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Lesions of the anterior ventral cochlear nucleus caused degeneration of synaptic endings in the anterior half of the nucleus of the trapezoid body. Lesions of the posterior ventral cochlear nucleus caused similar degeneration in the posterior half of the nucleus of the trapezoid body. Thus, the cochlea projects twice upon the nucleus of the trapezoid body.
Animal models of conductive hearing loss (CHL) show altered structure and function in the central auditory system (CAS), particularly following unilateral deprivation. Assessment of neuronal activity as measured by 2-deoxyglucose (2-DG) uptake following CHL has been reported by two groups of investigators, with different findings. Woolf and colleagues [Brain Res. 274 (1983) 119] found that 2-DG uptake increased in the cochlear nucleus ipsilateral to the CHL, while Tucci et al. [Laryngoscope 109 (1999) 1359] found a decrease in 2-DG uptake in the ipsilateral cochlear nucleus. One significant difference between the protocols in the two studies was that, in the first study, animals were maintained in silence following 2-DG injection, whereas in the Tucci et al. study, animals were exposed to sound. The current study was designed to replicate the protocol used by Woolf et al. Young adult gerbils underwent unilateral malleus removal with bilateral canal ligation (n=6) or a sham procedure (n=7) 48 h prior to 2-DG administration and sacrifice. Optical density measurements were made from CAS nuclei. 2-DG uptake decreased in the ipsilateral cochlear nucleus and contralateral inferior colliculus, and in nuclei of the superior olivary complex bilaterally, supporting the finding that CHL is associated with a decrease in CAS neuronal activity.
1. Recordings were made from single nerve fibers in barbiturate-anesthetized cats in the midline trapezoid body, a location that permits selective sampling of efferent cells of the ventral cochlear nucleus. Single units were localized to either the dorsal or ventral components of the trapezoid body. The fibers were physiologically classified on the basis of their peristimulus time histograms (PSTH) and receptive-field properties. In addition, low characteristic frequency (CF) units were probed for rapid rate and phase shifts with increases in intensity. The projection patterns of some fibers were traced by iontophoresing horseradish peroxidase (HRP) into their axons. 2. HRP-labeled fibers most likely originated from globular bushy cells of the ventral cochlear nucleus in that they sent a large branch into the contralateral medial nucleus of the trapezoid body which terminated in a calyceal ending and an ipsilateral branch into the lateral nucleus of the trapezoid body. A thin branch, usually starting from the large branch, wound its way through the medial nucleus of the trapezoid body to its termination in the ventral nucleus of the trapezoid body. Additional branches from the parent axon could pass through medial periolivary groups throughout the rostrocaudal extent of the superior olivary complex. The parent fiber was traced as far as the ventral lateral lemniscus where it faded before reaching its termination. 3. The majority of units were recorded in the ventral component of the trapezoid body. Although the ventral component is comprised of both large and small diameter fibers, our sample was biased to the larger diameter fibers representing the activity of axons originating from globular bushy cells in the ventral cochlear nucleus. Ventral component units were not tonotopically arrayed and had CFs that spanned the audible range for cats. HRP labeling of ventral component axons revealed that the section of the axon traveling through the midline shifted its dorsal-ventral location. This pattern was compatible with the lack of tonotopy found in the ventral component. Recordings were also made from the dorsal component of the trapezoid body, which contained medium diameter axons. These axons originated from spherical bushy cells in the ventral cochlear nucleus. Dorsal component units were tonotopically arrayed and had CFs less than 7 kHz. 4. Cells were characterized by their PSTH at CF. Primary-like and phase-locked units constituted most of the dorsal component units.(ABSTRACT TRUNCATED AT 400 WORDS)
The electrically evoked action potential (EAP) was recorded intra-operatively by use of neural response telemetry (NRT) on the Nucleus C124M cochlear implant. The aim of the present study was to investigate the EAP in young children immediately following implant surgery and whilst the children were still anaesthetized. The effect of data collection parameters on the reliability of the EAP was assessed and the relationships of the EAP findings to the intra-operative electrical auditory brainstem response (EABR) and early behavioural threshold levels (T-levels) were also investigated. The study data comprised intra-operative recordings in 60 children. Age at implantation was less than five years in 42 (70%) of the children. Aetiology of deafness was congenital in the majority of children (55, 92%), meningitic in four children and of unknown origin in one child. Optimum test parameters for the intra-operative EAP were an amplifier gain of 40 dB and a delay of 50 micros in order to minimize the effects of amplifier saturation due to stimulus artefact and to maximize the identification of the N1 component. An intra-operative protocol was established which involved recording four stimulus levels on each of the 22 electrodes of the electrode array, the range of stimulus levels being tailored towards the expected EAP thresholds and T-levels so as to identify response threshold. There was significant correlation between the intraoperative EAP thresholds and the early T-levels (Pearson's r = 0.93 ;p<0.01) when a correction factor was introduced based on a reliable behavioural measure of the threshold of electrical stimulation on electrode 10. The intra-operative EAP threshold, when combined with a limited amount of behavioural data, may therefore be used to predict the T-level with a useful degree of accuracy. This result is also supported by the significant correlation observed between the intra-operative thresholds of the EAP and EABR.
