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Evidence for curvilinear relationships between mu-opioid receptor labeling and undirected song rates. An inverted-U shaped curve showing the measure of undirected song on the x-axis and mean mu-opioid receptor total pixel area on the y-axis in A) POM, B) PAG, and C) BSTm. Individual birds are represented by a single black dot. Sample size indicated in the upper right corner of the figures. Presence of the red regression line indicates a significant relationship p<0.05.
