PONE-D-24-53823Symbolic and Non-Symbolic numbers differently affect centre identification in a number-line bisection taskPLOS ONE

Dear Dr. Porru,

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<!--StartFragment--> I have now received comments from two reviewers with expertise in the field and also read your paper carefully myself. The reviewers’ comments are appended below. Although both Reviewers concur there is merit in your work, they also raise several conceptual and methodological concerns that prevent publication in its current form. I encourage you to resubmit a revised version of your manuscript where to address each of the points raised by the Reviewers, paying particular attention to align the purposes with the methods used as pointed out by Reviewer 1 and providing robust justification for the methods as emphasized by both Reviewers. You should also try to extend the Discussion section so to provide a more comprehensive interpretation of your results. 

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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: No

Reviewer #2: Yes

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

Reviewer #1: No

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: Yes

Reviewer #2: 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: 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: Summary

The study evaluated effects of task-irrelevant numerical flanker features while 136 neurotypical adults bisected lines in 4 experiments. Experiment 1 (N=36) compared flanker pair 8-2 in symbolic and non-symbolic formats in two orders. Left bias was largest for large left non-symbolic condition. Experiment 2 (N=36) studied perceptually symmetrical symbolic and non-symbolic flankers. Bias was positive for symbolic and negative for non-symbolic flankers, and again largest for large left non-symbolic condition. Experiment 3 (N=36) studied font and spacing asymmetries for symbolic and non-symbolic 8. Again, largest left bias obtained for large left non-symbolic condition. The final Experiment 4 (N=36) replicated font and spacing asymmetries for symbolic and non-symbolic 2. Again, largest left bias obtained for large left non-symbolic condition. The results are taken to speculate about format-specific representation of number-space associations.

Evaluation

Although the experiment examined interesting perceptual conditions and wisely included a baseline assessment, I have conceptual and methodological problems with the current report that prevent me from recommending publication.

Main problems

0. Let me begin by providing relevant context that is missing from this manuscript. The line bisection task, naively adopted by neuropsychologists for efficient bed-side diagnostics (like other seemingly simple tasks, e.g., Corsi blocks), has long been studied to understand sensory, cognitive, and motor contributions to performance. Perceptual flankers were added to manipulate visual attention prior to bisection by demanding their verbal reporting. Ever since, perceptual and attentional contributions to flanked line bisection performance were discussed, but I have never encountered a discussion of conceptual representations in the context of flanker manipulations. This brings me to my first (conceptual) problem.

1. The objectives and methods seem misaligned. The main effect of number flanker format is taken to argue for distinct representations (e.g., line 327f, 402f). But the bisection task measures sensory/perceptual (and also motor) but not conceptual factors. This is evident in all four results (largest stimulus extent on the left creates largest left bias) and in the wider literature. Thinking of the bisector placement as the participant’s subjective balance point of a scale (the line), what can the effect of placing a picture of an elephant or ant vs the word elephant/ant reveal about the cognitive representation of elephants? Results will depend on the font size, picture size, etc. Consistent with this assessment, the present report identifies perceptual grouping effects, as did Fischer (1994). This study should be added to the review and the interpretation of results adjusted.

Fischer MH. Less attention and more perception in cued line bisection. Brain Cogn 1994;25:24–33

1. Turning to methodological issues, I believe statistics can be improved. Effects should be computed relative to the baseline bisection, not relative to zero. ANOVA should (also) be performed on subject means (as was the t-test, i.e. with 33 degrees of freedom). Importantly, the factor line length (cf line 211) should be included to reduce error variance and increase effect sizes (over which the authors complain in line 388).

2. The literature review is suboptimal because of unclear formulations (see my many minor comments below). Readers would benefit from stating explicitly the two main effect patterns reported in this literature, namely a tendency to orient towards larger surface flankers and towards larger symbolic magnitudes.

3. Hypotheses should also be stated more clearly. For Experiment 1, the expectation is not “similar results” but “similar bias towards the larger number”. For Experiment 2 the hypothesis is not accurate bisection because of the extensive evidence for pseudo-neglect, which should be explained better and explicitly credited to Jewell & Mc Court (2000) in line 82 as part of the literature review, and again in lines 130 and 144.

