Acute high-intensity sound exposure alters responses of place cells in hippocampus

Overstimulation is known to activate neural plasticity in the auditory nervous system causing changes in function and re-organization. It has been shown earlier that overstimulation using high-intensity noise or tones can induce signs of tinnitus. Here we show in studies in rats that overstimulation causes changes in the way place cells of the hippocampus respond as rats search for rewards in a spatial maze. In familiar environments, a subset of hippocampal pyramidal neurons, known as place cells, respond when the animal moves through specific locations but are relatively silent in others. This place-field activity (i.e. location-specific firing) is stable in a fixed environment. The present study shows that activation of neural plasticity through overstimulation by sound can alter the response of these place cells. Rats implanted with chronic drivable dorsal hippocampal tetrodes (four microelectrodes) were assessed for stable single-unit place-field responses that were extracted from multiunit responses using NeuroExplorer computer spike-sorting software. Rats then underwent either 30 min exposure to a 4 kHz tone at 104 dB SPL or a control period in the same sound chamber. The place-field activity was significantly altered after sound exposure showing that plastic changes induced by overstimulation are not limited to the auditory nervous system but extend to other parts of the CNS, in this case to the hippocampus, a brain region often studied in the context of plasticity.

Tinnitus and pain

Tinnitus has many similarities with the symptoms of neurological disorders such as paresthesia and central neuropathic pain. There is considerable evidence that the symptoms and signs of some forms of tinnitus and central neuropathic pain are caused by functional changes in specific parts of the central nervous system and that these changes are caused by expression of neural plasticity. The changes in the auditory nervous system that cause tinnitus and the changes in the somatosensory systems that cause central neuropathic pain may have been initiated from the periphery, i.e. the ear or the auditory nerve for tinnitus and receptors and peripheral nerves in the body for pain. In the chronic condition of tinnitus and pain, abnormalities in the periphery may no longer play a role in the pathology, but the tinnitus is still referred to the ear and central neuropathic pain is still referred to the location on the body of the original pathology. In this chapter we will discuss specific similarities between tinnitus and pain, and compare tinnitus with other phantom disorders. Since much more is known about pain than about tinnitus, it is valuable to take advantage of the knowledge about pain in efforts to understand the pathophysiology of tinnitus and find treatments for tinnitus.

Neurophysiologic basis for cochlear and auditory brainstem implants

The physiologic basis for cochlear and brainstem implants is discussed. It is concluded that the success of cochlear implants may be explained by assuming that the auditory system can adequately discriminate complex sounds, such as speech sounds, on the basis of their temporal structure when that is encoded in a few separate frequency bands to offer moderate separation of spectral components. The most important roles of the cochlea seems to be to prepare complex sounds for temporal analysis and to create separate channels through which information in different frequency bands is transmitted separately to higher nervous centers for decoding of temporal information. It is then pertinent to ask how many channels are needed. Because speech discrimination is very important, it is probably sufficient to use enough channels to separate formants from each other.

Symptoms and signs caused by neural plasticity

Plastic changes in the central nervous system are associated with hyperactivity, hypersensitivity, and spread of activity including activation of brain regions that are not typically involved. Symptoms and signs such as neuropathic pain and tinnitus and hyperactive disorders such as muscle spasm and synkinesis may result from such changes in function. Plastic changes that cause symptoms of diseases can be initiated by novel stimulations, overstimulation, or deprivation of input and the induced changes in the function of central nervous system structures may persist and aggravate after these events have ceased if the condition is not reversed. Disorders that are caused by neural plasticity are potentially reversible with treatment. However, the absence of morphologic abnormalities makes diagnosis of these conditions difficult and their treatment has been hampered by lack of understanding of their pathophysiology. Here the role of neural plasticity in the pathophysiology of several disorders is reviewed.

Vascular compression of cranial nerves: II: pathophysiology

The pathophysiology of trigeminal neuralgia, hemifacial spasm and other disorders that can be cured by microvascular decompression of cranial nerves, is reviewed and different hypotheses about its pathophysiology are discussed. It is found that the pathophysiology of these disorders is complex and other factors than vascular compression are necessary to cause symptoms. While the efficacy of the microvascular decompression (MVD) operation is indisputable, it is questionable if the symptoms and signs of these disorders are caused by abnormal neural activity in the respective cranial nerves that result from the compression from a blood vessel. Instead, studies point to hyperactivity and hyperexcitability of the respective nuclei as a cause of the symptoms and signs of these disorders. Results of several studies indicate that irritation of the cranial nerve in question from close contact with a blood vessel may promote such development, and it seems necessary that other factors in addition to the vascular contact must be present in order that such a condition develops.

