professional language editing service and provide a certificate of editing, but a signed letter from a colleague who is a native speaker of English is acceptable. Note the journal has arranged a number of discounts for authors using professional language editing services (https://royalsociety.org/journals/authors/benefits/language-editing/).

===PREPARING YOUR REVISION IN SCHOLARONE===
To revise your manuscript, log into https://mc.manuscriptcentral.com/rsos and enter your Author Centre -this may be accessed by clicking on "Author" in the dark toolbar at the top of the page (just below the journal name). You will find your manuscript listed under "Manuscripts with Decisions". Under "Actions", click on "Create a Revision".
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Decision letter (RSOS-200948.R2)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.

Dear Professor Conson,
It is a pleasure to accept your manuscript entitled ""NOT ONLY FACES": SPECIALIZED VISUAL REPRESENTATION OF HUMAN HANDS REVEALED BY ADAPTATION" in its current form for publication in Royal Society Open Science.
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