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PubMed2005

Tonotopic specialization of auditory coincidence detection in nucleus laminaris of the chick.

The interaural time difference (ITD) is a cue for localizing a sound source along the horizontal plane and is first determined in the nucleus laminaris (NL) in birds. Neurons in NL are tonotopically organized, such that ITDs are processed separately at each characteristic frequency (CF). Here, we investigated the excitability and coincidence detection of neurons along the tonotopic axis in NL, using a chick brainstem slice preparation. Systematic changes with CF were observed in morphological and electrophysiological properties of NL neurons. These properties included the length of dendrites, the input capacitance, the conductance of hyperpolarization-activated current, and the EPSC time course. In contrast to these gradients, the conductance of low-threshold K+ current and the expression of Kv1.2 channel protein were maximal in the central (middle-CF) region of NL. As a result, the middle-CF neuron had the smallest input resistance and membrane time constant, and consequently the fastest EPSP, and exhibited the most accurate coincidence detection. The specialization of middle-CF neurons as coincidence detectors may account for the high resolution of sound-source localization in the middle-frequency range observed in avians.

Action Potentials↗
PubMed2005

Neural sensitivity to interaural envelope delays in the inferior colliculus of the guinea pig.

Interaural time differences (ITDs) are important cues for mammalian sound localization. At high frequencies, sensitivity to ITDs, which are conveyed only by the envelope of the waveforms, has been shown to be poorer than sensitivity to ITDs at low frequencies, which are conveyed primarily by the fine structure of the waveforms. Recently, human psychophysical experiments have demonstrated that sensitivity to envelope-based ITDs in high-frequency transposed tones can be equivalent to low-frequency fine-structure-based ITD sensitivity. Transposed tones are designed to provide high-frequency auditory nerve fibers (ANFs) with similar temporal information to that provided by low-frequency tones. We investigated neural sensitivity to ITDs in high-frequency transposed and sinusoidally amplitude modulated (SAM) tones, in the inferior colliculus of the guinea pig. Neural sensitivity to ITDs in transposed tones was found to be greater than that to ITDs in SAM tones; in response to transposed tones, neural firing rates were more modulated as a function of ITD and discrimination thresholds were found to be lower than those in response to SAM tones. Similar to psychophysical findings, ITD discrimination of single neurons in response to transposed tones for rates of modulation <250 Hz was comparable to neural discrimination of ITDs in low-frequency tones. This suggests that the neural mechanisms that mediate sensitivity to ITDs at high and low frequencies are functionally equivalent, provided that the stimuli result in appropriate temporal patterns of action potentials in ANFs.

Acoustic Stimulation↗
PubMed2006

Binaural response properties of low-frequency neurons in the gerbil dorsal nucleus of the lateral lemniscus.

Differences in intensity and arrival time of sounds at the two ears, interaural intensity and time differences (IID, ITD), are the chief cues for sound localization. Both cues are initially processed in the superior olivary complex (SOC), which projects to the dorsal nucleus of the lateral lemniscus (DNLL) and the auditory midbrain. Here we present basic response properties of low-frequency (< 2 kHz) DNLL neurons and their binaural sensitivity to ITDs and IIDs in the anesthetized gerbil. We found many neurons showing binaural properties similar to those reported for SOC neurons. IID-properties were similar to that of the contralateral lateral superior olive (LSO). A majority of cells had an ITD sensitivity resembling that of either the ipsilateral medial superior olive (MSO) or the contralateral LSO. A smaller number of cells displayed intermediate types of ITD sensitivity. In neurons with MSO-like response ITDs that evoked maximal discharges were mostly outside of the range of ITDs the gerbil naturally experiences. The maxima of the first derivative of their ITD-functions (steepest slope), however, were well within the physiological range of ITDs. This finding is consistent with the concept of a population rather than a place code for ITDs. Moreover, we describe several other binaural properties as well as physiological and anatomical evidence for a small but significant input from the contralateral MSO. The large number of ITD-sensitive low-frequency neurons implicates a substantial role for the DNLL in ITD processing and promotes this nucleus as a suitable model for further studies on ITD-coding.

Acoustic Stimulation↗
PubMed1996

In vitro analysis of optimal stimuli for phase-locking and time-delayed modulation of firing in avian nucleus laminaris neurons.

