Учебники / Hearing - From Sensory Processing to Perception Kollmeier 2007

Fig. 1 Averaged AEPs from the ICs of three Chinese frog species in response to 10 tone bursts over 1–40 kHz presented at a rate of 0.5 bursts/s. The arrows in a, b and c indicate the responses to 34, 22 and 4 kHz, respectively, the highest tone frequencies that elicited AEPs above the noise level for each species

and pk is the probability of a success, p (60/15,000), q is the probability of a failure, equal to 1–p. Using Eqs. (1) and (2), and with an interval of 15 s between stimuli, the probability that five antiphonal responses out of 47 total responses occur within a 60-ms time window by chance is 1.3 × 10−6 (binomial probability). In light of this vanishingly small probability, the most parsimonious conclusion is that the antiphonal responses are not a result of chance, but rather that males of A. tormotus detect and respond to ultrasound. Similarly, the probability that three antiphonal responses out of 45 total responses occur within a 60-ms time window in response to the AUD stimulus by chance is 7.7 × 10−4 (binomial probability).

3Physiological Evidence

In order to verify ultrasonic sensitivity, auditory-evoked potentials (AEPs) and single-unit activity were recorded from the inferior colliculus of three species of frogs: A. tormotus and Odorrana livida living near the noisy Tau Hua Creek in Huangshan Hot Springs, and the black-spotted pond frog, Pelophylax nigromaculata, an inhabitant of rice fields throughout much of China. Frogs were anesthetized by immersion in tricaine methanesulfonate and the IC was surgically exposed. Tone bursts (50–100 ms duration, 5-ms rise/fall times, presented at 0.5–1 burst/s) were broadcast from a free-field US loudspeaker (Tucker-Davis ES-1, 1–100 kHz) located 10 cm from the frog’s contralateral eardrum. The stimulus delivery system was calibrated such that the frequency response was equalized ±6 dB between 2 and 40 kHz. AEPs were averaged over 10 trials; for single unit recordings, each tone/intensity was presented 20 times to construct a PSTH.

Representative AEPs are shown in Fig. 1. A. tormotus and O. livida, two sympatric frogs living near a fast-flowing creek that generates broadband noise up to 20 kHz (Feng et al. 2006), have clear US sensitivity; P. nigromaculata

Fig. 3 A. tormutus. Scale bar: 3.5 mm

contact with the head (Fig. 3). As a consequence of the recessed eardrum, the columella (stapes) and extracolumella (extrastapes) are physically smaller and less massive than those in the large majority of frogs which have tympanic membranes on the surface of the head (Wever 1985). The tympanic membrane, as in many species of Amolops, is transparent and exceedingly thin (3–4 m) at its edges (Feng et al. 2006).

This combination of lightweight ossicles and thin tympanic membranes may be viewed as an adaptation for detection of high frequencies. Assuming the ear cavity is a simple Helmholtz resonator, its resonant frequency was calculated to be 4.3 kHz, very close to the fundamental frequency of many of the advertisement calls of this species (Narins et al. 2004). This correlation may have implications both for the reception (Capranica and Moffat 1983) and broadcasting (Purgue 1997) of the animal’s calls.

Experiments are planned to examine the specific contributions of the hair cells, amphibian papilla, basilar papilla, tectorial membrane and Eustachian tubes of Amolops to its US sensitivity as well as the directional characteristics of its specialized ear. Moreover, since the female of this species lacks the recessed eardrum, it would be of interest to compare US sensitivity between the sexes.

Although it is tempting to ascribe the US sensitivity of A. tormotus and O. livida to the outcome of selection pressure from the noisy environment, it is clear this argument does not apply in all cases. For example, it has been recently shown that one sympatric species, the piebald odorous frog, Odorrana schmackeri, lacks US sensitivity (Yu et al. 2006). Additional behavioral and physiological studies are needed to identify this trait among species inhabiting such noisy environments.

Acknowledgments. We thank Chun-He Xu, Wen-Yu Lin, Zu-Lin Yu, Qiang Qiu, Zhi-Min Xu for their help with this work. We thank M. Kowalczyk for her help drawing Fig. 3. Supported by grants from the National Institutes of Health (R01DC-00222 to PMN and R01DC04998 to ASF), a UCLA Academic Senate Grant to PMN and a grant from the National Natural Sciences Foundation to J-XS.