Electrical auditory brain stem responses (EABR) and electrical middle latency responses (EMLR) were recorded from patients who had received the Nucleus multichannel cochlear implant system. Twenty-five sequential patients had either intraoperative or outpatient EABR testing. We also recorded EMLRs from several outpatients. EABR results were consistent among all patients tested. Wave V mean latencies were the shortest (3.82 msec) for the most apical electrode (E20) and increased slightly for the medial (E12) and basal (E5) electrodes (3.94 and 4.20 msec, respectively). Absolute latencies for all EABR component waves were observed to be 1 to 1.5 msec shorter than typical acoustic auditory brain stem response (ABR) mean latencies. We have examined the relationships between patients' EABR/EMLR and their behavioral responses to electrical stimulation. Generally, the behavioral threshold and comfort current levels were lower than the predicted values based on EABR/EMLR findings. This observation may be due in part to psychophysical loudness differences noted for pulse rates of 10 to 500 pulses per second in some of the patients that we have studied in greater detail.
The patterns of axonal degeneration following acoustic overstimulation of the cochlea were traced in the brainstem of adult chinchillas. The Nauta-Rasmussen method for axonal degeneration was used following survivals of 1-32 days after a 105 min exposure to an octave-band noise with a center frequency of 4 kHz and a sound pressure level of 108 dB. Hair-cell and myelinated nerve-fiber loss were assessed in the cochlea. The cochleotopic pattern of terminal degeneration in the ventral cochlear nucleus correlated with the sites of myelinated fiber and inner-hair-cell loss: this correlation was less rigorous with outer-hair-cell loss, especially in the dorsal cochlear nucleus. These results are consistent with a dystrophic process with a slow time course depending on hair-cell loss and/or direct cochlear nerve-fiber damage. However, in a number of cases with no damage in the apical cochlea, fine fiber degeneration occurred with a faster course in low-frequency regions in the dorsal cochlear nucleus and, transynaptically, in a non-cochleotopic pattern in the superior olive and inferior colliculus. These findings suggest that neuronal hyperactivity plays a role in the central degeneration following acoustic overstimulation, possibly by an excitotoxic process.
Kangaroo rats develop spongiform degeneration of the central auditory system similar to that seen in the gerbil. Light microscopic and transmission electron microscopic study of the cochlear nucleus and auditory nerve root (ANR) of Dipodomys deserti and D. merriami show that spongiform lesions develop in dendrites and oligodendrocytes of the cochlear nucleus and in oligodendrocytes of the ANR that are morphologically indistinguishable from those extensively described in the Mongolian gerbil, Meriones unguiculatus. As in Mongolian gerbils, the spongiform degeneration in Dipodomys were much more numerous in animals continually exposed to modest levels of low-frequency noise (< 75 dB SPL). The kangaroo rats with extensive spongiform degeneration also show slightly, but significantly, elevated auditory brainstem evoked response (ABR) thresholds to low-frequency stimuli, a result also found in Mongolian gerbils. These results suggest that the elevated ABR thresholds may be the result of spongiform degeneration. Because low-frequency noise-induced spongiform degeneration has now been shown in the cochlear nucleus of animals from separate families of Rodentia (Heteromyidae and Muridae), the possibility should be investigated that similar noise-induced degenerative changes occur in the central auditory system of other mammals with good low-frequency hearing.
This study investigated the effect of pulsatile stimulation rate on medial vowel and consonant recognition in cochlear implant listeners. Experiment 1 measured phoneme recognition as a function of stimulation rate in six Nucleus-22 cochlear implant listeners using an experimental four-channel continuous interleaved sampler (CIS) speech processing strategy. Results showed that all stimulation rates from 150 to 500 pulses/s/electrode produced equally good performance, while stimulation rates lower than 150 pulses/s/electrode produced significantly poorer performance. Experiment 2 measured phoneme recognition by implant listeners and normal-hearing listeners as a function of the low-pass cutoff frequency for envelope information. Results from both acoustic and electric hearing showed no significant difference in performance for all cutoff frequencies higher than 20 Hz. Both vowel and consonant scores dropped significantly when the cutoff frequency was reduced from 20 Hz to 2 Hz. The results of these two experiments suggest that temporal envelope information can be conveyed by relatively low stimulation rates. The pattern of results for both electrical and acoustic hearing is consistent with a simple model of temporal integration with an equivalent rectangular duration (ERD) of the temporal integrator of about 7 ms.
