Andrea Megela Simmons

Frequency selectivity of hearing in the green treefrog,Hyla cinerea

Summary1.Frequency selectivity of hearing was measured in the green treefrog,Hyla cinerea. A psychophysical technique based on reflex modification was used to obtain masked threshold estimates for pure tones (300–5,400 Hz) presented against two levels of broadband masking noise. A pure tone (S-1) presented 200 ms prior to a reflex-eliciting stimulus (S-2) inhibited the motor reflex response to S-2. The magnitude of this reflex modification effect varied systematically with the sound pressure level (SPL) of S-1, and threshold was defined as the SPL of S-1 at which the reflex modification effect disappeared.2.Masked thresholds were used to calculate critical ratios, an index of the auditory systems frequency selectivity. The frequency selectivity of the treefrogs hearing is greatest and critical ratios are lowest (22–24 dB) at about 900 and 3,000 Hz, the two spectral regions dominant in the male treefrogs species-specific advertisement call. These results suggest that the treefrogs auditory system may be specialized to reject noise at biologically relevant frequencies.3.As in other vertebrates, critical ratios remain constant when background noise level is varied; however, the shape of the treefrogs critical ratio function across frequencies differs from the typical vertebrate function that increases with increasing frequency at a slope of about 3 dB/octave. Instead, the treefrogs critical ratio function resembles its pure tone audiogram. Although the shape of the treefrogs critical ratio function is atypical, the critical ratio values themselves are comparable to those of many other vertebrates in the same frequency range.4.Critical ratio values here measured behaviorally do not match critical ratio values previously measured physiologically in single eighth nerve fibers. This suggests that frequency selectivity of hearing in the green treefrog is mediated in the central, rather than the peripheral, auditory system and probably does not arise directly from the tuning of the individual auditory nerve fibers.

Encoding of a spectrally complex communication sound in the bullfrog's auditory nerve

Summary1.A population study of eighth nerve responses in the bullfrog, Rana catesbeiana, was undertaken to analyze how the eighth nerve codes the complex spectral and temporal structure of the species-specific advertisement call over a biologically-realistic range of intensities. Synthetic advertisement calls were generated by Fourier synthesis and presented to individual eighth nerve fibers of anesthetized bullfrogs. Fiber responses were analyzed by calculating rate responses based on post-stimulustime (PST) histograms and temporal responses based on Fourier transforms of period histograms.2.At stimulus intensities of 70 and 80 dB SPL, normalized rate responses provide a fairly good representation of the complex spectral structure of the stimulus, particularly in the low- and mid-frequency range. At higher intensities, rate responses saturate, and very little of the spectral structure of the complex stimulus can be seen in the profile of rate responses of the population.3.Both AP and BP fibers phase-lock strongly to the fundamental (100 Hz) of the complex stimulus. These effects are relatively resistant to changes in stimulus intensity. Only a small number of fibers synchronize to the low-frequency spectral energy in the stimulus. The underlying spectral complexity of the stimulus is not accurately reflected in the timing of fiber firing, presumably because firing is ‘captured’ by the fundamental frequency.4.Plots of average localized synchronized rate (ALSR), which combine both spectral and temporal information, show a similar, low-pass shape at all stimulus intensities. ALSR plots do not generally provide an accurate representation of the structure of the advertisement call.5.The data suggest that anuran peripheral auditory fibers may be particularly sensitive to the amplitude envelope of sounds.

Periodicity extraction in the anuran auditory nerve. II: Phase and temporal fine structure

Discharge patterns of single eighth nerve fibers in the bullfrog, Rana catesbeiana, were analyzed in response to signals consisting of multiple harmonics of a common, low-amplitude fundamental frequency. The signals were chosen to reflect the frequency and amplitude spectrum of the bullfrogs species-specific advertisement call. The phase spectrum of the signals was manipulated to produce envelopes that varied in their shapes from impulselike (sharp) to noiselike (flattened). Peripheral responses to these signals were analyzed by computing the autocorrelation functions of the spike trains and their power spectra, as well as by constructing period histograms over the time intervals of the low-frequency harmonics. In response to a phase aligned signal with an impulsive envelope, most fibers, regardless of their characteristic frequencies or place of origin within the inner ear, synchronize to the fundamental frequency of the signal. The temporal patterns of fiber discharge to these stimuli are not typically captured by that stimulus harmonic closet to the fiber characteristic frequency, as would be expected from a spectral coding mechanism for periodicity extraction, but instead directly reflect the periodicity of the stimulus envelope. Changing the phase relations between the individual harmonics constituting the signal produces changes in temporal discharge patterns of some fibers by shifting predominant synchronization away from the fundamental frequency to the low-frequency spectral peak in the complex stimuli. The proportion of fibers whose firing is captured by the fundamental frequency decreases as the waveform envelope becomes less impulselike. Fiber characteristic frequency is not highly correlated with the harmonic number to which synchronization is strongest. The higher-harmonic spectral fine structure of the signals is not reflected in fiber temporal response, regardless of the shape of the stimulus envelope, even for those harmonics within the range of phase locking to simple sinusoids. Increasing stimulus intensity also shifts the synchronized responses of some fibers away from the fundamental frequency to one of the low-frequency harmonics in the stimuli. These data suggest that the synchronized firing of bullfrog eighth nerve fibers operates to extract the waveform periodicity of complex, multiple-harmonic stimuli, and this periodicity extraction is influenced by the phase spectrum and temporal fine structure of the stimuli. The similarity in response patterns of amphibian papilla and basilar papilla fibers argues that the frog auditory system employs primarily a temporal mechanism for extraction of first harmonic periodicity.

