The Masked ABR (mABR): a New Measurement Method for the Auditory Brainstem Response

Peripheral Auditory System Divergence Does Not Explain Species Differences in Call Preference

Receiver sensory systems have long been cited as an important source of variation in mate preferences that could lead to signal diversification and behavioral isolation between lineages, with a general assumption that animals prefer the most conspicuous signals. The matched filter hypothesis posits that tuning of the frog peripheral auditory system matches dominant frequencies in advertisement calls used to attract mates. However, little work has characterized species with frequency modulation in their calls. In this study, we extend prior work characterizing the lack of correlated evolution between auditory tuning and spectral properties of male calls in Engystomops (=Physalaemus) frogs. We analyze auditory sensitivity of three cryptic species that differ consistently in female mate preferences for calls of different frequencies. The audiograms of these species differ, but the frequency at which the frog is maximally sensitive is not the most relevant difference in tuning of the auditory periphery. Rather, we identify species differences in overall sensitivity within specific frequency ranges, and we model the effects of these sensitivity differences on neural responses to natural calls. We find a general mismatch between auditory brainstem responses and behavioral preferences of these taxa and rule out the matched filter hypothesis as explaining species differences in male calls and mate preferences in this group.

Bone conduction pathways confer directional cues to salamanders

Sound and vibration are generated by mechanical disturbances within the environment, and the ability to detect and localize these acoustic cues is generally important for survival, as suggested by the early emergence of inherently directional otolithic ears in vertebrate evolutionary history. However, fossil evidence indicates that the water-adapted ear of early terrestrial tetrapods lacked specialized peripheral structures to transduce sound pressure (e.g. tympana). Therefore, early terrestrial hearing should have required nontympanic (or extratympanic) mechanisms for sound detection and localization. Here, we used atympanate salamanders to investigate the efficacy of extratympanic pathways to support directional hearing in air. We assessed peripheral encoding of directional acoustic information using directionally masked auditory brainstem response recordings. We used laser Doppler vibrometry to measure the velocity of sound pressure-induced head vibrations as a key extratympanic mechanism for aerial sound reception in atympanate species. We found that sound generates head vibrations that vary with the angle of the incident sound. This extratympanic pathway for hearing supports a figure-eight pattern of directional auditory sensitivity to airborne sound in the absence of a pressure-transducing tympanic ear.

Three auditory brainstem response (ABR) methods tested and compared in two anuran species

Hearing sensitivity has been extensively investigated, often by measuring the auditory brainstem response (ABR). ABR measurements are relatively non-invasive, easy to reproduce, and allow the assessment of sensitivity when psychophysical data are difficult to obtain. However, the experimental methods differ greatly in respect to stimulation, which may result in different audiograms. We used three different methods in the same individual frogs: stimulating with brief tone bursts (tABR), long-duration tones (ltABR) and masked ABR (mABR), where transients are masked by a long-duration sinusoid, and the sensitivity is assessed by the difference between unmasked and masked ABR. We measured sensitivity in a range from 100 to 3500 Hz, and the resulting audiograms show two sensitivity peaks at 400-600 Hz and 1500-1600 Hz (both sensitive down to 30 dB re. 20 µPa). We found similar results below 1000 Hz, but when stimulating with long-duration tones, the sensitivity decreased more rapidly above this frequency. We showed that the frequency specificity of tone bursts becomes poorly defined with shorter duration at low frequencies. Comparisons between subjectively (visual inspection by researchers) and objectively (thresholds defined by signal-to-noise ratio) defined audiograms showed very little variation. In conclusion, the mABR method gave the most sensitive audiograms. The tABR method showed a similar audiogram when using relatively long-duration tone bursts (25 ms). The ltABR method is not a good choice for studying hearing thresholds above 1000 Hz because of the bias introduced by spike rate saturation in the nerve fibers and their inability to phase lock.

Seismic sensitivity and bone conduction mechanisms enable extratympanic hearing in salamanders

The tympanic middle ear is an adaptive sensory novelty that evolved multiple times in all the major terrestrial tetrapod groups to overcome the impedance mismatch generated when aerial sound encounters the air-skin boundary. Many extant tetrapod species have lost their tympanic middle ears, yet they retain the ability to detect airborne sound. In the absence of a functional tympanic ear, extratympanic hearing may occur via the resonant qualities of air-filled body cavities, sensitivity to seismic vibration, and/or bone conduction pathways to transmit sound from the environment to the ear. We used auditory brainstem response recording and laser vibrometry to assess the contributions of these extratympanic pathways for airborne sound in atympanic salamanders. We measured auditory sensitivity thresholds in eight species and observed sensitivity to low-frequency sound and vibration from 0.05-1.2 kHz and 0.02-1.2 kHz, respectively. We determined that sensitivity to airborne sound is not facilitated by the vibrational responsiveness of the lungs or mouth cavity. We further observed that, although seismic sensitivity probably contributes to sound detection under naturalistic scenarios, airborne sound stimuli presented under experimental conditions did not produce vibrations detectable to the salamander ear. Instead, threshold-level sound pressure is sufficient to generate translational movements in the salamander head, and these sound-induced head vibrations are detectable by the acoustic sensors of the inner ear. This extratympanic hearing mechanism mediates low-frequency sensitivity in vertebrate ears that are unspecialized for the detection of aerial sound pressure, and may represent a common mechanism for terrestrial hearing across atympanic tetrapods.