Cells that send ascending projections to the inferior colliculus were identified following injections of horseradish peroxidase into the colliculus. Labelled cells were found in all subcollicular auditory nuclei. Virtually all cells of the ipsilateral ventral nucleus of the lateral lemniscus and medial superior olive appear to project to the colliculus. Very few cells in these nuclei were labelled on the contralateral side. Heavy labelling on the contralateral side was found in the dorsal nucleus of the lateral lemniscus and cochlear nucleus, with less labelling being found ipsilaterally in these nuclei. The lateral superior olive was approximately evenly labelled on the two sides, with about half the cells from each side projecting to each colliculus. Cells in all periolivary cell groups were labelled, with most being found adjacent to the medial superior olive. An effort was made to identify individual cell types that were labelled and some 24 cell types were identified. In the cochlear nucleus there were marked differences between cell types in the extent of their labelling. Topographic projections matched previously described tonotopic organization of the colliculus and all major subcollicular nuclei except the ventral nucleus of the lateral lemniscus. A description of the cells in the nucleus is provided.
The speech perception abilities of 19 children with onset of deafness before age 3 years was examined after they received the Nucleus multichannel cochlear implant. The children were divided into two groups based on age at onset of deafness: children with congenital deafness (n = 8) and children with onset of deafness after birth but before age 3 (n = 11). There was no statistically significant difference between the scores of the two groups of subjects on 12 of the 13 speech perception tests administered. This finding suggests that children who are born deaf have the potential to derive the same benefit from cochlear implants as do children who have had some exposure to spoken language before the onset of their deafness. Examination of performance in terms of communication mode revealed that prelingually deafened children with implants who used oral communication obtained significantly higher scores on only 2 of the 13 speech perception measures than did children who used total communication. The data suggest that communication mode does not appear to account for large differences in speech perception performance among prelingually deafened children with multichannel cochlear implants.
Progression of ischemic damage was investigated immunohistochemically in neural dendrites using microtubule-associated protein 2 (MAP2) as a dendritic marker in the rat's brainstem. Neuronal soma and dendrites were clearly stained by this protein but some structures such as axonal bundles, glia and endothelial cells were not visualized. When the anterior inferior cerebellar artery (AICA) was occluded unilaterally for 30 min, a wide ischemic lesion was detected in the occluded side of the brainstem and was observed as a loss of reaction to MAP2. After ischemia for 2 h, loss of reaction in the perikarya and dendrites was seen to expand to the ipsilateral (occluded side) cochlear nucleus. When the basilar artery was blocked, ischemic damage in the vestibular nucleus was more intense than that in the cochlear nucleus. In all specimens studied, differences in anatomical blood supply demonstrated selective tissue vulnerability for ischemic damage.
In this study, differences between electrically evoked whole-nerve action potential (EAP) and electrically evoked auditory brainstem response (EABR) measurements within Nucleus CI24R cochlear implant recipients were evaluated. Precurved modiolus-hugging internal electrode arrays, such as the CI24R, are designed to provide more direct stimulation of neural elements of the modiolus. If the electrode array is closer to the modiolus, electrically evoked and behavioral levels might be lower than were previously recorded for the straight electrode array, the CI24M. EAP and EABR growth functions and behavioral levels were obtained for 10 postlingually deafened adults. Results revealed no significant differences between EAP and EABR threshold levels, and these levels were not significantly lower than those obtained using the CI24M.
AMPA receptors mediate rapid glutamatergic synaptic transmission. In the mammalian cochlear nuclei, neurons receive excitatory input from either auditory nerve fibers, parallel fibers, or both fiber systems. The functional correlates of differences in the source of input were examined by recording AMPA receptor-mediated, miniature EPSCs (mEPSCs) in whole-cell voltage-clamp mode from identified neurons. Bushy, octopus, and T-stellate cells of the ventral cochlear nucleus (VCN) and tuberculoventral cells of the dorsal cochlear nucleus (DCN) receive most of their excitatory input from the auditory nerve; fusiform cells receive excitatory inputs from both the auditory nerve and parallel fibers; cartwheel cells receive excitatory input from parallel fibers alone. mEPSCs from bushy, octopus, T-stellate, and tuberculoventral cells had significantly faster decay time constants (0.35-0.40 msec) than did those from fusiform and cartwheel cells (1.32-1.79 msec). Some fusiform cells had two populations of mEPSCs with distinct time courses. mEPSCs in cells with auditory nerve input alone were inhibited by philanthotoxin, a blocker of calcium-permeable AMPA receptors, whereas mEPSCs in cells with parallel fiber input were not. Thus AMPA receptors postsynaptic to the auditory nerve differ from those postsynaptic to parallel fibers both in channel-gating kinetics and in their permeability to calcium. These results confirm the conclusion that synaptic AMPA receptors are specialized according to the source of input (Hunter et al., 1993; Rubio and Wenthold, 1997; Wang et al., 1998).
