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Referee comments: Referee 1 (Constance Scharff)

Posted by PLOS_ONE_Group on 18 Mar 2008 at 09:50 GMT

Referee 1's review (Constance Scharff):

The paper 'Molecular Mapping of Movement-Associated Areas in the Avian Brain: A Motor Theory for Vocal Learning Origin' maps with unprecedented precision a number of movement-associated regions in three different avian vocal learners (zebra finch, parrot, hummingbird) as well as female zebra finches and ring doves, who do not learn their vocalizations by imitation. The authors interpret their data as supportive of a 'motor theory of vocal learning'.

A strong point of the manuscript is the careful dissection of ZENK (and c-fos) expression affected by different variables; vocalizing, hearing, seeing and moving. The ingenious methods employed to make birds move in a repetitive manner, factoring out the other variables, are exemplary and novel. The careful qualitative and quantitative documentation are the gold standard for these kinds of gene expression brain mapping studies. The study is clearly written and the ms easy to follow.

The theoretical part of the paper is useful because it recaps the various theories about the origin of vocal learning and points to the previous publications by (recent) proponents for each. The authors then state that their discovery of the proximity of movement-associated regions and song system nuclei support the notion that existing movement-associated brain regions were the substrates upon which auditory-guided vocal motor-learning evolved. While I am sympathetic to the idea, the argument is not stringent and could have been made stronger by being more specifically put into context of the various existing ideas of brain evolution. The theory also does not spell out sufficiently when and how according to the authors the auditory components came into play. Is the last sentence of the ms 'what makes vocal learning special is the motor component of the behaviour' supposed to imply that motor came first and auditory later? And if that was so, wouldn't the authors suppose that non-vocal learners like the pigeon have pallial song-motor nuclei? But overall I still commend the authors for putting their data into a larger speculative context which proposes testable hypotheses. This should make the paper attractive to a larger audience including readers interested in the evolution of speech and language. I anticipate that this study will be widely read, discussed and cited and serve as the departure for many important follow-up studies.
Minor comment: page 15 last paragraph: do the authors really mean 'interrelated'?

N.B. These are the comments made by the referee when reviewing an earlier version of this paper. Prior to publication the manuscript has been revised in light of these comments and to address other editorial requirements.