Figure 5.

Amplitude modulation of a brown thrasher vocalization is clearly visible as a correlated partial opening of the right valve formed by the adducted labia. (a) Illustration of the slight partial abduction (arrowhead) and fully closed position of the right side during one modulation cycle. On the left side the labia remain in close contact throughout the cycle (D, dorsal; V, ventral). (b) Series of representative vocalizations evoked by electrical brain stimulation is shown spectrographically (Upper) and as rectified sound amplitude (A) (Lower) together with the subsyringeal air sac pressure (P, horizontal line indicates ambient pressure). Sounds may be almost unmodulated (Left) or 100% amplitude modulated (Right). The blue arrow marks the modulation cycle illustrated in a.