Anne J. M. Moffat

Auditory sensitivity of the saccule in the American toad (Bufo americanus)

SummarySingle unit recordings in the posterior nerve branchlet from the saccule have shown that, in the American toad (Bufo americanus), approximately 30% of the fibers respond to airborne sounds in a way similar to fibers from the two known auditory organs, the amphibian and basilar papillae. In response to tones, saccule fibers have best excitatory frequencies which fall into two disjoint populations: units in the low-frequency-sensitive group (below 300 Hz) show tone-on-tone suppression while those in the high-frequency-sensitive group (700–1,200 Hz) show no evidence of peripheral inhibition. Saccule units have somewhat higher thresholds than those from the other auditory organs. It is suggested that the high-frequency-sensitive fibers might be useful for discriminating mating calls in an intense chorus while the low-frequency-sensitive units likely respond to other high intensity sounds in the environment.

Selectivity of the peripheral auditory system of spadefoot toads (Scaphiopus couchi) for sounds of biological significance

SummaryScaphiopus couchi is a primitive anuran whose vocal repertoire consists of a mating call and a release call. The two calls are distinct and differ in trill rate. Reception of airborne sound is achieved by means of a poorly differentiated region of skin on the head which serves as an eardrum.Whereas more modern anurans possessthree distinct types of auditory nerve fibers, spadefoot toads possess onlytwo types: a low-frequency-sensitive group which exhibits tone-on-tone inhibition and a high-frequency-sensitive group which is not inhibitable. The sharpness of frequency tuning of primary fibers in each group is comparable to more advanced vertebrate species. While the response properties of auditory fibers in the high-frequency-sensitive group are well matched to the spectral and temporal features in the spadefoots mating call and release call, the low-frequency-sensitive fibers do not respond to these calls. Instead they may be involved in detection of bodily transmitted sounds during clasping, as well as other low-frequency sounds in the environment. The two groups of auditory fibers probably derive from separate auditory organs within the inner ear. Thresholds of auditory nerve fibers in spadefoot toads are relatively poorer than in more advanced anurans, which likely is due to their less developed eardrum. The role of tone-on-tone inhibition in the peripheral auditory system is questioned with regard to its significance in processing sounds of biological value.

Nonlinear Properties of the Peripheral Auditory System of Anurans

The vertebrate ear is highly nonlinear. This is rather surprising since its vibrational amplitudes are so minute in response to normal sound pressures. Generally, one might expect a stable mechanical system to respond linearly when disturbed slightly from its resting state. Thus the nonlinear properties of the peripheral auditory system are of considerable interest inasmuch as they can provide valuable insight into the underlying transduction process in the ear. The two most prominent nonlinear properties are inter-modulation distortion and two-tone suppression. Their characteristics have been studied extensively in the mammalian auditory system by a number of investigators. To provide a comparative view, a series of electrophysiological experiments were conducted in order to determine the nonlinear behavior of the anuran’s peripheral auditory system. The results have interesting implications regarding the origin of nonlinearities, as well as the mechanical basis for frequency analysis, in the vertebrate inner ear in general. Before presenting these findings, several relevant studies of nonlinearities in the mammalian auditory system are summarized, followed by a brief review of the anatomy of the anuran’s ear.

Sonar gain control and echo detection thresholds in the echolocating bat, Eptesicus fuscus

The echolocating bat, Eptesicus fuscus, detects sonar echoes with a sensitivity that changes according to the time elapsed between broadcasting of each sonar signal and reception of echoes. When tested in an electronic target simulator on a two-choice echo-detection task, the bats threshold improved by 11.5 dB as echo delay changed from 2.3 to 4.6 ms (target ranges of 40 and 80 cm). Earlier experiments measured the change in detection threshold for delays from 1 to 6.4 ms (target ranges from about 17 to 110 cm) and obtained about 11 dB of improvement per doubling of delay. The new experiments used electronic delay lines to simulate echo delay, thus avoiding movement of loudspeakers to different distances and the possible creation of delay-dependent backward masking between stimulus echoes and cluttering echoes from the loudspeaker surfaces. The slope of the threshold shift defines an echo gain control that keeps echoes from point targets at a fixed sensation level--reducing sensitivity by 11 to 12 dB as echo amplitude increases by 12 dB per halving of range during the bats approach to the target. A recent experiment using loudness discrimination of echoes at 70 to 80 dB SPL (roughly 50 dB above threshold) found a slope of about 6 dB per halving of range, so the gain-control effect may be level dependent. The observed effect is operationally equivalent to forward masking of echoes by the transmission, but any events correlated with vocalization which impair hearing sensitivity for a short interval following transmissions could cause a decline in sensitivity to echoes. Contractions of the bats middle-ear muscles synchronized to transmissions may account for the observed threshold shift, at least for a span of echo delays associated with the most critical portion of the approach stage of pursuit. Forward masking by the sonar transmissions may contribute to the threshold shift, too, but middle-ear muscle contractions do occur and must be a significant part of the cause.

