Fig. 4. Effects of different audio-vocal feedback mechanisms on Doppler-shift compensation (DSC) behavior. Sensory information about different echo frequencies (B-D) is translated into motor activity that generates the corresponding call frequencies (A) using a purely inhibitory (B), all-excitatory (C) or combined excitatory/inhibitory (D) audio-vocal feedback mechanism. Note that the discussed non-linearity in the motor control system limiting call frequency increases (see Fig. 2; see also Schuller and Suga, 1976b; Suthers and Fattu, 1982) has been omitted for clarity. (A) In the motor nerve controlling the frequency of sound production by the larynx, vocalization (VOC) frequencies below resting frequency (RF) (VF₁; white arrow), such as during `normal' DSC behavior (see Fig. 2A), are caused by a level of motor activity (MA<sub>1</sub>) that is lower than that generated at RF. Conversely, call frequencies above RF (VF<sub>2</sub>; black arrow), such as during `inverse' DSC behavior (see Fig. 2B-D), require a level of motor activity (MA₂) that is above the resting value. (B-D) Illustrations of the three basic scenarios for how different sensory activity levels (SA₁ and SA₂) that are caused by echo frequencies above RF (EF₁; white arrows) or below RF (EF₂; black arrows) have to be integrated in a sensory-motor interface to yield the appropriate motor commands that ultimately lower and raise call frequencies (as shown in A).