The effect of contralateral stimulation on cochlear resonance and damping in the mustached bat: the role of the medial efferent system

Responses to Social Vocalizations in the Dorsal Cochlear Nucleus of Mice

Identifying sounds is critical for an animal to make appropriate behavioral responses to environmental stimuli, including vocalizations from conspecifics. Identification of vocalizations may be supported by neuronal selectivity in the auditory pathway. The first place in the ascending auditory pathway where neuronal selectivity to vocalizations has been found is in the inferior colliculus (IC), but very few brainstem nuclei have been evaluated. Here, we tested whether selectivity to vocalizations is present in the dorsal cochlear nucleus (DCN). We recorded extracellular neural responses in the DCN of mice and found that fusiform cells responded in a heterogeneous and selective manner to mouse ultrasonic vocalizations. Most fusiform cells responded to vocalizations that contained spectral energy at much higher frequencies than the characteristic frequencies of the cells. To understand this mismatch of stimulus properties and frequency tuning of the cells, we developed a dynamic, nonlinear model of the cochlea that simulates cochlear distortion products on the basilar membrane. We preprocessed the vocalization stimuli through this model and compared responses to these distorted vocalizations with responses to the original vocalizations. We found that fusiform cells in the DCN respond in a heterogeneous manner to vocalizations, and that these neurons can use distortion products as a mechanism for encoding ultrasonic vocalizations. In addition, the selective neuronal responses were dependent on the presence of inhibitory sidebands that modulated the response depending on the temporal structure of the distortion product. These findings suggest that important processing of complex sounds occurs at a very early stage of central auditory processing and is not strictly a function of the cortex.

Isoflurane increases amplitude and incidence of evoked and spontaneous otoacoustic emissions

The volatile anesthetic isoflurane was tested for its effect on cochlear function by means of measuring distortion product otoacoustic emissions (DPOAE) and spontaneous otoacoustic emissions (SOAE) in the mustached bat (Pteronotus parnellii parnellii). Averaged growth functions of DPOAE and spontaneous otoacoustic emissions were assessed and compared between the control group (no isoflurane application) and the isoflurane group (application of isoflurane at vaporizer settings sof about 1.5-2%). Isoflurane significantly increases the DPOAE amplitude, e.g. at a primary tone level l2 of 40 dB SPL by 10.7 dB. Additionally, the incidence of SOAEs was highly increased during application of isoflurane. The sound-evoked efferent effect on the generation of otoacoustic emissions was significantly reduced in the isoflurane group. We suggest that isoflurane might affect the postsynaptic action of acetylcholine (ACh) released by the efferent terminals of outer hair cells (OHCs). This could lead to the observed decrease of efferent suppression and to a disinhibition of cochlear amplification.

Sound-evoked efferent effects on cochlear mechanics of the mustached bat

The influence of the crossed medial efferent system on cochlear mechanics of the mustached bat was tested by measuring delayed evoked otoacoustic emissions (DEOAEs), cochlear microphonics, distortion product otoacoustic emissions (DPOAEs) and stimulus frequency otoacoustic emissions. Contralaterally delivered sinusoids, broadband noise and bat echolocation calls were used for acoustic stimulation of the efferent system. With all four measures we found a level-dependent suppression under stimulation with both broadband noise and echolocation calls. In addition, the sharply tuned cochlear resonance of the mustached bat which is involved in processing echolocation signals at 61 kHz shifted upward in frequency by several 100 Hz. Presentation of sinusoids did not have any significant effect. DEOAEs and DPOAEs were in some cases enhanced during contralateral presentation of the bat calls at moderate intensities. The most important function of the efferent system in the mustached bat might be the control of the extraordinarily fine-tuned resonator of this species, which is close to instability as evident from the very pronounced evoked otoacoustic emissions which sometimes convert into spontaneous otoacoustic emissions of high level.

