Fig. 11. Electromyogram (EMG) activity does not indicate a compensatory increase in muscle activity after injections. (A) Example of data for one zebra finch illustrates the decrease in air sac pressure (P) after two injections (green) compared with pre-injection (blue) to two injections and the accompanying EMG activity (E; rectified and integrated). The song is illustrated spectrographically (bottom). (B–D) Air sac pressure and corresponding EMG activity (integrated voltage) for the three zebra finches for quiet respiration and song. Colors indicate the three recording conditions (blue, pre-injection; red, one injection; green, two injections). The data in B are from the individual whose song is depicted in A, and numbers correspond to the four expiratory pulses of song. Arrows indicate the shift in air sac pressure after injections for the four expiratory pulses of song. C shows that the first injection was followed by an increase in EMG activity, which is most pronounced in the syllable in the upper right corner. However, a corresponding shift in EMG activity is seen in quiet respiration, at the bottom left of the graph, likely indicating an impedance change.