Representation of acoustic signals in the eighth nerve of the Tokay gecko. II. Masking of pure tones with noise.

Acoustic signals are generally encoded in the peripheral auditory system of vertebrates by a duality scheme. For frequency components that fall within the excitatory tuning curve, individual eighth nerve fibers can encode the effective spectral energy by a spike-rate code, while simultaneously preserving the signal waveform periodicity of lower frequency components by phase-locked spike-train discharges. To explore how robust this duality of representation may be in the presence of noise, we recorded the responses of auditory fibers in the eighth nerve of the Tokay gecko to tonal stimuli when masking noise was added simultaneously. We found that their spike-rate functions reached plateau levels fairly rapidly in the presence of noise, so the ability to signal the presence of a tone by a concomitant change in firing rate was quickly lost. On the other hand, their synchronization functions maintained a high degree of phase-locked firings to the tone even in the presence of high-intensity masking noise, thus enabling a robust detection of the tonal signal. Critical ratios (CR) and critical bandwidths showed that in the frequency range where units are able to phaselock to the tonal periodicity, the CR bands were relatively narrow and the bandwidths were independent of noise level. However, to higher frequency tones where phaselocking fails and only spike-rate codes apply, the CR bands were much wider and depended upon noise level, so that their ability to filter tones out of a noisy background degraded with increasing noise levels. The greater robustness of phase-locked temporal encoding contrasted with spike-rate coding verifies a important advantage in using lower frequency signals for communication in noisy environments.

Representation of acoustic signals in the eighth nerve of the Tokay gecko: I. Pure tones.

A systematic study of the encoding properties of 146 auditory nerve fibers in the Tokay gecko (Gekko gecko, L) was conducted with respect to pure tones and two-tone rate suppression. Our aim was a comprehensive understanding of the peripheral encoding of simple tonal stimuli and their representation by temporal synchronization and spike rate codes as a prelude to subsequent studies of more complex signals. Auditory nerve fibers in the Tokay gecko have asymmetrical, V-shaped excitatory tuning curves with best excitatory frequencies that range from 200-5100 Hz and thresholds between 4-35 dB SPL. A low-frequency excitatory 'tail' extends far into the low-frequency range and two-tone suppression is present only on the high frequency side of the tuning curve. The response properties to pure tones at different loci within a tuning curve can differ greatly, due to evident interactions between the representations of temporal, spectral and intensity stimulus features. For frequencies below 1250 Hz, pure tones are encoded by both temporal synchronization and spike rate codes, whereas above this frequency a fiber's ability to encode the waveform periodicity is lost and only a rate code predominates. These complimentary representations within a tuning curve raise fundamental issues which need to be addressed in interpreting how more complex, bioacoustic communication signals are represented in the peripheral and central auditory system. And since auditory nerve fibers in the Tokay gecko exhibit tonal sensitivity, selective frequency tuning, and iso-intensity and iso-frequency contours that seem comparable to similar measures in birds and mammals, these issues likely apply to most higher vertebrates in general. The simpler wiring diagram of the reptilian auditory system, coupled with the Tokay gecko's remarkable vocalizations, make this animal a good evolutionary model in which to experimentally explore the encoding of more complex sounds of communicative significance.

Measurement of temporal regularity of spike train responses in auditory nerve fibers of the green treefrog.

This paper describes a method which we have developed for quantifying the temporal regularity of neural spike trains in the sensory nervous system. Our method relies on the use of a modified correlation approach for identifying response firing patterns. We apply the concept of an ambiguity function and related coefficients to measure the tonic/phase character and statistical variability of spike patterns. We tested this method in recordings from auditory nerve fibers of the green treefrog (Hyla cinerea) in response to pure tone, multi-tone, and gaussian white noise. Our results indicate that there is a great deal of variability in the trains of spike times generated by any given fiber in response to identically repeated stimulus presentations. Nevertheless, despite this statistical jitter of the pattern to repetitive stimulation, the spike response trains from a single fiber maintain a high degree of individual detectability in signal metric space. The procedures in our method can be implemented in a relatively simple way on a Macintosh computer and the speed is fast enough for real-time spike analysis. This kind of quantification may especially be useful in studying habituation and plasticity in neural spike train data, as well as in judging the selectivity within the neural code of an individual fiber for particular stimulus features.