Neurons of the avian nucleus laminaris (NL) provide a neural substrate for azimuthal sound localization. We examined the optimal stimuli for NL neurons to maintain high discharge rates, reliable phase-locking, and sensitivity to time-delayed stimuli. Whole-cell recordings were performed in chick [embryonic days 19-21 (E19-E21)] NL neurons using an in vitro slice preparation. Variation of membrane properties along the tonotopic axis was examined. Computer-controlled intracellular current injection was used to mimic postsynaptic currents or conductances (PSCs) generated in NL neurons by the firing of nucleus magnocellularis (NM) neurons during acoustic stimulation. At various stimulus frequencies, the effects of varying the number of NM cells and PSC amplitudes on firing rate and phase-locking were examined. During high-frequency stimulation, the greatest firing rate and phase-locking occurred when the protocol contained few NM cells that generated large PSCs. Because the stimulus-evoked unitary PSCs are small, we propose that NM cells fire in synchrony to generate large PSCs. To mimic the arrival of PSCs during binaural stimulation, two stimulus trains were summed at different delays before injection. The firing rate of NL neurons was greatest with zero delay. A delay of half the stimulus period evoked firing that was less than that evoked with a single train. Neurons lacking strong outward rectification exhibited neither reliable phase-locking during high-frequency stimulation nor sensitivity to stimulus delays. These findings suggest that the firing responses of NL neurons are determined primarily by their membrane properties.

Acoustic Stimulation↗
PubMed2000

Neuronal sensitivity to interaural time differences in the sound envelope in the auditory cortex of the pallid bat.

Interaural time differences in the envelope of a sound (envelope ITDs) can potentially provide spatial information at high frequencies where interaural phase differences (IPDs) are not available. Interaural intensity differences (IIDs) also provide important spatial information at high frequencies. Both IIDs and envelope ITDs can influence spatial perception at high frequencies, but behavioral and physiological studies suggest that IIDs dominate perception. This study examines envelope ITD sensitivity in the auditory cortex of the pallid bat, a species that uses passive sound localization at the low end of its audible range to find prey. Its auditory system is entirely 'high-frequency' in that phase-locking does not occur at the low end of its audible range. If the bat uses ITDs, they must be derived from the envelope of the signal. A previous study of envelope ITD sensitivity in its inferior colliculus (IC) reported that neurons are sensitive to the small +/-70 micros range of available ITDs. This study extends these findings to the cortical level to assess the transformation of ITD sensitivity and the binaural response properties that underlie this sensitivity. Two measures of sensitivity were used. The dynamic ITD range measures the range of ITDs over which the maximum response of a neuron decreases by 80%. When presented with square-wave amplitude-modulated tones statically delayed in arrival time, the average dynamic ITD range in the IC is 304 micros, but dropped to 175 micros in auditory cortex. IC neurons average a 38% change in maximum response over the relevant ITD range, while cortical neurons average a 67% change. Also measured were time-intensity trading ratios, which index the extent to which a change in IID can cause a shift the dynamic ITD range. Average trading ratios are approximately the same in the IC and auditory cortex (17.9 micros/dB vs. 16.7 micros/dB, respectively). Binaural interactions changed from the IC to auditory cortex. In IC, ITD sensitivity is an inhibitory, subtractive process in which ITDs reduce the response evoked by contralateral monaural stimulation. In the auditory cortex, both binaural inhibition and facilitation occur. In the majority of cortical neurons, IID and ITD functions were remarkably similar in shape, having stepped, step-peaked or peaked functions. The binaural interactions (inhibition and/or facilitation) evoked by ITDs and IIDs were also typically similar. These results suggest that IIDs and envelope ITDs are having similar effects on output of the same binaural comparator system.

Acoustic Stimulation↗
PubMed2005

Minimum audible angle, just noticeable interaural differences and speech intelligibility with bilateral cochlear implants using clinical speech processors.

