Non-invasive auditory brainstem responses to FM sweeps in awake big brown bats.

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.

Amplitude discrimination is predictably affected by echo frequency filtering in wideband echolocating bats.

Big brown bats echolocate using wideband frequency-modulated (FM) ultrasonic pulses, perceiving target range from echo delay and target size from echo amplitude. Echolocation pulses contain two prominent down-sweeping harmonics (FM1, ∼55-22 kHz; FM2, ∼100-55 kHz), which are affected differently by propagation to the target and back to the bat. Previous work demonstrates that big brown bats utilize the low frequencies in FM1 for target ranging, while FM2 only contributes if FM1 is also present. The present experiments test the hypothesis that the bat's ability to discriminate echo amplitude is also affected by selectively attenuating FM1 or FM2 in target or nontarget echoes. Bats were trained to perform an amplitude discrimination task with virtual echo targets located 83 cm away. Echo delay was fixed and echo amplitude was varied, while either FM1 or FM2 was attenuated by highpass or lowpass filtering. Bats' performance decreased when lower frequencies were attenuated in target echoes and when higher frequencies were attenuated in nontarget echoes. Performance was reversed in the opposite filtering conditions. The bat's ability to distinguish between virtual targets varying in amplitude at the same simulated range indicates a high level of focused attention for perceptual isolation of target from non-target echoes.

Spatiotemporal patterning of acoustic gaze in echolocating bats navigating gaps in clutter.

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.