4. I was surprised to see the study was conducted on paper (136 participants x 132 trials with single lines per page = 17,952 sheets of paper!). Since several of the experiments reviewed as background were conducted on computer displays, the authors should justify their decision and discuss whether the medium of testing might have contributed to the alleged heterogeneity of results (in the Intro) as well as to their own findings (in the Discussion). Luh et al. (1995) is an example of a comparative bisection study.

Luh, K. E. (1995). Line bisection and perceptual asymmetries in normal individuals: What you see is not what you get. Neuropsychology, 9(4), 435–448. https://doi.org/10.1037/0894-4105.9.4.435

5. The authors feel compelled to invoke left-lateralized cortical symbol processing (lines 341 ff) to account for right-bias with symbolic numbers. First, this incomplete argument should be augmented by explaining to readers (a) the cross-lateralization of visual processing, (b) the relation between hemispheric activity and attention deployment, and (c) the link between attention deployment and subjective line extent. Secondly, the “right bias” is only a relative bias, depending on baseline performance (which incidentally depends on line length). The same relativity holds for “left bias”, which is on top of the hemi-neglect bias. Awareness of this compound nature of bisection effects should inform the interpretation.

Minor issues

Line 68: the subitizing range is irrelevant for symbolic number processing, so the authors need to clarify that the study used non-symbolic numbers. Moreover, 9 is not within the subitizing range.

Line 80: the authors should describe the number position effect in full to be more useful for the readers. In general, the entire description can be improved (e.g., line 83: near or far from what? Line 81: what is a lateral disparity?)

Line 99: what is the comparison task?

Line 110: closer to what?

Line 146: the expression “denser in the line proximity” contains two aspects (density and proximity) – please clarify.

Line 166: Was handedness self-reported or measured?

Line 182: Clarify whether the baseline was always the first four trials (as implied by the word “phase”) or whether these were within the four blocks.

Line 209: what is “gsm” and which part of a stimulus was printed in white (assuming the pages were white)?

Line 201: Lines were centered on the page –inviting use of the page as spatial reference and potentially reducing experimentally induced effects. The authors might want to acknowledge this.

Line 220: this is the only mention of an “invisible square” – what was its purpose etc? I assume the answer will also clarify in what sense flanker digits for horizontally displayed lines can be horizontally centered” (would this not mean they appear above the line?).

Line 262: Report baseline mean and add the unit mm to all means.

Line 284: this sentence is incomplete.

Reviewer #2: The manuscript (PONE-D-24-53823) presents a Research Article treating four line-bisection experiments which feature different kinds of numerical flankers, either symbolic or non-symbolic in format. Stronger leftward biases on line bisection are found in the non-symbolic format, leading the authors to conclude that processes akin to pseudoneglect might account for this; whereas the rightward biases encountered in the symbolic format are said to reflect an organized representation of numbers.

While the manuscript is well written, the design carefully thought and statistical analyses adequately done, I have some major concerns that should in my opinion be addressed before proceeding with the publication process. Therefore, my suggestion is to carry out a major revision.

1. I have some doubts about the methodology – detailed below – which do not necessarily question the soundness of the study but that should be addressed, nonetheless.

2. The manuscript is overly brief when detailing the experimental manipulations and their operational relevance to test the study’s hypothesis.

3. I did not see a great effort in discussing the two main results, which crucially differ depending on the flanker’s format (left/right-ward biases for symbolic/non-symbolic), jointly. In the end, each of them is traced back to theoretical explanations that are not novel (i.e., pseudoneglect and scanning habits). While this is not bad in principle, I think the great work carried out to design this study deserves to be discussed in little more depth.

Detailed comments

• Line 61-62: this statement is quite generic and seems unnecessary.

• Lines 65 – 66: what do the leftward and rightward biases substantiate in?

• Line 80: the results from this experiment are not clear. It is particularly not clear what “the position of the strings on the sheet” means.

