The hemispheres are not equal: How the brain is not symmetrical elife
Senior Editor; University of Oxford, United KingdomIn the interests of transparency, eLife publishes the most substantive revision requests and the accompanying author responses.Thank you for submitting your article"Asymmetry of cortical functional hierarchy in humans and macaques suggests phylogenetic conservation and adaptation" for consideration by.
To make this point clear, consider the following toy example. Say we have 4 brain regions A,B,C,D. Let us say that we have the following connectivity:So the connection between B and C is twice as strong in the right hemi, and everything else remains the same.Note how B,C are closer to each other in the RIGHT, but also that A,D have moved away from each other because the eigenvector has to have norm 1.B is higher on the RIGHTSo we have found asymmetry in all of our regions.
The gradient from resting-state functional connectome has been frequently used but mainly at the group level. The current study essentially applied the gradient comparison at the individual level. Biological interpretation for individual gradient score at the parcel level as well as its comparability between individuals and between hemispheres should be resolved. This is the fundamental rationale underlying the whole analyses.
The sample size of monkey is far less than human subjects . Such limitation raises severe concern on the validity of the currently observed gradient asymmetry pattern in the monkey group, as well as the similarity results with human gradient asymmetry pattern. Despite the marginal significance of G1 inter-hemisphere gradient between humans and monkeys, I feel overall there is no convincingly meaningful similarity between these two species.
Regarding my first comment, i.e. interpretation of a change in position along the gradients: I am not sure I understand your reply. You agreed that it is difficult to interpret these changes, given that they can represent changes occurring outside the region where the change is reported, but then the analysis you have done does not address this concern. Instead, you calculated some other measure and reported the asymmetry index using this new measure.
We thank the Reviewer for the appreciation of our work and the insightful comments, which we have addressed below. This illustrates the danger of using a global optimisation procedure to analyse and interpret local changes. One has to be very careful.
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