Time-variant spectral peak and notch detection in echolocation-call sequences in bats.

Bats are able to recognize and discriminate three-dimensional objects in complete darkness by analyzing the echoes of their ultrasonic emissions. Bats typically ensonify objects from different aspects to gain an internal representation of the three-dimensional object shape. Previous work suggests that, as a result, bats rely on the echo-acoustic analysis of spectral peaks and notches. Dependent on the aspect of ensonification, this spectral interference pattern changes over time in an object-specific manner. The speed with which the bats' auditory system can follow time-variant spectral interference patterns is unknown. Here, we measured the detection thresholds for temporal variations in the spectral content of synthesized echolocation calls in the echolocating bat, Megaderma lyra. In a two-alternative, forced-choice procedure, bats were trained to discriminate synthesized echolocation-call sequences with time-variant spectral peaks or notches from echolocation-call sequences with invariant peaks or notches. Detection thresholds of the spectral modulations were measured by varying the modulation depth of the time-variant echolocation-call sequences for modulation rates ranging from 2 to 16 Hz. Both for spectral peaks and notches, modulation-detection thresholds were at a modulation depth of approximately 11% of the centre frequency. Interestingly, thresholds were relatively independent of modulation rate. Acknowledging reservations about direct comparisons of active-acoustic and passive-acoustic auditory processing, the effectual sensitivity and modulation-rate independency of the obtained results indicate that the bats are well capable of tracking changes in the spectral composition of echoes reflected by complex objects from different angles.