Auditory brainstem responses in the bat Carollia perspicillata: threshold calculation and relation to audiograms based on otoacoustic emission measurement
An objective method to evaluate auditory brainstem-evoked responses (ABR) based on the root-mean-square (rms) amplitude of the measured signal and bootstrapping procedures was used to determine threshold curves (see Lv et al. in Med Eng Phys 29:191–198, 2007; Linnenschmidt and Wiegrebe in Hear Res 373:85–95, 2019). The rms values and their significance for threshold determination depended strongly on the filtering of the signal. Using the minimum threshold values obtained at three different low-frequency filter corner frequencies (30, 100, 300 Hz), ABR threshold curves were calculated. The course of the ABR thresholds was comparable to that of published DPOAE (distortion-product otoacoustic emission) thresholds based on a − 10 dB SPL threshold criterion for the 2f1–f2 emission (Schlenther et al. in J Assoc Res Otolaryngol 15:695–705, 2014, frequency range 10–90 kHz). For frequencies between 20 and 80 kHz, which is the most sensitive part of the bat’s audiogram, median thresholds ranged between 10 and 28 dB SPL, and the DPOAE thresholds ranged between 10 and 23 dB SPL. At frequencies below 20 kHz (5–20 kHz) and above 80 kHz (80–120 kHz), ABR thresholds increased by 20 dB/octave and 45 dB/octave, respectively. We conclude that the combination of objective threshold determination and multiple filtering of the signal gives reliable ABR thresholds comparable to cochlear threshold curves.