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

Abstract

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.

Keywords: Auditory brainstem response; Big brown bat; EEG; Echolocation; Scalp recordings.

Fig. 1

Spectrograms of FM-2H logarithmic down- and up-sweeping FM sweeps recorded at the midline of the bat’s head between the two ears, after passing through the TDT tweeter. Sweeps varied in duration as shown along the upper x-axis. FM-1H stimuli (not shown) did not include the second harmonic

Fig. 2

Example ABR waveforms recorded with pEEG electrodes (left column) and tEEG electrodes (right column). Stimuli are FM-upsweeps at 0.5 ms duration, with harmonic structure (FM-1H or FM-2H) and stimulus levels (90 dB peSPL, TDT tweeter; 98 dB peSPL, Kenwood tweeter) as indicated on the plots. pEEG data are from Bat J (top to bottom) 08-15-19, 08-15-19, and 06-24-29. tEEG data are from (top to bottom) Bat J (01-24-20), Bat J (01-28-20), and Bat T (03-14-19). The asterisk marks the second positive peak (latencies of around 4–5 ms) used for quantification of response amplitude for statistical testing (see Methods). Response amplitudes were not corrected for the different gains of pEEG and tEEG recordings

Fig. 3

Heatmaps of responses to FM-upsweeps and downsweeps presented at 90 dB peSPL (TDT tweeter) across sweep durations (y axes). Top panel: pEEG responses from Bat J (08-15-19), FM-1H sweeps. Middle panel: pEEG responses from Bat J (08-15-19), FM-2H sweeps. Bottom panel: tEEG responses from Bat J (01-28-20), FM-2H sweeps. The upsweep duration producing the largest peak-to-trough amplitude (measured from the second positive peak; see Methods) is 0.5 ms up (top), 0.7 ms up (middle), and 0.7 ms up (bottom)

Fig. 4

Peak-to-trough amplitudes vary with sweep duration (x-axis) and direction (legends). Because we are interested in responses to upsweeps vs downsweeps, these plots are restricted to matched pairs of responses to upsweeps and downsweeps at each tested sweep duration. There are unequal numbers of matched pairs across the different sweep durations and across recording days. Data are plotted as means and standard deviations. Stimulus levels are 86, 90, or 98 dB peSPL, depending on the tweeter used. Because signals had a raised cosine shape, rise time and FM sweep rate co-vary with stimulus duration; these parameters are not plotted separately here

Fig. 5

pEEG latencies to FM-upsweeps decrease with increases in stimulus levels, as expected from amplitude-latency trading. Data are plotted from four recording days in response to FM-1H and FM-2H upsweeps at two sweep durations, 1 and 0.5 ms. Data are from Bat J (legend order 06-27-19, 09-12-19, 09-13-19, 09-13-19)