The development of a single frequency place in the mammalian cochlea: the cochlear resonance in the mustached bat Pteronotus parnellii

Cochlear microphonic potentials (CMs) were recorded from the sharply tuned, strongly resonant auditory foveae of 1- to 5-week-old mustached bats that were anesthetized with Rompun and Ketavet. The fovea processes Doppler-shifted echo responses of the constant-frequency component of echolocation calls. During development, the frequency and tuning sharpness of the cochlear resonance increases, and CM ringing persists for longer after the tone. CM is relatively insensitive at tone onset and grows linearly with increased stimulus level. During the tone, the CM is more sensitive and grows compressively with increased stimulus level and phase leads onset CM by 90 degrees for frequencies below the resonance. CM during the ringing is also sensitive and compressive and phase leads onset CM by 180 degrees below the resonance and lags it by 180 degrees above the resonance. Throughout postnatal development, CMs measured during the tone and in the ringing increase both in sensitivity and compression. The cochlear resonance appears to be attributable to interaction between two oscillators. The more broadly tuned oscillator dominates the onset response, and the narrowly tuned oscillator dominates the ringing. Early in development, mechanical coupling between the oscillators results in a relatively broadly tuned system with several frequency modes in the CM at tone onset and in the CM ringing. Beating occurs between the resonance and the stimulus response during the tone and between two components of the narrowly tuned oscillator at tone offset. At maturity, the CM has three modes for frequencies within 10 kHz of the resonance at tone onset and a single, sharply tuned mode in the ringing.

Synchronization of a nonlinear oscillator: processing the cf component of the echo-response signal in the cochlea of the mustached bat

Cochlear microphonic potential (CM) was recorded from the CF2 region and the sparsely innervated zone (the mustached bat's cochlea fovea) that is specialized for analyzing the Doppler-shifted echoes of the first-harmonic (approximately 61 kHz) of the constant-frequency component of the echolocation call. Temporal analysis of the CM, which is tuned sharply to the 61 kHz cochlear resonance, revealed that at the resonance frequency, and within 1 msec of tone onset, CM is broadly tuned with linear magnitude level functions. CM measured during the ongoing tone and in the ringing after tone offset is 50 dB more sensitive, is sharply tuned, has compressive level functions, and the phase leads onset CM by 90 degrees: an indication that cochlear responses are amplified during maximum basilar membrane velocity. For high-level tones above the resonance frequency, CM appears at tone onset and after tone offset. Measurements indicate that the two oscillators responsible for the cochlear resonance, presumably the basilar and tectorial membranes, move together in phase during the ongoing tone, thereby minimizing net shear between them and hair cell excitation. For tones within 2 kHz of the cochlear resonance the frequency of CM measured within 2 msec of tone onset is not that of the stimulus but is proportional to it. For tones just below the cochlear resonance region CM frequency is a constant amount below that of the stimulus depending on CM measurement delay from tone onset. The frequency responses of the CM recorded from the cochlear fovea can be accounted for through synchronization between the nonlinear oscillators responsible for the cochlear resonance and the stimulus tone.

The innervation of outer hair cells: 3D reconstruction from TEM serial sections in the Japanese macaque

Transmission electron micrographs from serial sections were obtained from the neural pole of outer hair cells (OHCs) in the Japanese macaque (Macaca fuscata) and reconstructions of nerve terminals were made using computer software. Data are based on observations of six cells in the basal turn, eight in the middle turn and four in the apex. In general, the number of afferent (type II) terminals on each OHC increased from base to apex, and for a given turn, the numbers appeared unrelated to OHC row. On the other hand, the number of efferent terminals was greater in the middle turn than in other areas, and the number decreased from row 1 to row 3. Reciprocal synapses increased in frequency from the upper basal turn apicalward. The total number of terminals synapsing on an individual OHC increased from base to apex by nearly 100%. Three-dimensional reconstructions showed that nerve fibers terminating on basal and middle turn OHCs ascended directly from sub-OHC regions to synapse on the subnuclear regions of the OHC. In contrast, apical turn fibers ran horizontally at the level of the subnuclear region and the terminals appeared as en passant swellings along a single fiber. Although physiological data are wanting for the macaque, the anatomical findings suggest that functional differences may exist along the length of the cochlea.