Vascular compression of cranial nerves. I. History of the microvascular decompression operation

The development of the microvascular decompression (MVD) operation is reviewed. It is stressed that a few innovative neurosurgeons discovered the role of vascular compression of cranial nerves V and VII in trigeminal neuralgia (TGN) and hemifacial spasm (HFS) and developed an operation, later to be known as the MVD operation. While the understanding of the pathophysiology of these disorders has improved, the surgical procedure has undergone little change since Gardner described the operation about 1960.

Correlation between latency and amplitude of peak V in the brainstem auditory evoked potentials: intraoperative recordings in microvascular decompression operations

Intraoperative prolongation of the latency and decrement of the amplitude of peak V of brainstem auditory evoked potentials (BAEP) were studied in 38 microvascular decompression operations in which prolongation of the latency of peak V exceeded 1.0 msec. Postoperative hearing tests of all patients were compared with their preoperative hearing tests. Postoperative hearing loss was unrelated to the maximum prolongation of latency, but the amplitude decreased to lower values in patients with postoperative hearing loss compared to patients whose postoperative hearing was unchanged (P < 0.05). Twelve (32%) of 38 patients whose latency of peak V was prolonged more than 1.0 msec and 11 (61%) of 18 patients whose amplitude of peak V decreased more than 40% during the operations had decreased hearing postoperatively. In all patients, a prolongation of the latency of peak V was always accompanied by a decrease in the amplitude of peak V. The decrement of the amplitude was greater in the patients with decreased postoperative hearing thresholds than in the patients with unchanged postoperative hearing thresholds. The results of this study indicate that it would be valuable to monitor changes in the amplitude of peak V of BAEP in addition to monitoring the latency of peak V during operations where the VIIIth cranial nerve is manipulated.

Delayed hearing loss after microvascular decompression of the trigeminal nerve

OBJECTIVE

The development of sudden postoperative hearing loss as a complication of microvascular decompression (MVD) operations in the cerebellopontine angle has already been reported. A sudden hearing loss of vascular origin may also occur hours or days after such operations, but even in such cases an improvement of hearing over the following weeks is possible. Here we report on a gradual deterioration of hearing over a period of two weeks after MVD which has not been described in the literature up to now.

CLINICAL PRESENTATION

A MVD operation was performed twice on a 36 year old patient with trigeminal neuralgia. After the second operation the patient developed a slight hearing impairment 3 days postoperatively which increased over a period of two weeks and ended up with total deafness. The course of intra-operative brainstem auditory evoked potentials and postoperative audiograms is documented.

CONCLUSION

Because of gradual development of the delayed hearing loss, we conclude that postoperative tissue scarring may be the underlying pathology.

Temporal integration of pain from electrical stimulation of the skin

The threshold of sensation and the threshold of pain in response to electrical stimulation (impulses of 1 msec duration) of the skin on the forearm or hand in individuals without pain were compared with the thresholds of individuals with chronic pain in the range 1 to 100 pulses per second repetition rate. The threshold of sensation in patients without pain was little affected by the repetition rate of the stimulation within the range studied, and the threshold for pain decreased exponentially with increasing repetition rate. In individuals with chronic pain the threshold of sensation was similar to that of individuals without pain over the entire range of stimulus repetition rates studied, but the threshold of pain in patients with pain was lower and less affected by the stimulus rate than it was in the individuals without pain, thus closer to the threshold of sensation.

Similarities between chronic pain and tinnitus

OBJECTIVE

The aim of this study is to review hypotheses about the mechanisms of chronic pain and to compare them with that of tinnitus. Hypotheses about the pathophysiology of severe tinnitus and chronic pain have been of mainly two kinds: one of which claims that pathology located in the periphery (the ear for tinnitus, and peripheral nerves for pain) can explain the symptoms, while the other claims that the symptoms are caused by changes in the function of nuclei of the central nervous system.

DATA SOURCES

A search of the literature from the past 35 years was used.

