Fig. 4. Sound and (A) unilateral or (B) bilateral sonic muscle electromyogram (EMG) traces from voluntary disturbance calls recorded in air. In the bilateral trace, action potentials from the right sonic muscle deflect down, while those from the left muscle deflect up. Muscle action potentials occur before alternate sound pulses in the unilateral trace but before each sound in the bilateral trace (dotted lines). Note the action potentials failing to produce acoustic pulses early in both calls and the two attenuated sound pulses at the end of each call that are not associated with any action potentials (double arrows in both traces). Note also the sound pulse in the unilateral trace denoted with a single arrow. This sound indicates that the contralateral (off-trace) sonic muscle contracted last in this call.