Sound localization and speech intelligibility were assessed in 5 patients implanted bilaterally with Medel C40+ or Medel C40 cochlear implant (CI) systems. The minimum audible angle (MAA) around the head in the horizontal plane was assessed in patients with bilateral CI using white noise bursts of 1000 ms duration presented from a loudspeaker mounted on a rotating boom and compared with the MAA of age-matched normal hearing controls. Spatial discrimination was found to be good in front and in the back of the head with near-normal MAA values (patients: 3-8 degrees , controls: 1-4 degrees ). In contrast, poor performance on the sides was found (patients: 30 to over 45 degrees , controls 7-10 degrees ). Bilateral CI significantly improved spatial discrimination in front for all patients, when compared with the use of either CI alone. Just noticeable differences (JNDs) in interaural intensity and time were assessed using white noise bursts (1000 ms duration; 50 ms linear ramp). In addition, interaural time JNDs were assessed using click trains (800 ms duration, 40 mus clicks, 50 Hz) and noise bursts in which either only the envelope or only the fine structure was shifted in time. In comparison with normal hearing controls, patients with bilateral CI showed near-normal interaural intensity JNDs but substantially poorer interaural time JNDs depending on the type of stimulus. In contrast to envelope onset/offset cues, interaural fine structure time differences were not perceived by the patients using CI systems employing the continuous interleaved sampling strategy without synchronization between their pulse stimulation times. Speech intelligibility in quiet and CCITT noise from the side (+/-90 degrees ) was assessed using the German HSM sentence test and was significantly better when using bilateral CI in comparison with either unilateral CI, mainly due to a head shadow effect. These favorable results are in agreement with the patients' subjective experiences assessed with a questionnaire and support the use of bilateral CI.

Adolescent↗
PubMed1999

Shift from depolarizing to hyperpolarizing glycine action in rat auditory neurones is due to age-dependent Cl- regulation.

1. The inhibitory neurotransmitter glycine can elicit depolarizing responses in immature neurones. We investigated the changes in glycine responses and their ionic mechanism in developing neurones of the rat lateral superior olive (LSO), an auditory brainstem nucleus involved in sound localization. 2. Whole-cell and gramicidin perforated-patch recordings were performed from visually identified LSO neurones in brain slices and glycine was pressure applied for 3-100 ms to the soma. Glycine-evoked currents were reversibly blocked by strychnine. They were mostly monophasic, but biphasic responses occurred in approximately 30 % of P8-11 neurones in perforated-patch recordings. 3. In whole-cell recordings from P2-11 neurones, the reversal potential of glycine-evoked currents (EGly) was determined by the transmembranous Cl- gradient and corresponded closely to the Nernst potential for Cl-, regardless of age. This indicates that Cl- is the principle ion permeating glycine receptors, but is also consistent with a low relative (10-20 %) permeability for HCO3-. The Cl- gradient also determined the polarity and amplitude of glycine-evoked membrane potential changes. 4. Leaving the native intracellular [Cl-] undisturbed with gramicidin perforated-patch recordings, we found a highly significant, age-dependent change of EGly from -46.8 +/- 1.8 mV (P1-4, n = 28) to -67.6 +/- 3.3 mV (P5-8, n = 10) to -82.2 +/- 4.1 mV (P9-11, n = 18). The majority of P1-4 neurones were depolarized by glycine ( approximately 80 %) and spikes were evoked in approximately 30 %. In contrast, P9-11 neurones were hyperpolarized. 5. In perforated-patch recordings, EGly was influenced by the voltage protocol and the glycine application interval; it could be shifted in the positive and negative direction. For a given application interval, these shifts were always larger in P1-4 than in P8-11 neurones, pointing to less effective Cl- regulation mechanisms in younger neurones. 6. Furosemide (frusemide), a blocker of cation-Cl- cotransporters, reversibly shifted EGly in the negative direction in P2-4 neurones, yet in the positive direction in P8-10 neurones, suggesting the blockade of net inward and net outward Cl- transporters, respectively. 7. Taken together, age-dependent changes in active Cl- regulation are likely to cause the developmental shift from depolarizing to hyperpolarizing glycine responses. A high intracellular [Cl-] is generated in neonatal LSO neurones which decreases during maturation.

Aging↗
PubMed1990

A circuit for detection of interaural time differences in the brain stem of the barn owl.