CONCLUSIONS

There is considerable evidence that both chronic pain and some forms of tinnitus are caused by changes in the central nervous system and that the anatomic location of the physiologic abnormality causing the symptoms of chronic pain and some forms of tinnitus is not the same location to which the symptoms are referred, i.e., the ear for tinnitus and the location of injury for pain. Such changes in the central nervous system may have been induced by peripheral processes such as tissue damage, but the changes can persist a long time after complete healing of a peripheral lesion. Different forms of tinnitus may respond to different treatments as is the case for chronic pain. If the different forms of tinnitus cannot be separated, then the results of studies of the efficacy of different kinds of drugs may be misleading.

Contralateral evoked brainstem auditory potentials as an indicator of intraoperative brainstem manipulation in cerebellopontine angle tumors

Brainstem auditory evoked potentials (BAEP) were studied during operations to remove acoustic tumors using the retromastoid approach. BAEP were elicited from the side contralateral to the tumor, and changes in the latencies of peaks III and V of the BAEP were compared with changes in cardiovascular parameters throughout the operation. When the changes in the determined cardiovascular parameters were related to surgical manipulations, the related changes in the latencies of peaks III and V of the BAEP were more consistent than the changes in the cardiovascular parameters and they tended to occur earlier than the changes noted in the cardiovascular parameters.

Effects of (-)-baclofen, clonazepam, and diazepam on tone exposure-induced hyperexcitability of the inferior colliculus in the rat: possible therapeutic implications for pharmacological management of tinnitus and hyperacusis

Recent investigations in the authors' laboratory have shown that acute tone exposure (4 kHz continuous tone, 104 dB sound pressure level (SPL), 30-min duration) induces increases in the amplitude of click-evoked potentials in the inferior colliculus (IC). These increases have been attributed to a decrease in GABAA-mediated inhibition on IC neurons. In the present study, we examined the effects of three compounds (diazepam, clonazepam, and (-)-baclofen) that are known to enhance GABAergic inhibition on these tone exposure-induced increases and on changes in temporal integration in the IC. (-)-Baclofen was the only one of the three compounds tested that reversed in a dose-dependent manner the effects of tone exposure on both the amplitude of the click-evoked potentials recorded from the IC and on measures of the changes in temporal integration based on these potentials. Diazepam and clonazepam exhibited remarkably different effects on the click-evoked potentials recorded from the surface of the IC. Diazepam caused a dose-dependent decrease in one of the components of the IC potentials that reflects postsynaptic activity in the IC, whereas clonazepam caused a dose-dependent decrease in a peak that reflects input to the IC from the superior olivary complex (SOC). At dosages up to 40 mg/kg, neither diazepam nor clonazepam reversed the changes in temporal integration in the IC that were induced by the tone exposure; diazepam caused a small, but statistically significant, enhancement of the effects of tone exposure on this function. The results of this study show that (-)-baclofen is a potent modulator of both the excitability of neurons in the ascending auditory pathway and the processing of auditory information by IC neurons. The finding of the present study that two benzodiazepines (clonazepam and diazepam) have remarkably different effects on evoked potentials, which reflects both input to the IC and postsynaptic events in the IC neurons, suggests heterogenicity of the GABAA receptor from one structure to another in the ascending auditory pathway. We suggest that (-)-baclofen may be clinically useful in treating disorders of the auditory system that are caused by plasticity in the ascending auditory pathway.

Monitoring auditory function during operations to remove acoustic tumors

The use of auditory evoked potentials in the preservation of hearing is reviewed, and the feasibility of using brainstem auditory evoked potentials, electrocochleography, and compound action potentials directly recorded from the eighth nerve is discussed. The method of recording directly from the surface of the cochlear nucleus using an electrode placed in the lateral recess of the fourth ventricle is described, and the advantages of its use are demonstrated.

Evidence of neuronal plasticity within the inferior colliculus after noise exposure: a study of evoked potentials in the rat