References

Capranica RR, Moffatt A (1983) Neurobehavioral correlates of sound communication in anurans. In: Ewert J-P, Capranica RR, Ingle DJ (eds) Advances in vertebrate neuroethology. Plenum Press, New York, pp 701–730

Feng AS, Narins PM, Xu C-H (2002) Vocal acrobatics in a Chinese frog, Amolops tormotus. Naturwissenschaften 89:352–356

Feng AS, Narins PM, Xu C-H, Lin W-Y, Yu Z-L, Qiu Q, Xu Z-M, Shen J-X (2006) Ultrasonic communication in frogs. Nature 440:333–336

Gridi-Papp M, Narins PM (2007) Sensory ecology of hearing. In: Dallos P, Oertel D, Hoy RR (eds) Handbook of the senses. Academic Press, London

Narins PM, Feng AS, Schnitzler H-U, Denzinger A, Suthers RA, Lin W, Xu C-H (2004) Old world frog and bird vocalizations contain prominent ultrasonic harmonics. J Acoust Soc Am 115:910–913

Purgue AP (1997) Tympanic sound radiation in the bullfrog Rana catesbeiana. J Comp Physiol 181:438–445

Wever EG (1985) The amphibian ear. Princeton University Press, Princeton, NJ

Yu Z-L, Qiu Q, Xu Z-M, Shen J-X (2006) Auditory response characteristics of the piebald odorous frog and their implications. J Comp Physiol 192:801–806

2Methods

We used a telemetric single-neuron recording technique, which allowed recording of hearingand vocalization-related activity in freely moving animals (for a detailed description of the method: Grohrock et al. 1997; Jürgens and Hage 2006). Neuronal activity was recorded during all call types uttered. Quantitative data analysis was done for a highly frequency-modulated type with a specific repetitive character (trill). To test whether the recorded neurons showed a consistent auditory response, bursts of white noise were used as acoustic stimuli, beside the animal’s own vocalization and vocalizations from its group mates.

For the identification of vocal-motor and auditory units, conventional perievent time histograms (PETH) and peri-stimulus time histograms (PSTH), respectively, were constructed after the original recording had passed a spikesorting procedure (template-based spike-clustering). Verification of the recording sites was carried out histologically at the end of the experiments by staining the brain sections for glial fibrillary acidic protein, allowing the identification of electrode tracks even many months after withdrawal of the electrodes (Benevento and McCleary 1992).

3Results

A total of 322 units were isolated in the ventrolateral pontine brainstem of three squirrel monkeys showing various response patterns to noise bursts. Neurons were located in SOC, the ventral nucleus of the lateral lemniscus (vLL), the lateral lemniscus (LL) and the adjacent pontine reticular formation (FRP; Fig. 1A). Most of these neurons (n = 295) did not show activity prior to the onset of own vocalizations and, therefore, were defined as auditory neurons. A small group of the isolated neurons (n =27) showed modulations of their activity (increases or decreases) to external acoustic stimuli as well as prior to and during self-produced vocalizations (for examples, see Fig. 1B). These audio-vocal units (AVU) were found at locations not described before, namely in the POR of the SOC and the adjacent pontine reticular formation. Two-thirds of the recorded AVU showed an increase of activity immediately before and during self-produced vocalization (excitatory AVU). The remaining AVU were characterized by suppression of spontaneous activity prior and during self-produced vocalization (inhibitory AVU). About one-third of recorded AVU showed activity correlated with the repetition of the trill syllables, as shown in Fig. 1B. The locations of excitatory and inhibitory AVU showed little overlap. Almost all excitatory AVU were located more laterally in the POR and the adjacent pontine reticular formation than the inhibitory AVU (see Fig 1A). A comparison of the auditory response types with those to self-produced vocalizations showed a significant non-homogeneous distribution (Fisher’s exact-test, P<0.05). Most neurons with excitatory responses to

Fig. 1 A Frontal views of the squirrel monkey’s brainstem showing the spatial distribution of recorded excitatory ( filled black circles) and inhibitory audio-vocal neurons (open circles) and the purely auditory neurons (grey dots). Scale, 500 m. B Peri-event time histograms and raster plots of excitatory and inhibitory AVU showing similar activity patterns to self-produced vocalizations and bursts of white noise. Black bars below the trill-related activity indicate the onset of all trill vocalizations (time 0.0) and the duration of the shortest trill emitted; the gray bars indicate the duration of the longest trill. Different call durations are mainly due to different numbers of syllables in trill vocalizations. Black bars below the noise-related activity indicate the onset and duration of the noise bursts (300 ms). The relationship between neuronal activity and trill syllables of a representative trill call is shown. Bin size, 5 ms. (modified from Hage et al. 2006)

self-produced vocalizations had a tonic response pattern to noise bursts (13/18). Tonic activity in inhibitory AVU, in contrast, was very rare (1/9). Inhibitory AVU mainly showed phasic (4/9) or inhibited responses (4/9). In other words, most AVU (17/27) responded similarly to external and selfproduced acoustic stimuli, with the only difference that the change in neural activity started before the acoustic stimulus when self-produced, as shown in Fig. 1B. When we compared auditory responses of excitatory and inhibitory AVU with those of close-by purely auditory neurons in the same electrode tract (Fig. 1A), we found a statistically significant relationship (P<0.01, Fisher’s exact test): except for the very caudal periolivary region, excitatory AVU were more frequently co-localized with tonically responding auditory neurons, while inhibitory AVU were more frequently co-localized with phasically responding auditory neurons.

