Assessing stimulus and subject influences on auditory evoked potentials and their relation to peripheral physiology in green treefrogs (Hyla cinerea)

Abstract

Anurans (frogs and toads) are important models for comparative studies of communication, auditory physiology, and neuroethology, but to date, most of our knowledge comes from in-depth studies of a relatively small number of model species. Using the well-studied green treefrog (Hyla cinerea), this study sought to develop and evaluate the use of auditory evoked potentials (AEPs) as a minimally invasive tool for investigating auditory sensitivity in a larger diversity of anuran species. The goals of the study were to assess the effects of frequency, signal level, sex, and body size on auditory brainstem response (ABR) amplitudes and latencies, characterize gross ABR morphology, and generate an audiogram that could be compared to several previously published audiograms for green treefrogs. Increasing signal level resulted in larger ABR amplitudes and shorter latencies, and these effects were frequency dependent. There was little evidence for an effect of sex or size on ABRs. Analyses consistently distinguished between responses to stimuli in the frequency ranges of the three previously-described populations of afferents that innervate the two auditory end organs in anurans. The overall shape of the audiogram shared prominent features with previously published audiograms. This study highlights the utility of AEPs as a valuable tool for the study of anuran auditory sensitivity.

Hearing conspecific vocal signals alters peripheral auditory sensitivity

“…Specifically, we measured the auditory brainstem response, a type of AEP which is generated by the auditory periphery, occurs in response to the onset of sound and is seen within 10 ms of the arrival of the stimulus at the ear. The green treefrog response consists of two peaks, P1 and P2 [21]. We found that P1 was only consistently measurable from frequencies of 0.8 to 1.6 kHz (figure 2).…”

“…The threshold is then estimated as lying halfway between this stimulus intensity and the next lowest intensity (in our case, 2.5 dB below the stimulus intensity of the last detectable response). This method consistently produces evoked potential thresholds that provide the greatest agreement with thresholds estimated with single unit or behavioural estimates [17,21,29].…”

“…At low frequencies (0.4-0.63 kHz) and higher frequencies (2-5 kHz), we could reliably identify P2 but could not reliably identify P1 (figure 2). It has recently been suggested that these differences in the appearance of the evoked potential may be related to the three populations of afferent fibres [21,29]. Two populations of fibres, one at low and one at middle frequencies originate from the amphibian papilla, while a third population tuned to higher frequencies originates from the basilar papilla [18,19,21].…”

“…It has recently been suggested that these differences in the appearance of the evoked potential may be related to the three populations of afferent fibres [21,29]. Two populations of fibres, one at low and one at middle frequencies originate from the amphibian papilla, while a third population tuned to higher frequencies originates from the basilar papilla [18,19,21]. Therefore, we chose to analyse amplitude for P1 when the stimulus frequency was between 0.8 and 1.25 kHz, which are also the lower frequencies found in green treefrog calls [22][23][24].…”

“…In the current study, we ask whether social experience in the form of exposure to conspecific vocal signals over several days could modify the auditory periphery's response to novel calls or other acoustic stimuli. We examined this by measuring auditory evoked potentials (AEPs) in the green treefrog (H. cinerea), a species with a well-described auditory system [18][19][20][21]. The H. cinerea advertisement call is a short, pulsatile, broad band signal with spectral peaks around 900 and 3000 Hz [22][23][24][25].…”