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(a-h) Low and high power views of darkfield images showing PVALB mRNA expression in the core region of the NAO song nucleus in eight parrot species. Silver grains is white; red-brown is cresyl violet staining. Brain sections are not to scale, due to large size differences between species. Sections are in the coronal plane; medial is to the left, dorsal is top. See list for Abbreviations.