Encoding of phase spectra by the peripheral auditory system of the bullfrog.

In this study we have examined the sensitivity of auditory nerve fibers in the bullfrog (Rana catesbeiana) to changes in the phase spectrum of an equal-amplitude multi-harmonic stimulus which spanned the bullfrog's range of hearing. To assess peripheral auditory phase sensitivity, changes in the response properties of VIIIth nerve fibers were measured when the relative phase angle of a single harmonic component nearest a unit's best excitatory frequency was systematically varied. The results revealed that shifts in the phase spectrum are encoded in at least 3 different ways by the peripheral auditory system of the bullfrog: 1) by changes in the degree of spike synchronization of fibers from both inner ear organs (the amphibian papilla and the basilar papilla) to the fundamental waveform period; 2) by changes in the shapes of period histograms of fibers from both organs; and 3) by changes in the spike rates of amphibian papilla fibers. The presence of phase sensitivity in the peripheral auditory system of the bullfrog indicates that information regarding the fine-temporal waveshape and the underlying phase spectrum of an acoustic signal is contained within the spike trains of VIIIth nerve fibers. Similar sensitivities to changes in the phase spectra and temporal waveshapes of acoustic signals may also be present in the peripheral auditory system of other vertebrates. Such studies could provide valuable insight into the role that phase spectra and temporal waveshape may play in bioacoustic communication.

A comparison of anesthetic agents and their effects on the response properties of the peripheral auditory system.

Anesthetic agents were compared in order to identify the most appropriate agent for use during surgery and electrophysiological recordings in the auditory system of the tokay gecko (Gekko gecko). Each agent was first screened for anesthetic and analgesic properties and, if found satisfactory, it was subsequently tested in electrophysiological recordings in the auditory nerve. The following anesthetic agents fulfilled our criteria and were selected for further screening: sodium pentobarbital (60 mg/kg); sodium pentobarbital (30 mg/kg) and oxymorphone (1 mg/kg); 3.2% isoflurane; ketamine (440 mg/kg) and oxymorphone (1 mg/kg). These agents were subsequently compared on the basis of their effect on standard response properties of auditory nerve fibers. Our results verified that different anesthetic agents can have significant effects on most of the parameters commonly used in describing the basic response properties of the auditory system in vertebrates. We therefore conclude from this study that the selection of an appropriate experimental protocol is critical and must take into consideration the effects of anesthesia on auditory responsiveness. In the tokay gecko, we recommend 3.2% isoflurane for general surgical procedures; and for electrophysiological recordings in the eighth nerve we recommend barbiturate anesthesia of appropriate dosage in combination if possible with an opioid agent to provide additional analgesic action.

Hormone-induced vocal behavior and midbrain auditory sensitivity in the green treefrog, Hyla cinerea.

Twenty four castrated male, 6 intact male, and 11 intact female Hyla cinerea were injected subcutaneously with 25 micrograms arginine-vasotocin (AVT) and induced to call 1 h later in response to the playback of a conspecific mating call. Eighteen castrated males and 8 intact females were implanted 5 mg androgen pellets for 3 weeks prior to the neuropeptide injection. Among castrated males, 6/9 testosterone (T) implanted, 4/9 dihydrotestosterone (DHT) implanted and 2/6 non implanted individuals produced calls after being administered AVT. 5/6 intact non implanted males and 6/8 T intact implanted females also called, and 3 intact non implanted females remained silent after the injection. Evoked calls had a mid-frequency spectral peak at about 1900 Hz which is absent in field-recorded mating calls of this species. Calls of implanted females and castrated non implanted males were shorter than those of castrated implanted and intact non implanted males. Audiograms measured before hormone implants showed dips of enhanced sensitivity at about 0.5, 0.9 and 3.0 kHz in males and females. After AVT injection, thresholds at frequencies within the 0.7-1.5 kHz range were increased in castrated males. Such reduction in sensitivity points to an inhibition of the auditory system during hormone induced vocal activation.