Detection of interaural time differences underlies azimuthal sound localization in the barn owl Tyto alba. Axons of the cochlear nucleus magnocellularis, and their targets in the binaural nucleus laminaris, form the circuit responsible for encoding these interaural time differences. The nucleus laminaris receives bilateral inputs from the cochlear nucleus magnocellularis such that axons from the ipsilateral cochlear nucleus enter the nucleus laminaris dorsally, while contralateral axons enter from the ventral side. This interdigitating projection to the nucleus laminaris is tonotopic, and the afferents are both sharply tuned and matched in frequency to the neighboring afferents. Recordings of phase-locked spikes in the afferents show an orderly change in the arrival time of the spikes as a function of distance from the point of their entry into the nucleus laminaris. The same range of conduction time (160 mu sec) was found over the 700-mu m depth of the nucleus laminaris for all frequencies examined (4-7.5 kHz) and corresponds to the range of interaural time differences available to the barn owl. The estimated conduction velocity in the axons is low (3-5 m/sec) and may be regulated by short internodal distances (60 mu m) within the nucleus laminaris. Neurons of the nucleus laminaris have large somata and very short dendrites. These cells are frequency selective and phase-lock to both monaural and binaural stimuli. The arrival time of phase-locked spikes in many of these neurons differs between the ipsilateral and contralateral inputs. When this disparity is nullified by imposition of an appropriate interaural time difference, the neurons respond maximally. The number of spikes elicited in response to a favorable interaural time difference is roughly double that elicited by a monaural stimulus. Spike counts for unfavorable interaural time differences fall well below monaural response levels. These findings indicate that the magnocellular afferents work as delay lines, and the laminaris neurons work as co-incidence detectors. The orderly distribution of conduction times, the predictability of favorable interaural time differences from monaural phase responses, and the pattern of the anatomical projection from the nucleus laminaris to the central nucleus of the inferior colliculus suggest that interaural time differences and their phase equivalents are mapped in each frequency band along the dorsoventral axis of the nucleus laminaris.

Acoustic Stimulation↗
PubMed1992

Patterns of glutamate, glycine, and GABA immunolabeling in four synaptic terminal classes in the lateral superior olive of the guinea pig.

The goal of this study was to correlate synaptic ultrastructure with transmitter specificity and function in the lateral superior olive (LSO), a nucleus that is thought to play a major role in sound localization. This was accomplished by means of postembedding immunogold immunocytochemistry. Four classes of synaptic terminals were identified in the LSO. They were distinguishable from one another both morphologically and on the basis of their different patterns of immunolabeling for glutamate, glycine, and gamma-aminobutyric acid (GABA). The highest level of glutamate immunoreactivity was found in terminals that contained round vesicles (R) and formed synaptic contacts with asymmetric synaptic junctions. Round-vesicle terminals predominated on small caliber dendrites by a ratio of at least 2:1 over the other classes combined. The thinnest dendrites were typically contacted by R terminals only. The ratio of R terminals to the other types decreased as the caliber of the dendritic profiles they apposed increased so that on the soma, R terminals were outnumbered by at least 2:1 by the other types. Terminals containing flattened vesicles (F) exhibited intense immunoreactivity for both glycine and glutamate, although the glutamate immunolabeling was not as high as that in the R terminals. Flattened-vesicle terminals formed symmetric synaptic contacts with their targets and their distribution was the reverse of that described for R terminals; i.e., they were most abundant on LSO perikarya and fewest on small caliber dendrites. Two terminal types, both containing pleomorphic vesicles and forming symmetric synaptic junctions, were found in far fewer numbers. One group contained large pleomorphic vesicles (LP) and was immunoreactive for both glycine and GABA. The other group contained small pleomorphic vesicles (SP) along with a few dense-core vesicles and labeled for GABA only. The LP terminals were preferentially distributed on somata and large-caliber dendrites, while the SP terminals most often contacted smaller dendrites. Previous work suggests that a large percentage of the R terminals arise from spherical cells in the ipsilateral cochlear nucleus and are excitatory in action. This pathway may use glutamate as a transmitter. Many of the F terminals are thought to originate from the ipsilateral medial nucleus of the trapezoid body and appear to be the inhibitory (glycinergic) terminals from a pathway that originates from the contralateral ear. The origins and functions of LP and SP terminals are unknown, but a few possibilities are discussed along with the significance of cocontainment of neuroactive substances in specific terminal types.

Animals↗
PubMed1984

Responses of neurons in inferior colliculus to variations in sound-source azimuth.