In most mammals, the superior olive is the first stage for binaural interaction. Neurons in the medial superior olive (MSO) receive excitatory input from both ears and are sensitive to interaural time or phase differences of low-frequency sounds. The mustached bat (Pteronotus parnellii parnellii), a small echolocating species with high-frequency hearing, probably does not use interaural time or phase differences as cues for sound localization. Although the mustached bat has a large MSO, there is some evidence that it is functionally different from the MSO in nonecholocating mammals. Most MSO neurons in the mustached bat are monaural, excited by a contralateral sound. Their responses are phasic and correlated with either the onset or the offset of a sound. As a first step in determining the origin of these phasic monaural responses, we traced the connections of the MSO by using both retrograde and anterograde transport methods. Excitatory inputs to the MSO originate from spherical cells in the anteroventral cochlear nucleus, almost exclusively from the contralateral side. Glycinergic inhibitory input is relayed from the contralateral cochlear nucleus through the medial nucleus of the trapezoid body. To investigate the interactions of the contralateral excitatory and inhibitory inputs at the level of the MSO cell, we recorded sound-evoked responses and applied glycine or its antagonist by using microiontophoresis. The results show that the phasic response to a contralateral sound is created by interaction of a sustained excitatory input with a sustained inhibitory input, also from the contralateral ear. Whether the response is to the onset or offset of a sound is determined by the relative timing between the excitatory and inhibitory inputs. Thus, in MSO of the mustached bat, the ipsilateral excitatory pathway from the cochlear nucleus seen in animals with low-frequency hearing is virtually absent, and the MSO is adapted for timing analysis by using input from only the contralateral ear.
An intraoperative recording of a direct compound action potential of the 8th cranial nerve with pre, intra and postoperative monitorings of auditory brainstem evoked response (ABR) was carried out for the purpose of clarifying mechanism and prevention of postoperative hearing dysfunction following microvascular decompression surgery as a treatment for hyperdysfunction syndrome of cranial nerves. In 221 patients with hemifacial spasm and tic douloureux out of 510 patients operated on by microvascular decompression surgery, ABR was monitored before, during and after surgery and furthermore, in the recent 94 patients among them, an intraoperative direct recording of a compound action potential of the acoustic nerve was performed simultaneously. Among these 94 patients, postoperative mild hearing dysfunction was encountered in 11 patients and severe reduction of hearing acuity in 2. As a result of our studies of monitoring action potential of the 8th cranial nerve and ABR, the following conclusions were drawn. When a latency of component V of ABR was delayed by more than 1.5-2.0 msec during surgery, and delay of latency of main negative peak (N1) of an action potential of the acoustic nerve exceeded by more than 1.0 msec, the occurrence rate of postoperative hearing dysfunction became positively high. There were two different types of changes in both ABR and action potentials; one type was a delay of an action potential in combination with that of ABR latencies and the other one was merely a delay in ABR components without following a remarkable change in action potentials. The former change might due to be an insult of whole neural pathways of the 8th cranial nerve including a cochlear nucleus and the latter change may due to be functional changes of the structures proximal to the cochlear nucleus. The delay of latencies in ABR and action potentials could be imposed either by a traction of the 8th cranial nerve or by a compression of the region of cochlear nucleus by a blain spatula as well as a vascular insufficiency of the tiny vessels which supplied these neural structures. To avoid the surgical insult to the acoustic nerve itself and more proximal neural pathways, the traction of any neural structures by a brain spatula should be lasted less than 5 minutes with more than 2 minutes intervals when judged from the results of our monitoring studies and the pressure of the compression on the cerebellar cortex is much preferable to be less than 20 mmHg for the safety throughout whole surgical procedures.