Tympanic and extratympanic sound transmission in the leopard frog.

The inner ear of the leopard frog, Rana pipiens, receives sound via two separate pathways: the tympanic-columellar pathway and an extra-tympanic route. The relative efficiency of the two pathways was investigated. Laser interferometry measurements of tympanic vibration induced by free-field acoustic stimulation reveal a broadly tuned response with maximal vibration at 800 and 1500 Hz. Vibrational amplitude falls off rapidly above and below these frequencies so that above 2 kHz and below 300 Hz tympanic vibration is severely reduced. Electrophysiological measurements of the thresholds of single eighth cranial nerve fibers from both the amphibian and basilar papillae in response to pure tones were made in such a way that the relative efficiency of tympanic and extratympanic transmission could be assessed for each fiber. Thresholds for the two routes are very similar up to 1.0 kHz, above which tympanic transmission eventually becomes more efficient by 15-20 dB. By varying the relative phase of the two modes of stimulation, a reduction of the eighth nerve response can be achieved. When considered together, the measurements of tympanic vibration and the measurements of tympanic and extratympanic transmission thresholds suggest that under normal conditions in this species (1) below 300 Hz extratympanic sound transmission is the main source of inner ear stimulation; (2) for most of the basilar papilla frequency range (i.e., above 1.2 kHz) tympanic transmission is more important; and (3) both routes contribute to the stimulation of amphibian papilla fibers tuned between those points. Thus acoustic excitation of the an uran's inner ear depends on a complex interaction between tympanic and extratympanic sound transmission.

Correlation between auditory thalamic area evoked responses and species-specific call characteristics. II. H. Hyla cinerea (Anura: Hylidae).

Evoked potentials were recorded from the posterior dorsal thalamus of green treefrogs (Hyla cinerea) in response to single tones and combinations of two and three tones. 1. The responses to two tones were largest when one of the component tones was 500 Hz and when the second component was between 2000 and 4000 Hz (Fig.3). 2. The response to 500 + 3000 Hz showed nonlinear facilitation; i.e., the amplitude of the response was greater than the sum of the responses to the component tones alone (Figs. 4, 5). This result provides evidence that cells functioning as 'AND' gates will be found in this center. 3. When a third tone around 1200 Hz was added to a stimulus of 500 + 3000 Hz a 65% decrease in the evoked response amplitude occurred (Fig. 6). 4. The largest evoked response amplitude to a two-tone stimulus (500 + 3000 Hz) occurred when the rise-time was less than 50 ms (Fig. 7). 5. The two-tone tuning was found to be temperature dependent. The optimal lower frequency tone shifted downward with decreasing temperatures (Fig. 8). 6. When the temperatures of the neurophysiological and the behavioral experiments are matched, the optimal stimuli for evoking a large response are closely correlated to the parameters of the acoustic stimuli preferred by gravid H. cinerea females in discrimination tests. This center therefore appears to be very important for the processing of complex species-specific sounds.

Correlation between auditory evoked responses in the thalamus and species-specific call characteristics. I. Rana catesbeiana (Anura: Ranidae).

This evoked potential study of the bullfrog's auditory thalamic area (an auditory responsive region in the posterior dorsal thalamus) shows that complex processing, distinct from that reported in lower auditory regions, occurs in this center. An acoustic stimulus consisting of two tones, one which stimulates either the low-frequency or the mid-frequency sensitive population of auditory nerve fibers from the amphibian papilla and the other the high-frequency sensitive population of fibers from the basilar papilla, evoked a maximal response. The amplitude of the response to the simultaneous stimulation of the two auditory organs was, in some locations, much larger than the linear sum of the responses to the individual tones presented separately. Bimodal spectral stimuli that had relatively long rise-times (greater than or equal to 100 ms) evoked much larger responses than similar sounds with short rise-times. The optimal rise-times were close to those occurring in the bullfrog's mating call. The response was dependent on the waveform periodicity and harmonic content, with a fundamental frequency of 200 Hz producing a larger response than those with fundamentals of 50, 100 or 300 Hz. Six of the natural calls in the bullfrog's vocal repertoire were tested and the mating call and warning call were found to evoke the best responses. Each of these calls stimulate the two auditory organs simultaneously. The evoked response had a long refractory period which could not be altered by lesioning the efferent telencephalic pathways. The type of spectral and temporal information extracted by the auditory thalamic area suggests that this center is involved in processing complex sounds and likely plays an important role in the bullfrog's detection of some of its vocal signals.