This study aimed to classify the responses of single units in the auditory midbrain to acoustic stimuli presented in the free field in order to characterize those units likely to have a role in sound localization in the horizontal plane. The responses of 131 single units in the inferior colliculus of the cat and the brush-tailed possum were studied using tone and noise-burst stimuli presented from a speaker capable of movement at any point along a plane 10 degrees above the horizontal plane. Speaker positions along this plane are referred to as speaker azimuths; those on the same side as the recorded inferior colliculus as ipsilateral, and on the opposite side as contralateral, azimuths. For each unit, spike counts were measured as a function of azimuth either at the best frequency (BF) or using noise bursts. These functions are referred to as azimuth functions and were usually measured for at least two intensities, between 10 and 70 dB above threshold. The recording sites of most units were identified histologically with the aid of microlesions and were related to the major subdivisions of the inferior colliculus: the central nucleus (ICC), the lateral part of the external nucleus (ICX), and the rostroventral process (R-ICX). Two units were located in the pericentral nucleus and two in the dorsal nucleus of the lateral lemniscus. Two major classes of neuron were identified: omnidirectional and directionally sensitive. Omnidirectional units exhibited azimuth functions that were either flat or that declined gradually at progressively ipsilateral azimuths. For the latter units, discharge rates at all points monotonically increased with stimulus intensity. There was no indication, for either type of omnidirectional unit, of significant binaural interaction. A good correlation was found between the summed proportions of excitatory-excitatory (EE) and monaural (EO) units observed in dichotic studies (46-55%) and the proportion of omnidirectional units in the present study (47%). A subgroup of directionally sensitive units (36% of the total) displayed azimuth functions for which the azimuthal position of the discharge border or peak firing azimuth remained essentially unaltered over a range of stimulus intensities. These azimuth-selective units are likely to have a role in the detection of the location of stimuli in the horizontal plane and appear to include units that would be considered excitatory-inhibitory (EI) or delay sensitive in dichotic studies. The azimuths over which directionally sensitive units showed their marked directional effects were influenced by the position of the contralateral pinna.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1994

Response selectivity for multiple dimensions of frequency sweeps in the pallid bat inferior colliculus.

1. While hunting, the pallid bat uses passive sound localization at low frequencies to find terrestrial prey, and echolocation for general orientation. It must therefore process two different types of acoustic input at the same time. The pallid bat's echolocation pulse is a downward frequency-modulated (FM) sweep from 60 to 30 kHz. This study examined the response selectivity of single neurons in the pallid bat's central nucleus of the inferior colliculus (ICC) for FM sweeps, comparing the response properties of the high-frequency population, tuned to the biosonar pulse, with the low-frequency population, tuned below the pulse. The working hypothesis was that the high-frequency population would exhibit a response selectivity for downward FM sweeps that was not present in the low-frequency population. 2. Neurons were tested for their selectivity for FM sweep direction, duration, frequency range and bandwidth, and rate of frequency change. The extent to which they responded exclusively to tones, noise, and FM sweeps was also examined. Significant differences in the response properties of neurons in the two populations were found. In the low-frequency population, all neurons responded to tones, but only 50% responded to FM sweeps. Only 23% were selective for sweep direction. In the high-frequency population, all neurons responded to FM sweeps, but 31% did not respond to tones. Over one-half of this population was selective for sweep direction, and of those that were selective, all preferred the downward sweep direction of the biosonar pulse. A large percentage (31%) responded exclusively to downward sweeps, and not to tones or upward sweeps. None of the cells in either population responded to noise, or did so only at very high relative thresholds. 3. Both populations contained neurons that were selective for short stimulus durations that approximated the duration of the biosonar pulse, although the percentage was greater in the high-frequency population (58% vs. 20%). In the high-frequency population, 31% of the neurons tested for duration responded exclusively to both the sweep direction and duration of the biosonar pulse. 4. Downward FM-selective neurons, with one exception, were generally insensitive to the rate of frequency change of the FM sweep, as well as the frequency range and bandwidth of the sweep. They responded similarly to both the full 60- to 30-kHz sweep and to 5-kHz bandwidth portions of the full sweep.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1989

A specialization for speech perception.

The processes that underlie perception of consonants and vowels are specifically phonetic, distinct from those that localize sources and assign auditory qualities to the sound from each source. This specialization, or module, increases the rate of information flow, establishes the parity between sender and receiver that every communication system must have, and provides for the natural development of phonetic structures in the species and in the individual. The phonetic module has certain properties in common with modules that are "closed" (for example, sound localization or echo ranging in bats) and, like other members of this class, is so placed in the architecture of the auditory system as to preempt information that is relevant to its special function. Accordingly, this information is not available to such "open" modules as those for pitch, loudness, and timbre.

Animals↗
PubMed2005

Hyperpolarization-activated cyclic nucleotide-gated cation channels regulate auditory coincidence detection in nucleus laminaris of the chick.

