Auditory brainstem responses in Cope’s gray treefrog (Hyla chrysoscelis): effects of frequency, level, sex and size

Abstract

Our knowledge of the hearing abilities of frogs and toads is largely defined by work with a few well-studied species. One way to further advance comparative work on anuran hearing would be greater use of minimally invasive electrophysiological measures, such as the auditory brainstem response (ABR). This study used the ABR evoked by tones and clicks to investigate hearing in Cope's gray treefrog (Hyla chrysoscelis). The objectives were to characterize the effects of sound frequency, sound pressure level, and subject sex and body size on ABRs. The ABR in gray treefrogs bore striking resemblance to ABRs measured in other animals. As stimulus level increased, ABR amplitude increased and latency decreased, and for responses to tones, these effects depended on stimulus frequency. Frequency-dependent differences in ABRs were correlated with expected differences in the tuning of two sensory end organs in the anuran inner ear (the amphibian and basilar papillae). The ABR audiogram indicated two frequency regions of increased sensitivity corresponding to the expected tuning of the two papillae. Overall, there was no effect of subject size and only small effects related to subject sex. Together, these results indicate the ABR is an effective method to study audition in anurans.

Evolutionary adaptations for the temporal processing of natural sounds by the anuran peripheral auditory system

“…The observation of larger ASSRs in females suggests better recruitment of nerve fibers across the two papillae in females than in males. This result is consistent with previous results from recordings of AEPs in these species Schrode et al, 2014). In those studies, the amplitudes of P1 of tone-evoked ABRs were larger in females than males when tones had intermediate frequencies (1.5 to 2.0 kHz).…”

“…The primary generator of the first peak (P1) in the ABRs of all animals studied to date is the auditory nerve (Achor and Starr, 1980;Brown-Borg et al, 1987;Buchwald and Huang, 1975;Lev and Sohmer, 1972;Seaman, 1991). The timing of P1 of the ABR in both gray and green treefrogs corresponds well to the expected latencies of anuran auditory nerve fibers Schrode et al, 2014). In support of this view, the minimum resolvable ICIs of between 1.5 and 2.0 ms measured in the present study are comparable with the average gap detection times of between 1.2 and 2.2 ms reported previously for anuran auditory nerve fibers (Feng et al, 1994).…”

“…Instead, we observed frequency-dependent sex differences in which females tended to have relatively larger ASSRs than males at the middle carrier frequency. Previous behavioral studies (Gerhardt, 2005) and recordings of AEPs Schrode et al, 2014) in treefrogs indicate that sound frequencies between the two spectral peaks of advertisement calls, and correspondingly between the two frequency regions of greatest auditory sensitivity, are able to stimulate simultaneously both auditory papillae in the anuran inner ear. The observation of larger ASSRs in females suggests better recruitment of nerve fibers across the two papillae in females than in males.…”

“…This finding is noteworthy, as it suggests that specializations in temporal processing are related to differences in how the spectral properties of the vocalizations of the two species are transduced. Recall that in both species, each of the two spectral peaks in the advertisement call is primarily transduced by one of the two inner ear sensory papillae (the AP or the BP) Capranica and Moffat, 1983;Gerhardt, 1974c;Hillery, 1984;Schrode et al, 2014). In gray treefrog advertisement calls, the relative amplitude of the high spectral peak (2.5 kHz) is approximately 11 dB greater than the low spectral peak (1.25 kHz; Ward et al, 2013).…”

Automatic Recognition of Speech-evoked Brainstem Responses to English Vowels

“…The second difference is in the stimuli used in this study. In the majority of the previous works, the stimuli used were either consonant-vowel syllables (Cunningham et al, 2001;Wible et al, 2005;Russo et al, 2004;Song et al, 2006;Wible et al, 2004) or tones and clicks (Schrode et al, 2014;Starr et al, 1996;Sininger, 1993), but in this study we used pure vowels as stimuli. The main reason for choosing vowels as stimuli in this study was that different vowels have distinct spectral features; and since our goal was to use automatic speech recognition methods for discriminating between speechevoked ABRs, the vowels were chosen over other stimuli for an initial investigation of the proposed approach.…”