Auditory-Feedback Control of Temporal Call Patterns in Echolocating Horseshoe Bats

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Abstract
During flight, auditory feedback causes horseshoe bats to adjust the duration and repetition rate of their vocalizations in a context-dependent manner. As these bats approach a target, they make finely graded adjustments in call duration and interpulse interval (IPI), but their echolocation behavior is also characterized by abrupt transitions in overall temporal calling patterns. We investigated the relative contributions of two prominent acoustic cues, echo frequency and delay, toward the control of both graded and transitional changes in call duration and IPI. Echoes returning at frequencies above the emitted call frequency caused bats to switch from long single calls to pairs of short calls (doublets). Alternatively, increasing echo delay caused progressive increases in IPI but caused no accompanying changes in call duration. When frequency shifts were combined with changing echo delays, echo delay altered the IPIs occurring between doublets but not the IPI within a doublet. When the echo mimic was replaced by presentation of either an artificial constant-frequency (CF) stimulus or a frequency-modulated (FM) stimulus, each designed to mimic major components of the echo acoustic structure, we found that CF stimuli could trigger the switch to doublets, but changing CF delay had no influence on IPI, whereas the timing of an FM-sweep presentation had a strong effect on IPI. Because CF and FM sounds are known to be processed separately in the bat auditory system, the results indicate that at least two distinct neural feedback pathways may be used to control the temporal patterns of vocalization in echolocating horseshoe bats.