Coincidence detection of bilateral acoustic signals in nucleus laminaris (NL) is the first step in azimuthal sound source localization in birds. Here, we demonstrate graded expression of hyperpolarization-activated cyclic nucleotide-gated (HCN) cation channels along the tonotopic axis of NL and its role in the regulation of coincidence detection. Expression of HCN1 and HCN2, but not HCN3 or HCN4, was detected in NL. Based on measurement of both subtype mRNA and protein, HCN1 varied along the tonotopic axis and was minimal in high-characteristic frequency (CF) neurons. In contrast, HCN2 was evenly distributed. The resting conductance was larger and the steady-state activation curve of Ih was more positive in neurons of middle to low CF than those of high CF, consistent with the predominance of HCN1 channels in these neurons. Application of 8-Br-cAMP or noradrenaline generated a depolarizing shift of the Ih voltage activation curve. This shift was larger in neurons of high CF than in those of middle CF. The shift in the activation voltage of Ih depolarized the resting membrane, accelerated the EPSP time course, and significantly improved the coincidence detection in neurons of high CF, suggesting that Ih may improve the localization of sound sources.

Animals↗
PubMedDate not supplied

[The functional characteristics of ototopia during the monaural localization of a mobile sound object in space].

Ototopical function was studied in subjects with normal hearing by their mono- and binaural perception of the sound from a mobile source differently located in space. There was a relationship between ototopicometric parameters and acoustic system function. The discussion covers opportunities of ototopicometry in audiological evaluation of space hearing.

Adolescent↗
PubMed2004

Benefits of bilateral electrical stimulation with the nucleus cochlear implant in adults: 6-month postoperative results.

OBJECTIVE: To evaluate the benefits of bilateral electrical stimulation for hearing-impaired adult subjects using the Nucleus 24 cochlear implant in a multicenter study, and to compare and quantify performance on speech perception measures in quiet and in noise and localization ability for unilateral and bilateral cochlear implant use. DESIGN: : Repeated single subject measures were carried out for each subject, with each subject serving as their own control. Assessment of unilateral and bilateral listening conditions for performance on tests of speech comprehension and sound localization were performed. Speech comprehension measures were performed in quiet at 0 degree azimuth and in the presence of background noise simultaneously presented from the same speaker and spatially separated by 90 degrees, at S+45 degrees N45 degrees and at S-45 degrees N+45 degrees . Test materials included Freiburger monosyllabic words, Oldenburger sentences, and the Hochmair-Schulz-Moser sentences. Tests of localization were performed in the horizontal plane with 12 speaker locations 30 degrees apart using a shortened sentence stimulus from the Hochmair-Schulz-Moser sentences at two possible presentation levels of 55 and 70 dB sound pressure level for assessment of directionality. The binaural advantage provided by bilateral stimulation was calculated with respect to each ear separately, classified as either the better or poorer performing ear for each speech material in quiet and in noise test conditions. For localization of sound, the binaural advantage was compared with left and right ears separately. Paired comparisons for performance data in all conditions were carried out by considering measurements for each subject in different conditions as paired observations and applying the Student's t test to determine the statistical difference between the data sets. SETTING: Tertiary referral centers with a cochlear implant program. PATIENTS: Thirty-seven profoundly hearing-impaired adults were enrolled in the study, 22 simultaneously and 15 sequentially bilaterally implanted. All patients received the Nucleus 24 cochlear implant and used the Nucleus SPrint or ESPrit 3G speech processor, with the vast majority using the ACE speech coding strategy. RESULTS: For spatially separated speech in noise conditions, an interaural performance advantage for the ear closest to the speech source (i.e., with a superior signal to noise ratio) compared with that for the ear closest to the noise source (i.e., with an inferior signal to noise ratio) is consistently demonstrated regardless of whether it is the better or poorer performing ear closest to the speech signal. This is referred to as a significant binaural head-shadow benefit, resulting in a mean improvement between -10 dB and -11.4 dB in the critical signal to noise ratio required for 50% speech comprehension for the Olden-burger sentences and a mean improvement in the maximum score of 42% to 55% for the ear closest to the speech signal over the ear farthest away for the Hochmair-Schulz-Moser sentences. Bilateral stimulation is always observed to provide a performance advantage over the unilateral listening condition for either ear when ipsilateral to the noise source. In addition, as demonstrated by approximately half the subjects tested in noise with the Hochmair-Schulz-Moser sentences, a performance advantage of bilateral stimulation may be observed over the better ear alone when positioned ipsilateral to the speech signal, which is referred to as a binaural squelch effect. On average, for the group, this resulted in a statistically significant improvement in speech comprehension scores of 8% in the bilateral listening condition compared with the scores for the better ear alone. Through assessment of comprehension of coincidental speech in noise and speech in quiet, a significant benefit of binaural redundancy was noted for the group for Oldenburger sentence scores in noise and in quiet compared with unilateral scores for either ear and for the Freiburger monosyllabic words in quiet in comparison with the better ear alone scores. Binaural stimulation also led to a significant improvement in localization ability over either monaural condition, with the root mean square degrees of error reduced by 38 degrees compared with that observed for unilateral stimulation. CONCLUSION: Similar to what has been observed for bilateral acoustic stimulation in the past, bilateral electrical stimulation provides the foundation for the potential advantages of the head-shadow effect, providing a binaural head-shadow benefit and binaural auditory processing such as binaural redundancy and binaural squelch effects, all of which combine to lead to improved speech comprehension over unilateral listening conditions. The combination of improved speech comprehension and improved localization ability made available through bilateral electrical stimulation provides the necessary foundation to further assist the hearing-impaired listener to better cope with communication in the everyday listening situation both in noise and in quiet.