4Discussion

4.1Modulation of the Auditory System by Vocalization

AVU showed vocalization-related excitation or inhibition prior to vocal onset, indicating that they got input from the vocalization pathways or from AVU in the upper auditory pathways. Since more than half of the recorded AVU did not follow the rhythm of trill syllables and thus did not show an activity correlated with call patterns, we suggest that these neurons received their vocalization-related input rather indirectly, possibly via projections from the auditory cortex (e.g. Mulders and Robertson 2001) or the inferior colliculus (e.g. Huffman and Henson 1990), both of which are known to modulate the activity of the olivocochlear system (e.g. Groff and Liberman 2003; Mulders and Robertson 2005; Xiao and Suga 2002).

The AVU described here may be involved in a general modulation of cochlear sensitivity to self-produced and external sounds. In the pontine brainstem, two systems are known to modulate cochlear sensitivity: the OCS, having its efferent neurons mainly in the POR of the SOC, and the middle-ear reflex, having its motoneurons ventrolateral to the trigeminal motor nucleus and ventromedial to the facial nucleus in primates (M. stapedius: Thompson et al. 1985; M. tensor tympani: Rouiller et al. 1986). Since the motoneurons of the middle-ear muscles differ in their activity patterns from the neurons recorded here (Suga and Jen 1975) and are located outside the explored area, we can exclude the middle-ear reflex as a possible function represented by the AVU activity. However, all except the two neurons dorsal of vLL could be part of the OCS.

The neurons of the lateral OCS in the squirrel monkey are located lateral and caudal of the medial nucleus of the SOC (MSO); the neurons of the medial OCS are located medial, rostral and ventral of the MSO (Thompson

and Thompson 1986). With this division, the six neurons located in the POR medial of the MSO would belong to the medial OCS, probably together with the four neurons dorsal of the rostral MSO (stereotaxic coordinate 0.5 in Fig. 1A), while the 13 other neurons in the POR and the two neurons dorsal of the POR in between the MSO and the lateral nucleus of SOC (LSO) would belong to the lateral OCS. The lateral OCS projects in the cochlea directly to the afferent fibers from the inner hair cells and/or the inner hair cells themselves, the medial OCS innervates the outer hair cells (e.g. Guinan et al. 1984).

Seven of the 10 AVU belonging to the medial OCS (as defined above) were inhibitory AVU. Their contribution to OCS function could be a tonic inhibitory influence on the olivocochlear neurons as found by Liberman (1988). The reduced activity of the inhibitory AVU during vocalization, in this case, would lead to an increased activity (disinhibition) of the olivocochlear neurons leading to a suppression of cochlear output (e.g. Wiederhold and Kiang 1970) and, thus, to a reduced cochlear sensitivity to self-produced trills. Since trills are quite loud (about 86 dB sound pressure level at a distance of 0.5 m) and have their main energy in a frequency range of high sensitivity in the squirrel monkey (Wienicke et al. 2001), the inhibitory AVU of the medial OCS could have a protective effect on the cochlea against overstimulation by self-produced trills, as has been proposed for external sounds before (e.g. Patuzzi and Thompson 1991). Since most inhibitory AVU responded to loud external noise bursts (80 dB) by inhibition or a weak phasic excitation followed by inhibition, this response would lead to the same protective effect as assumed for the selfproduced trills.

Fourteen of the 15 AVU belonging to the lateral OCS (as defined above) were excitatory AVU, responding either tonically or weakly phasically (sometimes followed by inhibition) to external sounds. The tonically responding excitatory AVU were found in the rostral (stereotaxic coordinates rostral to AP 0), the phasically responding excitatory AVU in the very caudal brainstem. The lateral OCS can have increasing and decreasing effects on the amplitude of the cochlear output by excitatory and inhibitory influences on the afferents from the cochlear inner hair cells (e.g. Mulders and Robertson 2005). Hence, tonically active excitatory AVU may sensitize or desensitize auditory nerve fibers when processing selfproduced vocalizations. Excitatory AVU responding tonically to external sounds should, according to our results, exert the same modulatory effects on auditory nerve fibers during self-produced trills. Excitatory AVU with a weak phasic or even inhibitory response to external sounds should have little or even a reversed effect compared to that of self-produced trills. Thus, AVU of the lateral OCS are expected to have diverse effects on cochlear sensitivity, partly depending on whether they become activated by self-produced and/or external sounds. Such a diversity of effects of the lateral OCS is in agreement with existing evidence (e.g. Groff and Liberman 2003).