Masking patterns in the bullfrog (Rana catesbeiana). I: Behavioral effects.

A psychophysical technique based on reflex modification was used to measure masked auditory thresholds in the bullfrog (Rana catesbeiana). Masked thresholds were converted to critical ratios to describe the frequency selectivity of the bullfrogs hearing. The bullfrogs critical ratio function is complex. At low frequencies (100-600 Hz), it increases gradually with tone frequency at a rate of about 4 dB/octave. At high frequencies (1000-2500 Hz), the rate of increase with tone frequency is steeper (9 dB/octave). Critical ratios are lowest at frequencies around 1000 Hz (about 18 dB). Unlike effects previously reported in another anuran species, the green treefrog, the bullfrogs critical ratio function is not parallel to its pure-tone audiogram. Moreover, the relationship of the bullfrogs critical ratio function to the spectral composition of its species-specific vocalizations is not as compelling as in the green treefrog.

Lateral line-mediated rheotactic behavior in tadpoles of the African clawed frog (Xenopus laevis)

Tadpoles (Xenopus laevis) have a lateral line system whose anatomical structure has been described, but whose functional significance has not been closely examined. These experiments tested the hypothesis that the lateral line system is involved in rheotaxis. Tadpoles in developmental stages 47–56 oriented toward the source of a water current. Orientation was less precise after treatment with cobalt chloride or streptomycin, but was similar to that of untreated animals after exposure to gentamicin. In no current conditions, tadpoles exhibited a characteristic head-down posture by which they held themselves in the water column at an angle around 45°. This body posture became significantly less tilted in the presence of water current. Treatment with cobalt chloride or streptomycin increased the angle of tilt close to that seen in no current conditions, while gentamicin treatment tended to decrease tilt angle. The data are consistent with anatomical and physiological findings that tadpole neuromasts are similar to superficial, but not canal, neuromasts in fishes, and they suggest that the lateral line system is involved in both directional current detection and current-related postural adjustments in Xenopus.

Cell Proliferation in the Forebrain and Midbrain of the Adult Bullfrog, Rana catesbeiana

The distribution of proliferating cells in the midbrain, thalamus, and telencephalon of adult bullfrogs (Rana catesbeiana) was examined using immunohistochemistry for the thymidine analog 5-bromo-2′-deoxyuridine (BrdU) and DNA dot-blotting. At all time points examined (2 to 28 days post-injection), BrdU-labeled cells were located in ventricular zones at all levels of the neuraxis, but with relatively more label around the telencephalic ventricles. Labeled cells, some showing profiles indicative of dividing and migrating cells, were present in brain parenchyma from 7 to 28 days post-injection. These labeled cells were particularly numerous in the dorsal and ventral hypothalamus, preoptic area, optic tectum, and laminar and principal nuclei of the torus semicircularis, with label also present, but at qualitatively reduced levels, in thalamic and telencephalic nuclei. Double-label immunohistochemistry using glial and early neural markers indicated that gliogenesis and neurogenesis both occurred, with new neurons observed particularly in the hypothalamus, optic tectum, and torus semicircularis. In all brain areas, many cells not labeled with BrdU were nonetheless labeled with the early neural marker TOAD-64, indicating that these cells were postmitotic. Incorporation of DNA measured by dot-blotting confirms the presence of DNA synthesis in the forebrain and brainstem at all time points measured. The pattern of BrdU label confirms previous experiments based on labeling with 3H-thymidine and proliferating cell nuclear antigen showing cell proliferation in the adult ranid brain, particularly in hypothalamic nuclei. The consistent appearance of new cells in the hypothalamus of adult frogs suggests that proliferative activity may be important in mediating reproductive behaviors in these animals.

Representation of waveform periodicity in the auditory midbrain of the bullfrog, Rana catesbeiana.