Middle ear sensitivity in anurans and reptiles measured by light scattering spectroscopy

Summary1.We have used a light scattering spectroscopy technique to measure the vibration amplitude, in response to sound stimuli, of the eardrums of two species of anurans (Bufo americanus andHyla cinerea) and one reptile species (Chrysemys scripta elegans).2.The amplitude of displacement as a function of frequency of the eardrums for constant sound intensity varies in a species-specific manner (Figs. 2, 5, and 7).3.The middle ear responds like a low-pass filter with the cut-off frequency differing in each species. This frequency is quite well correlated with the highest best excitatory frequencies of eighth nerve fibers in the same species, suggesting that the middle ear may limit the frequency range detected by the auditory system.4.In anurans the tympanum overlying the site of attachment of the extracolumella vibrates with a smaller amplitude than does the surrounding ‘free’ membrane (Figs. 3 and 6). This indicates that the tympanum can act as a curved membrane lever thereby increasing the force transmitted by the plectrum to the inner ear and, by so doing, aid in impedance matching.5.Calculations based on data obtained in this study and information available from the literature suggest that, at least over their range of maximum sensitivity, the middle ears of nonmammalian vertebrates can be as efficient as those of mammals.

A functional explanation of top-bottom asymmetry in vertical orbwebs

Abstract We tested the hypothesis that orb-weaving spiders (Nuctenea sclopetaria) improve their ability to catch entangled prey by locating the hub of their web (the location where they often sit and wait for prey to strike the web) somewhat above the webs centre. In both field and laboratory tests spiders reached a vibration source below them faster than they reached the same source located an equal distance above them. The difference was apparently due to the greater amount of time required to run in the upward direction, since the time taken to react to the stimulus was the same for both stimulus locations. Thus, if the spiders aim is to build a web in which the average time to reach prey is minimized, it should locate the hub such that the distances from hub to top and bottom of the web are directly proportional to the running speeds in the two directions. This is, in fact, what we found.

Noise masking of tone responses and critical ratios in single units of the mouse cochlear nerve and cochlear nucleus

Responses of single units in the cochlear nerve and cochlear nucleus to tone bursts in a background of continuous white broadband noise were recorded. Tone and noise intensities ranged from threshold to saturation levels. Masking of the tone response by the noise was demonstrated by comparing peristimulus-time histograms and spike rates recorded during the tone and between tone presentations. The response of a unit to masking was found to be predictable based upon the difference in its rate of response to the tone and to the noise when the tone was masked. Several nonlinearities of the masking process are described. The most prominent one is an increase in the difference between tone and noise levels at the threshold of masking with increasing tone levels, i.e. neural critical ratios increase with increasing tone level. On the average, the frequency dependence of single unit effective bandwidths and of critical ratio bandwidths is similar to that of mean behavioral critical ratio bands.

Two-tone suppression in auditory nerve fibers of the green treefrog (Hyla cinerea)

The phenomenon of two-tone suppression was studied quantitatively in the peripheral auditory system of the green treefrog (Hyla cinerea). Linear relationships were found between best excitatory and best suppressor frequency, between response thresholds at these frequencies, between Q 10dB-values of excitatory and suppressor tuning curves and best excitatory frequency, and between both Q 10dB-values.

Behavioral determination of frequency resolution in the ear of the cricket, Teleogryllus oceanicus

Summary1.We used the flying phonotactic behavior of tethered female Australian field crickets,Teleogryllus oceanicus, to measure frequency filtering (determined from CR-bands, critical bands, and “effective” bandwidths) in the auditory system at frequencies of 4.5, 20, 40 and 70 kHz. Till now such measurements have been made only in vertebrates.2.In CR-band determinations the spectrum level of the noise at the masked threshold increased monotonically with increasing tone level (Fig. 2). At 20, 40 and 70 kHz the regression lines had slopes very close to 1 indicating that, at least over the intensity range tested (5–20 dB above tone threshold), the masking process is linear. At 4.5 kHz, however, the slope of the regression line was only 0.56 showing a strong non-linearity in the masking process at this frequency. Such a nonlinearity has not been found in vertebrates.3.At 4.5 kHz the three measures of filter bandwidths, CR-band, critical band and “effective” bandwidth, were not significantly different from each other, whereas at 40 kHz the “effective” bandwidth was significantly smaller than CR-band and critical band.4.The width of the CR-band filter around 4.5 kHz increases with tone intensity, an effect which has not been observed in vertebrates.5.Both CR-bands and critical bands indicate that the cricket auditory system is sharply tuned around 4.5 kHz, may show some tuning at 40 kHz, and is completely untuned at 20 and 70 kHz.6.These results are discussed with respect to frequency tuning at the single unit level in crickets.

Phase cancellation of auditory nerve fiber responses to combination tones, f2–f1

Low‐frequency sensitive fibers from the amphibian papilla of anurans (Bufo americanus and Rana pipiens) respond to combination tones of the form f2–f1 in the same way as do mammalian cochlear nerve fibers, even though the structure of the receptor organs is quite different in the two classes. In anurans the shapes of the combination tone and pure tone tuning curves are similar, suggesting that the amphibian papilla fibers are responding to energy generated at the difference frequency. Furthermore, the response of a fiber to a tone pair, whose frequencies are harmonically related and whose fundamental is near the fibers best excitatory frequency, can be almost totally eliminated by the addition of energy at the fundamental provided it is of appropriate intensity and relative phase. Systematically varying the phase of the fundamental produces a sharp minimum in firing rate with a maximum occurring about 180° later. These results suggest that a basilar membrane is not necessary for propagation of energy at ...