Bilateral syringeal coupling during phonation of a songbird.

The syrinx of oscine birds ("true songbirds") is a double vocal organ, and each side has generally been presumed to function independently under separate neural control during phonation. A significant counterexample is demonstrated here in the production of a common vocalization by the black-capped chickadee (Parus atricapillus), in which the 2 acoustic sources of the syrinx interact in a nonlinear fashion. The chickadee produces a sound with multiple frequency components that superficially resemble harmonics. An analysis of vocal production after unilateral and bilateral syringeal denervation shows instead that these frequency components are sum and difference frequencies, or heterodyne frequencies, resulting from cross-modulation between the 2 syringeal sides. A limited form of this bilateral coupling may be achieved after unilateral denervation of either syringeal side but not after bilateral denervation. Unilaterally denervated birds are capable of significant improvement in coupling after 10 d, too short a period for neural regrowth. These results suggest that coupling arises from a passive physical interaction between the 2 syringeal sources which is activated or regulated in some fashion by neural control from either side.

Bilateral syringeal interaction in vocal production of an oscine bird sound.

The vocal organ, or syrinx, of oscine birds has two parts, each of which has generally been presumed to operate independently of the other. A significant counter-example is now demonstrated in the production of a common vocalization by the black-capped chickadee (Parus atricapillus), in which the two acoustic sources interact in a nonlinear fashion. This bird produces a sound with multiple frequency components that are heterodyne products resulting from cross-modulation between two signals, thus providing evidence that avian phonation can involve cooperative coupling between the two syringeal sources.

Species specificity and temperature dependency of temporal processing by the auditory midbrain of two species of treefrogs.

The mating (advertisement) calls of two sibling species of gray treefrogs, Hyla versicolor and Hyla chrysoscelis, are spectrally identical but differ in trill rate; being higher for H. chrysoscelis. Single-unit recordings were made from the torus semicircularis of both species to investigate the neural mechanisms by which this species-specific temporal feature is analyzed. Using sinusoidally amplitude-modulated (AM) white noise as a stimulus, the temporal selectivity of these midbrain auditory neurons could be described by five response categories: 'AM nonselective' (34%); 'AM high-pass' (7%); 'AM low-pass' (6%); 'AM band-suppression' (12%); 'AM tuned' (40%). The distributions of temporal tuning values (i.e., modulation rate at which each AM-tuned unit responds maximally) are broad; in both species, neurons were found which were tuned to modulation rates greater than those found in their advertisement calls. Nevertheless, the temporal tuning values for H. versicolor (median = 25 Hz) were significantly lower than those for H. chrysoscelis (median = 32.5 Hz). The temporal selectivities of AM band-suppression neurons were found to be temperature dependent. The modulation rate at which a response minimum was observed shifted to higher values as the temperature was elevated. These results extend our earlier findings of temperature-dependent temporal selectivity in the gray treefrog. The selectivity of band-suppression and AM-tuned neurons to various rates of amplitude modulation was largely, but not completely, independent of whether sinusoidal or natural forms of AM were used.

Neural correlates of temperature coupling in the vocal communication system of the gray treefrog (Hyla versicolor).

The central nervous system of the gray treefrog includes a class of auditory neurons which respond selectively to particular rates of amplitude modulation. When body temperature is increased, the temporal tuning of these neurons shifts to higher rates of modulation. This shift in tuning parallels the temperature-dependent shift in the trill rate in the male's mating call, thus constituting the neural correlate of temperature coupling between the female's behavioral selectivity and the temporal signal characteristics in the male's call.