Recent investigations have implicated that the central nervous system has a role in the changes that occur in auditory function following acoustic trauma caused by noise exposure. These investigations indicate that the inferior colliculus may be the primary anatomical location in the ascending auditory pathway where noise-induced neuronal plasticity occurs, thereby resulting in changes in the neuronal processing of auditory information. In the present investigation, we show that the amplitudes of all peaks in the click-evoked response from the external nucleus of the inferior colliculus decrease during a 30 min exposure to a tone (104 dB sound pressure level (SPL) at 4 kHz and 8 kHz). After tone exposure, the amplitudes of two of the peaks of the response from the external nucleus of the inferior colliculus that reflect the input from more caudal structures slowly returned to baseline levels, whereas the amplitudes of the two peaks reflecting neuronal activity in the inferior colliculus increased above baseline levels and remained at the increased levels for at least 90 min following exposure to the tone. We also show that exposure to a 4 kHz tone at 104 dB SPL causes changes in the neuronal processing of tonebursts in the form of changes in the temporal integration function for one of the peaks of the response from the external nucleus of the inferior colliculus that originates in the inferior colliculus. Before tone exposure the amplitude of this peak decreased with increasing stimulus duration, but after tone exposure the amplitude of this peak was independent of the duration of the toneburst stimulus. We interpret these changes as evidence that noise exposure (tone exposure) causes changes in the excitability of the inferior colliculus that are not seen in more caudal structures, and these changes are probably a result of a change in the balance between inhibition and excitation in the inferior colliculus.

Evidence of decreased GABAergic influence on temporal integration in the inferior colliculus following acute noise exposure: a study of evoked potentials in the rat

Many investigations have shown that modulation of sensory input, either by over stimulation or sensory deprivation, can cause a reorganization of structures located high in the central nervous system (CNS). Although most of these studies had focused on studying changes in the function and tonotopic organization of the sensory cortex, recent evidence has suggested that plastic changes in specific subcortical nuclei of sensory systems may also occur in response to modulation of sensory input, and may be partially responsible for changes reflected at the level of the cortex. In the present study we investigated the effects of noise exposure (4-kHz continuous tone at 104 dB sound pressure level (SPL) for 30 min duration) on the processing of auditory information at the level of the inferior colliculus (IC). We studied how evoked potentials recorded from the surface of the IC changed as a function of the duration of the tone bursts used as stimuli. We measured the amplitude of a peak that is generated postsynaptically in the IC in response to tone bursts between 1 and 6 ms duration. In animals that were not exposed to the tone, the amplitude of this peak decreased with increasing stimulus duration, but after tone exposure, the decrease in the amplitude of this peak was significantly less than in the animals not exposed to the tone. A microinjection of the GABAA antagonist, bicucullene, into the IC in the animals not exposed to the tone caused the amplitude of the peak to be less dependent on tone burst duration, which indicates that the decrease in the amplitude of this component of the response from the IC with increasing stimulus duration is a result of GABAA mediated inhibition on IC neurons. The tone exposure caused a similar decrease in amplitude of this component of the response from the IC, thus indicating that noise exposure reduced the GABAA mediated component of this function. This is supported by the finding that microinjections of bicucullene into the IC of noise-exposed animals did not significantly change the relationship between the amplitude of this peak and the stimulus duration.

Microsurgical anatomy around the foramen of Luschka in relation to intraoperative recording of auditory evoked potentials from the cochlear nuclei

Three cadaveric heads were dissected to investigate the microsurgical anatomy around the foramen of Luschka. It was found possible to place a recording electrode in proximity to the cochlear nuclei by inserting it in the lateral recess of the fourth ventricle through the foramen of Luschka. In operations of the cerebellopontine angle using the retromastoid approach, access to the foramen of Luschka and the lateral recess is obtained by retracting the biventral lobule of the cerebellum in a caudal-rostral direction under a caudal-rostral/medial field of vision. The craniectomy might need to be enlarged a few millimeters in the caudal direction. A wick electrode can be inserted in the lateral recess beneath the choroid plexus in a rostromedial direction and to a depth of approximately 3 to 5 mm from the foramen of Luschka without excessive retraction of the cerebellum. The optimum position for the recording electrode is in the triangle formed by the axis of the cochlear nerve and the glossopharyngeal nerve and by the lip of the foramen of Luschka. The caudal retromastoid approach is more suitable than the translabyrinthine technique for recording from the cochlear nuclei as well as for implantation of stimulating electrodes into the cochlear nuclei for use as hearing prostheses.

