We introduce two EEG techniques, one based on conventional monopolar electrodes and one based on a novel tripolar electrode, to record for the first time auditory brainstem responses (ABRs) from the scalp of unanesthetized, unrestrained big brown bats. Stimuli were frequency-modulated (FM) sweeps varying in sweep direction, sweep duration, and harmonic structure. As expected from previous invasive ABR recordings, upward-sweeping FM signals evoked larger amplitude responses (peak-to-trough amplitude in the latency range of 3–5 ms post-stimulus onset) than downward-sweeping FM signals. Scalp-recorded responses displayed amplitude-latency trading effects as expected from invasive recordings. These two findings validate the reliability of our noninvasive recording techniques. The feasibility of recording noninvasively in unanesthetized, unrestrained bats will energize future research uncovering electrophysiological signatures of perceptual and cognitive processing of biosonar signals in these animals, and allows for better comparison with ABR data from echolocating cetaceans, where invasive experiments are heavily restricted.

Summary We challenged four big brown bats to maneuver through abrupt turns in narrow corridors surrounded by dense acoustic clutter. We quantified bats' performance, sonar beam focus, and sensory acquisition rate. Performance was excellent in straight corridors, with sonar beam aim deviating less than 5° from the corridor midline. Bats anticipated an upcoming abrupt turn to the right or left by slowing flight speed and shifting beam aim to “look” proactively into one side of the corridor to identify the new flightpath. All bats mastered the right turn, but two bats consistently failed the left turn. Bats increased their sensory acquisition rate when confronting abrupt turns in both successful and failed flights. Limitations on biosonar performance reflected failures to switch beam aim and to modify a learned spatial map, rather than failures to update acquisition rate.

When flying in clutter, big brown bats actively modify their biosonar broadcasts by successively alternating the lowest frequencies in the first harmonic of their FM broadcasts in a phenomenon called “frequency hopping;” and by grouping broadcasts into “sonar sound groups,” with short time intervals within a group and longer intervals between groups. These modifications can minimize pulse-echo ambiguity. We analyzed the relationship between frequency hopping and emission of sonar sound groups while bats flew through chain corridors with different clutter densities and through circular hoop tunnels. In more difficult flight tasks, bats emit more sounds in groups of triplets and quadruplets, often with frequency shifts of 1–6 kHz between each sound in the group, creating not only sound but frequency triplets and quadruplets. Echoes of successive sounds remain highly correlated and not separable by conventional receivers according to which broadcast causes them. The consistent pairing of frequency shifting with sonar sound grouping may reflect an auditory streaming mechanism whereby coupled shifts in both frequency and timing can link streams of received echoes to their corresponding emitted calls to disambiguate overlapping echo streams for closely spaced broadcasts. [Work supported by ONR.]