• Lines 76 – 91: I do not think that dedicating an entire paragraph to one single study is justified here, especially because it comprises many studies, the results of which cannot be addressed in detail for obvious reasons. I suggest to briefly outline the rationale behind the study and skipping to the general conclusion as at lines 89 – 91.

• Lines 110 – 111: the relationship between Gebuis and Gevers’ and the claim that introduces the paragraph (“distinction between symbolic and non-symbolic numbers’ representations” – line 106) is not clear.

• Lines 185 – 187: “Moreover … of the line”. This is not clear. What is the most “prominent” flanker? Why would it be placed on the left side of the line only? Please articulate.

• Figure 1 is in low resolution; moreover, it is overly concise. I think splitting it into two figures would make more sense.

• Lines 221 – 236: referencing to the figure and panel would help. Please also add a title for the very first column in figure 1 panel a – in which are the definitions Baseline, Small-left, Large-left…

• Lines 221, 223, 226 and any other occurrence: Please be consistent when referencing to the flankers: either use flankers, flanker stimuli or stimuli.

• Lines 223 – 224: “The stimuli “2-2” were arbitrarily considered the Small-left orientation and the “8-8” were considered the Large-left orientation” I do not understand this arbitrary choice, more so because I did not understand the rationale for this manipulation as explained at lines 185 – 187.

• Lines 332 – 335 “The behaviour in the Non-symbolic format is compatible with pseudoneglect” Was the leftward bias at all present in the symbolic format, though less intense? If that is the case, then I suggest rephrasing.

• Lines 357 – 364: I found this paragraph quite confusing, particularly so because there is an overlap between the terminology used by the authors (i.e., “scanning from left-to-right”) and what we usually refer to when talking about cultural-linguistic scanning. The authors should clarify if they are treating a cultural habit or a situational behaviour. In the latter case, I still believe the cultural bias for that should at least be acknowledged.

• Lines 368 – 369: “a right-to-left direction bias possibly counteracts the pseudoneglect phenomenon, yielding responses closer to the midpoint”. This was addressed a little too superficially. What would motivate such a bias?

General comments

• Procedure – this part could go into little more detail. How long were the lines? How big were the flankers? What were the numerosity of the flankers (both for symbolic and non-symbolic) How long on average did it take to hand the single sheets? Why was a pencil-and-paper version choosing instead of a digital one?

• *Edit* - while continuing reading, I realized the details missing in the procedure are actually in the Stimuli and Materials. I am leaving my prior comment because readers could get confused like me for not being able to read these details before the procedure. Please consider switching the two sections (Stimuli first – procedure second)

• I think a further justification for the paper-and-pencil methodology other than citing De Hevia and Spelke is needed. This choice leaves me puzzled as I believe precision in collecting and coding data has been undermined by this choice. How have potential biases or human errors been prevented? For instance, I am worried that letting participants carry the bisection manually could have let them adjust their hand position as to securely bisect it more or less at the same spot. Please elaborate on this.

• I would have appreciated a very brief introduction and/or discussion section for each experiment, summarizing the conditions in focus and the relevance of the results from each experiment for the hypothesis that informed its design. It would help focus the relevance of each experiment for the general research question.

Typos and minor concerns

• Line 73: “systematically bias”

• Line 192: You used “numerosity 8” and “numerosity two”. Please homologate the format.

• Line 284: “Moreover, deviations from the objective center of the line.” I guess this sentence is incomplete.

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Reviewer #1: No

Reviewer #2: No

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Symbolic and Non-Symbolic numbers differently affect centre identification in a number-line bisection task

PONE-D-24-53823R1

Dear Dr. Porru, I am 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.

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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 #2: 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

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

Reviewer #1: Yes

Reviewer #2: 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

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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 #2: 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: The authors did a good job of addressing the reviews and have improved the ms accordingly. The revised Introduction is much more useful and appropriate, especially in clarifying the impact of conceptual influences on bisection. Hypotheses now flow more naturally from this review.

My two minor observations can easily be addressed during proof-reading.

L 397-9 seem contradictory, please check

L 501 The Fias reference lacks co-authors.

Reviewer #2: (No Response)

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Reviewer #1: No

Reviewer #2: No

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