Contribution from crossed and uncrossed brainstem structures to the brainstem auditory evoked potentials: a study in humans

The neural generators of the brainstem auditory evoked potentials (BAEPs) in humans are not completely known. Attempts to identify the anatomical location of the neural generators of the human BAEP based on the results of studies in animals commonly used in auditory experimentation have been difficult because of the considerable anatomical differences between the ascending auditory pathways in humans and animals. The authors of this study compared recordings obtained from different locations on the lateral side of the brainstem in six patients undergoing microvascular decompression surgery for a cranial nerve disorder affecting the fifth cranial nerve (i.e., trigeminal neuralgia). Ipsilateral click stimulation evoked prominent responses from the caudal aspect of the pons up to the junction between the pons and the midbrain, but all components of the responses with latencies shorter than 8 msec had smaller amplitudes when recorded at more rostral locations. Components with latencies in the range of peak V elicited by contralateral click stimulation had their largest amplitudes when recorded from the lateral brainstem at the level of the fourth cranial nerve (thus, close to the inferior colliculus). Earlier components of the contralateral responses (latencies in the range of the latency of peak III) had their largest amplitudes when recorded from the caudal lateral brainstem. The results of this study indicate that the part of the uncrossed auditory pathway that is located rostral to the cochlear nucleus contributes little to the farfield potentials (i.e., BAEP), and it is doubtful whether the contralateral response that can be recorded at the level of the cochlear nucleus contributes noticeably to the BAEP.

Transcranial magnetic stimulation of the trigeminal nerve: intraoperative study on stimulation characteristics in man

We studied responses from the masseter and nasalis muscles following magnetic stimulation (magStim) and compared these responses with those obtained by direct electrical stimulation of the trigeminal (NV) and facial (NVII) nerve near the root exit zone during microvascular decompression operations of NVII. We found that (1) magStim threshold to excite the nerve is high for NV and low for NVII; (2) excitation of all motor fibers is impossible for NV, and easy for NVII; (3) optimal coil placement is critical for NV, but not critical for NVII; and (4) between and within subjects, the excitation site is variable on NV, but stable on NVII. We estimated that the anatomical location of magStim to be either within or outside the cerebrospinal fluid for NV, and to be in the labyrinthine segment of the facial canal for NVII. Physical models explain and clinical lesion models support these differences found between NV and NVII.

Effects of L-baclofen and D-baclofen on the auditory system: a study of click-evoked potentials from the inferior colliculus in the rat

The drug baclofen is a potential treatment for severe tinnitus, but its action in relieving tinnitus is not known. Baclofen is available as an approved drug only in racemic form with about equal content of the two enantiomers. In the present paper we show that L-baclofen causes a considerable (40.7%) suppression of the amplitude of the second peak in the click-evoked response from the cochlear nucleus. Bipolar recordings from the external nucleus of the inferior colliculus showed that L-baclofen caused a reduction in the amplitude of three or four distinct peaks in this response. D-Baclofen had no detectable effect on the response from the cochlear nucleus, and had only a slight effect on one component of the response from the external nucleus of the inferior colliculus. The demonstrated effect of L-baclofen on excitation in the ascending auditory pathway indicates that this drug may be a potential treatment for hyperactive auditory disorders such as tinnitus and hyperacusis.

Recordings from the facial motonucleus in rats with signs of hemifacial spasm

We recorded evoked potentials from the facial motonucleus of rats in response to electrical stimulation of the temporal branch of the facial nerve in which chronic irritation from a blood vessel had caused the development of an abnormal muscle response. The abnormal muscle response that can be recorded from face muscles that are innervated by one branch of the facial nerve in response to electrical stimulation of a different branch is regarded to be a sign of hemifacial spasm. In the recordings from the motonucleus in rats that showed such an abnormal muscle response (model rats) there was a late component at a latency of about 5 msec, in addition to the early component with a latency of 1.5-2.5 msec that is also observed in normal rats. The latency of the electromyographic potentials recorded from the mentalis-orbicularis oris muscles in response to stimulation of the facial motonucleus was about 2 msec. The latency of the abnormal muscle response obtained from the mentalis muscle in the model rats was about 7 msec. This value is close to the sum of the conduction time from the motonucleus to the mentalis muscle (2 msec) and the latency of the late response from the motonucleus (5 msec). Similar results were obtained in rats in which the facial nerve had been chronically stimulated electrically and which had developed an abnormal muscle response. The results of this study further support the hypothesis that the hyperactivity of the facial motonucleus is the pathophysiology of hemifacial spasm.

Chronic vascular irritation of the facial nerve causes facial spasm in rats

The abnormal muscle response, elicited by electrical stimulation of one branch of the facial nerve and recorded from muscles innervated by another branch, has been used previously as an objective sign of hemifacial spasm in the development of animal models of this disorder. In the present study we recorded spontaneous electromyographic activity from the orbicularis oculi muscle from both sides in rats in which a demyelination of the peripheral portion of the facial nerve and vascular contact had been made previously. The root mean square value of the electromyographic activity on the affected side was significantly larger than that on the unaffected side in all rats in which the vascular irritation had caused the abnormal muscle response to appear. The results support our earlier finding that vascular contact together with demyelination of the peripheral facial nerve can cause the development of signs of hemifacial spasm, including involuntary muscle contractions.

