Structural Connectivity of the Substantia Nigra: A Comprehensive Review of Diffusion Imaging and Tractography Studies

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors of the paper titled Structural Connectivity of the Substantia Nigra: A Comprehensive Review of Diffusion Imaging and Tractography Studies have significantly advanced our understanding of SN connectivity and its crucial role in motor, cognitive, and emotional brain functions. The authors also stated that tractography can be a promising tool for both clinical practice and research. Thirty-two studies were included in the analysis for this review. In order to improve the manuscript, please correct the following points.

Point 1. Please change the order of Figures 1 and 2, and adjust the text in the introduction section accordingly.

Point 2. Including the number of papers analyzed in the materials and methods section would be helpful.  You can move the paragraphs from the result section here.

Point 3. Please remove text from lines 132 to 137. It is not necessary to put these lines in the result section.

Author Response

We sincerely thank the reviewer for the careful reading of our manuscript.  We appreciate the positive feedback regarding our contribution to understanding substantia nigra connectivity and the potential of tractography for clinical and research purposes.

Below, we provide detailed responses to each of the reviewer’s comments. We have revised the manuscript accordingly to address all points raised.

Point 1. Please change the order of Figures 1 and 2, and adjust the text in the introduction section accordingly.

We have revised the order of Figures 1 and 2 as requested. The text has been updated to ensure that figure references are consistent and accurate.

Point 2. Including the number of papers analyzed in the materials and methods section would be helpful.  You can move the paragraphs from the result section here.

We have moved the relevant paragraphs from the Results section to the Materials and Methods section, and clarified the number of papers analyzed to improve the structure of the manuscript.

Point 3. Please remove text from lines 132 to 137. It is not necessary to put these lines in the result section.

We have removed the text from lines 132 to 137 as suggested to improve the clarity and focus of the Results section.

Reviewer 2 Report

Comments and Suggestions for Authors

In this manuscript, the authors review the structural connectivity of the substantia nigra (SN) in humans based on findings from diffusion tensor imaging and tractography. The review summarizes the classical pathways and recently identified connections, providing insights into the role of the SN in motor control and neuropsychiatric disorders. Overall, the manuscript is well-written and informative.

I have two suggestions that may strengthen the manuscript:

• Expand the discussion of connectivity changes in disease state: How specific pathways are altered in pathological conditions, and how the disruptions may contribute to clinical symptoms.
• Consider including a discussion on how structural connectivity studies could advance individualized diagnosis and therapeutic strategies.

Author Response

We are grateful for the reviewer’s thoughtful and constructive feedback on our manuscript. We appreciate the recognition that the review is informative and well-prepared, and we are pleased that our summary of classical and newly identified pathways, as well as the discussion on tractography’s potential in clinical and research contexts, was found valuable.

Comments 1: I have two suggestions that may strengthen the manuscript: Expand the discussion of connectivity changes in disease state: How specific pathways are altered in pathological conditions, and how the disruptions may contribute to clinical symptoms. Consider including a discussion on how structural connectivity studies could advance individualized diagnosis and therapeutic strategies.

We have expanded the Discussion (lines 419-434) to describe how specific SN pathways are altered in pathological conditions and how these changes relate to clinical symptoms. We also discuss the potential of structural connectivity mapping to support individualized diagnosis and therapeutic strategies.

 

Reviewer 3 Report

Comments and Suggestions for Authors

Bublíková and colleagues propose an interesting review of the studies regarding the structural connectivity of the two components of the substantia nigra (SN), the pars compacta (SNc) and the pars reticulata (SNr) component. The authors focused on tractography techniques for investigating anatomical connectivity and the available studies with these techniques in humans, following the recent criteria for preparing a systematic review. Using this approach, the authors reported evidence demonstrating an extended direct connectivity of SN, in addition to the classical nigro-striatal and -thalamic connections, with limbic, cortical, subcortical areas, and the hippocampus.

I particularly appreciated the inclusion of studies highlighting a relationship between the integrity of structural connectivity and the symptomatology in Parkinson's disorder (PD), which led the authors to suggest the potential value of the reviewed techniques as tools for the diagnosis and insights into potential benevolent rearrangements of connectivity in response to therapy for PD.

However, I have a major comment related to some limitations that should be addressed to avoid reducing the overall impact of the manuscript. I therefore recommend major revisions before further consideration for publication.

Major comment:

I think the manuscript would improve its impact by including a section in the discussion suggesting brain areas that are potential candidates for connectivity with SN and its components, and are relevant to the pathologies cited. This would provide a valuable framework for directing future studies in humans toward investigating such potential connectivity.