The generators of the short-latency auditory evoked potentials (SLAEPs) in the monkey have been defined by intracranial mapping from cochlea to auditory cortex. SLAEP components other than 1a and the slow negativity (SN) following wave 7 derive from compound action potentials propagated in subcortical auditory pathways. The component generators are complex due to the presence of two bursts of activity in the eighth nerve, to the fact that the ascending auditory fibers both synapse on and bypass specific relay nuclei, and to the differences in orientation of segments of the auditory pathways. Most SLAEP components recorded at the surface reflect the summation of activity from multiple generators. However, much of the activity seen within subcortical structures cannot be traced to the surface of the brain. Component 1a is identified with the cochlear summating potential, while 1b reflects the initial afferent volley in the distal portion of the eighth nerve. Component 2 represents the initial depolarization of the eighth nerve terminals within the ipsilateral cochlear nucleus. Component 3h reflects the second volley of activity in the distal portion of the eighth nerve and the outflow of the cochlear nucleus which decussates in the trapezoid body. Component 3v represents the initial cochlear nucleus outflow volley ascending the lateral lemniscus. Component 4 principally reflects the second volley of activity within the eighth nerve terminals, and outflow from the ipsilateral superior olivary complex ascending in that lateral lemniscus, with a possible contribution from activity in the contralateral CNC. Component 5 represents the outflow of the contralateral superior olivary complex ascending in that lateral lemniscus. Component 6 reflects another volley from the ipsilateral superior olivary complex ascending in that lateral lemniscus, as well as outflow from both inferior colliculi propagating in their brachii. The generators of component 7 are the most complex encountered, representing volleys in both lateral lemnisci, activity of the contralateral inferior colliculus, and activity in both auditory radiations. A component that follows wave 7, seen best in mastoid-to-mastoid recording linkage, represents outflow from both inferior colliculi propagating in their brachia. Components 8 and 9 principally reflect propagated action potentials in the auditory radiations bilaterally, with an additional contribution from activity of both inferior colliculi. The SN mainly represents volume-conducted postsynaptic potentials from both inferior colliculi and cochlear nuclei.
OBJECTIVES: To review our intraoperative and postoperative testing protocol for cochlear implant patients. This study describes the methodologies and applications of a new technique called neural response telemetry (NRT) for the Nucleus CI24M cochlear implant system. NRT uses radiofrequency telemetry technology to measure the action potentials of the auditory nerve. STUDY DESIGN: We have developed a specific protocol for intraoperative testing of the implant device before, during, and after implantation. This includes device integrity tests, visual detection of electrical stapedius muscle reflexes (VESR), and NRT. METHODS: Our methodologies use the commercial software (Windows-based Diagnostic and Programming System [WIN-DPS] and NRT) for the Nucleus CI24M. We describe the details of our protocol used on all of the patients (14 adults and 14 children) who received CI24M implants at Mayo Clinic (Rochester, MN). Our protocol correlates the NRT threshold with the behavioral responses for each patient on at least four electrodes. RESULTS: From August 1, 1998, to December 31,1998, we completed electrode integrity tests, NRT, and VESR testing intraoperatively on 12 patients with the Nucleus CI24M. We have measured normal implant function on all 28 of our CI24M patients with one exception. One of our children had a device failure after approximately 4 months as a result of head trauma. We have also obtained NRT results from an additional 10 patients postoperatively. CONCLUSIONS: The measurement of device and electrode array function is quite simple with the CI24M software. These measurements can be obtained intraoperatively as well as postoperatively. We conclude that VESR and NRT measurements can be very helpful in programming for patients with cochlear implants, especially children, because they provides us with target settings for the speech processor.
This report is the fourth in a series describing the results of ablation-behavior experiments directed to the ascending output of the cochlear nuclei as it is conducted centrally within the acoustic striae. This fourth report focuses on the unique physiology of the fusiform or 'output' cells of the dorsal cochlear nucleus whose axons course through the dorsal acoustic stria (DAS). Because electrophysiological studies have shown that the cues for sensing the elevation of a sound source would seem to be best analyzed by the dorsal cochlear nucleus and projected centrally via its DAS, we tested normal cats and cats deprived of DAS for their ability to orient to elevated sources of broad-band noise. For behavioral testing, we made use of reflexive or unconditioned orienting responses to elevated sound sources using a similar method to one we have used previously for azimuth testing (Thompson GC, Masterton RB. Brainstem auditory pathways involved in reflexive head orientation to sound. J Neurophysiol 1978;41:1183-1202). The results show that cats deprived of their DAS do indeed have a marked deficit in their ability to orient to an elevated sound source. Further behavioral testing indicated that this deficit is not the secondary result of an attentional or peripheral motor deficit. Although the present results do not prove that the reflexive deficit is strictly auditory in nature, the deficit is notable in that it is the only one yet known to result from a lesion of the dorsal cochlear nucleus or its central projections.