Neural conduction velocity of the human auditory nerve: bipolar recordings from the exposed intracranial portion of the eighth nerve during vestibular nerve section

We measured the conduction velocity of the intracranial portion of the auditory nerve in 3 patients undergoing vestibular nerve section to treat Ménière's disease. The conduction velocity varied from patient to patient, with an average value of 15.1 m/sec. The latency of peak III of the brain-stem auditory evoked potentials (BAEPs) increased by an average of 0.5 msec as a result of exposure of the eighth nerve, and if that increase is assumed to affect the entire length of the auditory nerve (2.6 cm) evenly, then the corrected estimate of conduction velocity would be 22.0 m/sec. Estimates of conduction velocity based on the interpeak latencies of peaks I and II of the BAEP, assuming that peak II is generated by the mid-portion of the intracranial segment of the auditory nerve, yielded similar values of conduction velocities (about 20 m/sec).

Facial nerve demyelination and vascular compression are both needed to induce facial hyperactivity: a study in rats

It is generally assumed that hemifacial spasm (HFS) is caused by vascular compression of the facial nerve at the root exit zone (REZ), but the mechanism for the development of HFS is not known. Evidence has been previously presented that the signs of HFS are caused by hyperactivity of the facial motonucleus that is caused by the irritation to the facial nerve from the vascular contact. This assumption has been supported by the finding that daily electrical stimulation of the facial nerve in the rat facilitates the development of an abnormal muscle response that is a characteristic sign of HFS in man and is an indication of an abnormal cross-transmission that makes it possible to elicit a contraction of muscles innervated by one branch of the facial nerve by electrically stimulating another branch of the facial nerve. In the present study we show that close contact between a peripheral branch of the facial nerve and an artery also facilitates the development of an abnormal muscle response, but only if the facial nerve has previously been slightly injured (by a chromic suture) at the location of the arterial contact. We also show that blocking neural conduction in the facial nerve proximal to the artificial vascular compression abolishes the abnormal muscle contraction, which supports the assumption that the anatomical location of cross-transmission that is causing the abnormal muscle response is central to the vascular compression, most likely in the facial motonucleus. These findings may explain why the facial nerve is only susceptible to vascular compression near its REZ, where an injury to its myelin is more likely to occur than where the nerve is covered with schwann cell myelin.

Click-evoked responses from the cochlear nucleus: a study in human

Recordings from the vicinity of the cochlear nucleus in 9 patients undergoing microvascular decompression operations to relieve hemifacial spasm, trigeminal neuralgia, tinnitus, and disabling positional vertigo were conducted by placing a monopolar electrode in the lateral recess of the fourth ventricle (through the foramen of Luschka), the floor of which is the dorsolateral surface of the dorsal cochlear nucleus. The click-evoked potentials recorded by such an electrode display a slow negative wave with a peak latency of about 6-7 msec on which several sharp peaks are superimposed. None of the peaks in the recordings from the vicinity of the cochlear nucleus coincided with any vertex-positive peaks of the brain-stem auditory evoked potentials. In recordings from the lateral aspect of the floor of the fourth ventricle near the cochlear nucleus 1 patient showed 2 positive peaks, the earliest of which had a latency close to that of peak II and the second of which had a latency close to the negative peak between peaks III and IV of the brain-stem auditory evoked potentials. There is a distinct negative peak in the responses recorded from the midline of the floor of the fourth ventricle, the latency of which is only slightly shorter than that of peak V of the brain-stem auditory evoked potentials, supporting earlier findings that the sharp tip of peak V of the brain-stem auditory evoked potentials is generated by the termination of the lateral lemniscus in the inferior colliculus.

Auditory neurophysiology

The anatomy and the physiology of the ear and the auditory system are reviewed. The differences between the anatomy and physiology in humans and animals commonly used in auditory research are discussed. The clinical importance of brainstem auditory evoked potentials and their neural generators are discussed as well as the anatomy and physiology of the acoustic middle ear reflex. The possible clinical relevance of the extralemniscal auditory system is also addressed.