AbstractThe period of complex signals is encoded in the bullfrog’s eighth nerve by a synchrony code based on phase-locked responding. We examined how these arrays of phase-locked activity are represented in different subnuclei of the auditory midbrain, the torus semicircularis (TS). Recording sites in different areas of the TS differ in their ability to synchronize to the envelope of complex stimuli, and these differences in synchronous activity are related to response latency. Cells in the caudal principal nucleus (cell sparse zone) have longer latencies, and show little or no phase-locked activity, even in response to low modulation rates, while some cells in lateral areas of the TS (magnocellular nucleus, lateral part of principal nucleus) synchronize to rates as high as 90–100 Hz. At midlevels of the TS, there is a lateral-to-medial gradient of synchronization ability: cells located more laterally show better phase-locking than those located more medially. Pooled all-order interval histograms from short latency cells located in the lateral TS represent the waveform periodicity of a biologically relevant complex harmonic signal at different stimulus levels, and in a manner consistent with behavioral data from vocalizing male frogs. Long latency cells in the caudal parts of the TS (cell sparse zone, caudal magnocellular nucleus) code stimulus period by changes in spike rate, rather than by changes in synchronized activity. These data suggest that neural codes based on rate processing and time domain processing are represented in anatomically different areas of the TS. They further show that a population-based analysis can increase the precision with which temporal features are represented in the central auditory system.

Selectivity for harmonic structure in complex sounds by the green treefrog (Hyla cinerea)

Summary1.A psychophysical technique based on reflex modification was used to study the detection of two-tone complexes in background noise by the green treefrog (Hyla cinerea). Three different twotone complexes were synthesized and presented to measure detection thresholds — a harmonic complex of 900 + 3000 Hz (periodicity of 300 Hz, mimicking the structure of the natural advertisement call); an inharmonic complex of 830+ 3100 Hz; and a second harmonic complex of 828+2760 Hz (periodicity of 276 Hz).2.Masked thresholds and ‘critical ratios’ (signal-to-noise ratios at threshold) were lowest for the two harmonic complexes (900+3000 Hz, mean ‘critical ratio’ of 16 dB; 828+2760 Hz, mean ‘critical ratio’ of 14 dB). For the inharmonic complex, for which there is no stable first-harmonic periodicity, the mean ‘critical ratio’ was 24 dB. These data suggest that the green treefrog is sensitive to the harmonic structure of complex sounds as a specific acoustic feature.3.Because of the unique structure of the treefrogs inner ear, the heightened behavioral sensitivity to harmonic complexes must be due to processing in the central, rather than peripheral, auditory system.

ATMOSPHERIC AND UNDERWATER PROPAGATION OF BULLFROG VOCALIZATIONS

ABSTRACT Male bullfrogs vocalize while partially submerged in shallow freshwater ponds. This imposes two potential propagation pathways, atmospheric and underwater, on transmission of their communication sounds. Propagation of pure tones, amplitude modulated (AM) broadband noise and natural calls was measured in air and underwater at three bullfrog breeding sites. In air, propagation losses were consistent with spherical spreading. No excess attenuation was observed for any tone frequency at any site. Both temporal envelope modulations and spectral cues are available to conspecific receivers at biologically realistic distances. The bullfrogs advertisement call is thus well adapted for transmission in air at the air/water interface. Underwater signal propagation differed at the three sites, consistent with substrate effects. Tone propagation showed the high-pass frequency window characteristic of shallow water. Broadband signals underwent propagation losses greater than expected by cylindrical spreading. ...

Perception of complex sounds by the green treefrog, Hyla cinerea: envelope and fine-structure cues

Abstract1.The envelope periodicity of communication signals is an important feature distinguishing advertisement and aggressive calls for the green treefrog (Hyla cinerea). Envelope periodicity, a cue for periodicity pitch perception in humans, is affected by the fine-structure of the signal, a cue for timbre perception in humans. The present study examined perception of two acoustic features affecting waveform fine-structure — harmonic structure and phase structure — in male green treefrogs.2.We analyzed evoked vocal responses of male green treefrogs living in laboratory arenas to playbacks of digitally-generated signals resembling either conspecific advertisement or aggressive calls in their first harmonic periodicity. Systematic changes in the harmonic structure of these signals were achieved by varying the harmonic relations between frequency components in the signals, and changes in phase structure were achieved by varying the starting phases of harmonically-related components.3.Calling was significantly influenced by the first harmonic periodicity of the signals. Males vocalized more to signals with the periodicity of the advertisement than the aggressive call. There were no differences in response to harmonic and inharmonic signals with similar spectral content. Phase structure did not significantly influence vocal responses.4.These results suggest that the fine-structure (“timbre”) of complex acoustic signals is not a significant feature guiding behavior tested using a communication response in this species.