Adolescent↗
PubMed1999

Coding of sound pressure level in the barn owl's auditory nerve.

Rate-intensity functions, i.e., the relation between discharge rate and sound pressure level, were recorded from single auditory nerve fibers in the barn owl. Differences in sound pressure level between the owl's two ears are known to be an important cue in sound localization. One objective was therefore to quantify the discharge rates of auditory nerve fibers, as a basis for higher-order processing of sound pressure level. The second aim was to investigate the rate-intensity functions for cues to the underlying cochlear mechanisms, using a model developed in mammals. Rate-intensity functions at the most sensitive frequency mostly showed a well-defined breakpoint between an initial steep segment and a progressively flattening segment. This shape has, in mammals, been convincingly traced to a compressive nonlinearity in the cochlear mechanics, which in turn is a reflection of the cochlear amplifier enhancing low-level stimuli. The similarity of the rate-intensity functions of the barn owl is thus further evidence for a similar mechanism in birds. An interesting difference from mammalian data was that this compressive nonlinearity was not shared among fibers of similar characteristic frequency, suggesting a different mechanism with a more locally differentiated operation than in mammals. In all fibers, the steepest change in discharge rate with rising sound pressure level occurred within 10-20 dB of their respective thresholds. Because the range of neural thresholds at any one characteristic frequency is small in the owl, auditory nerve fibers were collectively most sensitive for changes in sound pressure level within approximately 30 dB of the best thresholds. Fibers most sensitive to high frequencies (>6-7 kHz) showed a smaller increase of rate above spontaneous discharge rate than did lower-frequency fibers.

Acoustic Stimulation↗
PubMed2005

Auditory localization abilities in bilateral cochlear implant recipients.

OBJECTIVE: To quantify binaural advantage for auditory localization in the horizontal plane by bilateral cochlear implant (CI) recipients. Also, to determine whether the use of dual microphones with one implant improves localization. METHODS: Twenty subjects from the UK multicenter trial of bilateral cochlear implantation with Nucleus 24 K/M device were recruited. Sound localization was assessed in an anechoic room with an 11-loudspeaker array under four test conditions: right CI, left CI, binaural CI, and dual microphone. Two runs were undertaken for each of five stimuli (speech, tones, noise, transients, and reverberant speech). Order of conditions was counterbalanced across subjects. RESULTS: Mean localization error with bilateral implants was 24 degrees compared with 67 degrees for monaural implant and dual microphone conditions (chance performance is 65 degrees). Normal controls average 2 to 3 degrees in similar conditions. Binaural performance was significantly better than monaural performance for all subjects, for all stimulus types, and for different sound sources. Only small differences in performance with different stimuli were observed. CONCLUSIONS: Bilateral cochlear implantation with the Nucleus 24 device provides marked improvement in horizontal plane localization abilities compared with unilateral CI use for a range of stimuli having different spectral and temporal characteristics. Benefit was obtained by all subjects, for all stimulus types, and for all sound directions. However, binaural performance was still worse than that obtained by normal hearing listeners and hearing aid users with the same methodology. Monaural localization performance was at chance. There is no benefit for localization with dual microphones.

Acoustic Stimulation↗