In this regard, a subthalamic area, called the “Zona Incerta” (ZI), has raised interest as a structure possessing anatomical connectivity with SN in animal models and as a potential target for the therapeutic treatment of PD in humans. Ossowska (2020, 10.1007/s00415-019-09486-8) by considering some findings in humans on the therapeutic effect on PD motor symptomatology caused by deep brain stimulation (DBS) on ZI, she proposes in light of the widespread anatomical connectivity between ZI and SNr and SNc in animal models that such therapeutic effects are mediated by restoring the normal functioning of the SN–ZI–thalamus–striatum network induced by DBS on ZI. Consistent with this hypothesis, Londei et al. (2024, https://doi.org/10.1016/j.isci.2023.108761) demonstrated that ZI neurons exhibit functional connectivity with the SNr and thalamic nuclei, as well as with the caudatoputamen, forming recurrent connectivity mechanisms reentrant to ZI or in the aforementioned areas. Collectively, these evidences, along with other brain areas potentially connected with the SN components and relevant to the pathologies discussed, would strengthen the manuscript.

 

Minor comments

• Figure 2 is called before Figure 1 in the text.
• Figure 1 legend: Panel A appears to be an illustration taken from reference 28. I recommend reporting this information more clearly in the legend.
• Line 159: If I did not misunderstand, there should be a typo in the word “imag-hese”.
• Line 239: missing reference for “Carmichael et al.” study.
• Line 251: Double space.

Author Response

We sincerely thank the reviewer for the positive and encouraging feedback on our manuscript. We are pleased that the systematic approach, the emphasis on tractography techniques, and the exploration of both classical and extended SN connectivity patterns were appreciated. In the following, we address the reviewer’s specific comments and suggestions in detail.

Comments 1:

I think the manuscript would improve its impact by including a section in the discussion suggesting brain areas that are potential candidates for connectivity with SN and its components, and are relevant to the pathologies cited. This would provide a valuable framework for directing future studies in humans toward investigating such potential connectivity.

In this regard, a subthalamic area, called the “Zona Incerta” (ZI), has raised interest as a structure possessing anatomical connectivity with SN in animal models and as a potential target for the therapeutic treatment of PD in humans. Ossowska (2020, 10.1007/s00415-019-09486-8  by considering some findings in humans on the therapeutic effect on PD motor symptomatology caused by deep brain stimulation (DBS) on ZI, she proposes in light of the widespread anatomical connectivity between ZI and SNr and SNc in animal models that such therapeutic effects are mediated by restoring the normal functioning of the SN–ZI–thalamus–striatum network induced by DBS on ZI. Consistent with this hypothesis, Londei et al. (2024, https://doi.org/10.1016/j.isci.2023.108761) demonstrated that ZI neurons exhibit functional connectivity with the SNr and thalamic nuclei, as well as with the caudatoputamen, forming recurrent connectivity mechanisms reentrant to ZI or in the aforementioned areas. Collectively, these evidences, along with other brain areas potentially connected with the SN components and relevant to the pathologies discussed, would strengthen the manuscript.

Response: 

- We thank you for the insightful suggestion. We have expanded the Discussion (lines 444-455) to include a paragraph highlighting the zona incerta (ZI) as a potential node within SN related networks, referencing Ossowska et al. (2020) and Londei et al. (2024). This addition emphasizes the relevance of ZI–SN connectivity in the context of Parkinson’s disease and supports future research directions.

Minor comments:

Figure 2 is called before Figure 1 in the text.

- We appreciate the reviewer’s careful reading. We have thoroughly reviewed the manuscript and ensured that all figure references appear in the correct order in the text. Specifically, we have adjusted the citations so that Figure 1A, Figure 1B, and Figure 2 are referred to in proper sequence.

Figure 1 legend: Panel A appears to be an illustration taken from reference 28. I recommend reporting this information more clearly in the legend.

- We have revised the Figure 1 legend to provide a clearer attribution. It now reads: “Schematic representation of the nigro-thalamic system. Reproduced from Cirillo et al., 2025, Frontiers in Neural Circuits, CC BY 4.0.”

Line 159: If I did not misunderstand, there should be a typo in the word “imag-hese”.

- We have corrected “imag-hese” to the intended term “imaging” in the revised manuscript.

Line 239: missing reference for “Carmichael et al.” study.

- We have added the reference for the “Carmichael et al.” 2021 study in the revised manuscript and revised the manuscript reference list.

Line 251: Double space.

- We have removed the unintended double space in line 251.

 

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have addressed my concerns.