Sensitivity to amplitude modulated sounds in the anuran auditory nervous system.

Auditory responses were recorded from single units in the eighth nerve and in the midbrain torus semicircularis of the leopard frog (Rana pipiens). Acoustic stimuli included sinusoidally amplitude-modulated (AM) tones and noise, as well as pure tones. Mean spike rates were measured at various rates of AM, and the degree to which a unit's spikes were restricted to a particular phase of the modulation cycle was described by a synchronization coefficient. The firing rate of eighth-nerve fibers was largely independent of the rate of AM over the modulation range 10 to 150 Hz. Further, the general shape of the spike rate vs. AM-rate function was invariant with either depth of modulation or sound-pressure level (SPL). Although virtually all eighth-nerve fibers exhibited significant synchronization to the envelope of AM, the shape of the synchronization function depended on the unit's best-excitatory frequency (BEF). Fibers with highest BEF's, presumed to innervate the basilar papilla, generally showed greater synchronization as the AM rate was increased (up to 100-150 Hz). Fibers tuned to the low-and midfrequency region, which innervate the amphibian papilla, exhibited low-pass synchronization characteristics. As the depth of modulation was reduced, the degree of synchronization of eighth-nerve fibers decreased. For a given depth of modulation an increase in sound level tended to decrease the degree of synchronization, but significant synchronization could still be observed at stimulus intensities at least 65 dB above threshold. On the basis of the spike rate vs. AM-rate functions, the temporal selectivity of single cells in the torus could be characterized by five response types: AM nonselective (spike rate was largely independent of the AM rate); AM high-pass (activity increased as the AM rate was increased); AM low-pass (response was greatest for slow AM rates and decreased at high rates); AM band-suppression (these neurons responded well to low and high AM rates, but responded weakly to intermediate rates); and AM-tuned (spike rate was greatest over a narrow range of modulation rates). In these measurements the depth of modulation was held constant at 100%. The five response categories are not discrete, but rather reflect representative examples along a continuum with regard to temporal selectivity. The temporal selectivity exhibited by toral units in their firing rates was not evident in their AM-synchronization functions.(ABSTRACT TRUNCATED AT 400 WORDS)

Auditory input to a vocal nucleus in the frog Rana pipiens: hormonal and seasonal effects.

In Rana pipiens, single unit recordings from the pretrigeminal nucleus (pre-V), a nucleus involved in call production, demonstrated neurons that respond to auditory stimuli. Most of these neurons had V-shaped tuning curves, with best excitatory frequencies between 200-1400 Hz, thresholds between 31-87 dB SPL, latencies between 10-50 ms, and Q (10 dB) between 1.3-5.1. The number of pre-V neurons that responded to acoustic stimulation increased after gonadotropin injections, and appeared to increase during the breeding season. In addition, a small number of neurons with more complex response properties, such as W-shaped tuning curves or sensitivity to white noise but not to pure tones, appeared after hormone treatments. The hormonal and possible seasonal effects on pre-V auditory activity suggest that the auditory input to this vocal nucleus may play a role in reproductive behavior.

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.

Temporal selectivity in the central auditory system of the leopard frog.

Amplitude modulation is a predominant temporal feature in many vocal signals. The leopard frog, Rana pipiens, has a class of neurons in the central auditory system that respond selectively to particular rates of amplitude modulation; these neurons can be characterized by a temporal tuning curve. Such selectivity is absent in the peripheral auditory system. This type of transformation may be fundamental in processing temporal information in the vertebrate sensory nervous system.

Aggressive signal in "courtship" chirps of a gregarious cricket.

Unlike other known species of crickets, Amphiacusta maya in Central America mates in groups. Experimentally silenced males experience reduced mating success, not owing to decreased receptivity by females, but owing to increased time spent fighting with other males that persistently interrupt silent courtships. Thus, the data indicate that "courtship" chirping functions as a warning to other males, rather than as a signal to females.