Tonic efferent-induced cochlear damping in roosting and echolocating mustached bats

The activity of the medial olivocochlear (MOC) efferent system in mustached bats, Pteronotus p. parnellii, was studied by monitoring changes in the mechanical properties of the cochlea. The changing properties were expressed by the decay time (DT) of cochlear microphonic potentials produced by transient-induced ringing (Henson et al., 1995). Tape-recorded roost noise (biosonar and communication sounds) produced sudden, marked decreases in DT when presented to the contralateral ear of animals adapted to the quiet. When the animals were first removed from their roosts the DT was relatively short (1.2-1.5 ms) but this gradually lengthened by about 0.5-1.0 ms as they rested in a quiet chamber. The time required to reach a stable, quiet-adapted state after noise exposure varied with SPL and exposure time; in many experiments recovery was in the range of 90-120 min. When quiet-adapted bats were isolated and allowed to fly and echolocate for 20 min, the DTs measured within a few minutes after the end of the flight were also short and only slowly returned to longer preflight values. The administration of a single dose of gentamicin, which blocks MOC effects, greatly reduced the amount of suppression (damping) observed after periods of noise and echolocation sound exposure. We conclude that tonic MOC activity is induced by the natural vocalizations and roost noise and this activity probably regulates and protects the highly resonant cochlear partition.

Changes in cochlear mechanics during vocalization: evidence for a phasic medial efferent effect

The mustached bat, Pteronotus p. parnellii, has a finely tuned cochlea that rings at its resonant frequency in response to an acoustic tone pip. The decay time (DT) and frequency of these damped oscillations can be measured from the cochlear microphonic potential (CM) to study changes in cochlear mechanics. In this report, we describe phasic changes that occur in synchrony with communication sound vocalizations of the bat. Three animals with chronically implanted electrodes were studied. During the experiments, 1-2 ms tone pips were emitted from a speaker every 200 ms. This triggered a computer analysis of the resulting CM to determine the DT and cochlear resonance frequency (CRF) of the ringing. The time relative to vocalizations was determined by monitoring the output of a microphone placed near a bat's mouth. Similar results were obtained from all three bats tested. In a representative case, the average DT was 2.33 +/- 0.25 ms while the bat was quiet, but it decreased by 46% to 1.26 +/- 0.75 during vocalizations, which indicates a greater damping of the cochlear partition. Sometimes, DT started decreasing immediately before the bat vocalized. After the end of a vocalization, the return to baseline values varied from rapid (milliseconds) to gradual (1-2 seconds). The CRF also changed from baseline values during vocalization, although the amount and direction of change were not predictable. When gentamicin was administered to block the action of medial olivocochlear (MOC) efferents, DT reduction was still evident during vocalization but less pronounced. We conclude that phasic changes in damping occur in synchrony with vocalization, and that the MOC system plays a role in causing suppression. Since suppression can begin prior to vocalization, this may be a synkinetic effect, mediated by neural outflow to the ear in synchrony with neural outflow to the middle ear muscles and the muscles used for vocalization.

The course and distribution of medial efferent fibers in the cochlea of the mustached bat

The course and distribution of medial olivocochlear (MOC) nerve fibers were studied in the cochlea of the mustached bat. This animal is of interest because of the very sharp tuning of the ear and fine frequency resolution in small frequency bands near 60 and 90 kHz. The MOC fibers arise from about 400 cells in the dorsomedial periolivary (DMPO) nucleus and they are distributed to approximately 4500 outer hair cells (OHCs), resulting in an average OHC unit size of 11.25. Individual fibers appear to have a small number of branches and each branch entering the tunnel of Corti terminates on a patch of OHCs. The patch size is typically 1-3 OHCs with the smallest average patch sizes in the regions tuned to 60 and 90 kHz. The majority of the MOC terminals are derived from the contralateral DMPO. Contralateral vs. ipsilateral projecting fibers are not preferentially distributed within any of the three rows of OHCs or within specific regions throughout most of the cochlea. It can be concluded that the main differences between the mustached bat's MOC system and that of most other mammals are: (1) origin from a single nucleus; (2) relatively small sizes of the patches; (3) a single terminal on each OHC; (4) a gradient in the size of the terminals but not in the number of terminals from row to row or from base to apex.