Preservation of hearing in operations on acoustic tumors: an alternative to recording brain stem auditory evoked potentials

The monitoring of auditory function by recording brain stem auditory evoked potentials in patients undergoing removal of acoustic tumors is hampered by the small amplitude of the brain stem auditory evoked potentials. Because several thousands of responses must be added, it takes several minutes to obtain an interpretable record. Recordings done directly from the exposed eighth nerve have much higher amplitudes, and, therefore, interpretable responses can be obtained after only a few responses have been added. However, it is difficult to place the recording electrode in an optimal position and the electrode may interfere with the removal of the tumor. In this report, we show that evoked potentials from the cochlear nucleus, which can be recorded by placing an electrode in the lateral recess of the fourth ventricle, have a large amplitude, and that the electrode placed in this way does not interfere with the removal of the tumor. This way of monitoring, therefore, yields interpretable responses within 15 to 20 seconds, or less, and makes it possible to detect injuries to the entire intracranial portion of the eighth nerve, just as brain stem auditory evoked potentials do, but 20 to 50 times faster.

Conduction pathways and generators of magnetic evoked spinal cord potentials: a study in monkeys

Evoked spinal cord potentials (ESCPs) following transcranial magnetic stimulation were recorded from the spinal cord in monkeys anesthetized with ketamine. Isopotential maps of the earlier negative deflection of the magnetic ESCP (N1 wave) revealed a distribution of negative field potentials, the maximum of which were located within the medial dorsolateral funiculus, which corresponds to the dorsolateral corticospinal tracts. The N1 wave of the magnetic ESCP had the same latency as the D-wave of the electrical ESCP elicited by either direct cortical or transcranial electrical stimulation. We assumed that the N1 wave was generated by direct excitation of pyramidal axons. Isopotential maps of the waves that followed the N1 peak (waves N2, N3, N4, and N5) of the magnetic ESCP showed a negative field potential distribution, the maximum of which was at the ventromedial funiculus as well as within the medial dorsolateral funiculus. Later waves of magnetic ESCP were suggested to reflect not only the dorsal corticospinal tracts but also the ventromedial spinal cord function.

Late waves in the response recorded from the intracranial portion of the auditory nerve in humans

We showed in previous studies that the click-evoked responses from the exposed eighth nerve in some patients contain quasi-periodic components that appear in the interval between 4 and 16 milliseconds after the click stimulus, and that the phase of these oscillatory components reverses when the click polarity is reversed. When the responses to clicks of opposite polarity are subtracted from each other, these late waves appear as a prolonged quasi-periodic oscillation with a frequency around 700 Hz. These late components appear more frequently in patients with high-frequency hearing losses than in patients with normal hearing. We have attributed these components to prolonged and in-phase oscillations of a large portion of the basilar membrane, possibly caused by the generation of standing waves on the basilar membrane. The results of the present study show that these oscillations correspond closely in both frequency and phase to the late oscillations that are seen in the cross-correlograms of the responses from the exposed eighth nerve to pseudorandom noise. The finding that similar quasi-periodic components can be retrieved from the responses from the exposed eighth nerve to both transient and continuous sounds is taken as an indication that the neural activity that these components represent reflects some general property of the cochlear frequency analyzer. We also found that the speech discrimination is not noticeably different in patients with such waves compared with what it is in patients with similar hearing loss who do not have such waves. This is taken as an indication that spectral analysis in the cochlea is less important in speech discrimination than previously assumed.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction between auditory and somatosensory systems: a study of evoked potentials in the inferior colliculus

It is known that some neurons in the external nucleus of the inferior colliculus (ICX) receive both auditory and somatosensory input. In recordings of evoked potentials from the ICX in the rat we show evidence that somatosensory activation can modify the auditory response to click stimulation. Electrical stimulation of the median nerve affects the click-evoked response that is recorded from the surface of the ICX and different components of the response were affected differently. The magnitude of the changes was dependent on the time interval between electrical stimulation of the median nerve and presentation of the click stimulation. The amplitude of the two earliest peaks in the click-evoked ICX response increased slightly (16% and 20%, respectively; average of recordings in 10 animals) as a result of stimulation of the median nerve, while the amplitudes of two later peaks decreased as a result of stimulation of the median nerve (the average decrease in amplitude of these two peaks in recordings in 10 animals was 18 and 26%, respectively). The latencies of the 3 peaks we studied were only slightly affected, and the maximal change was 0.07 and 0.25 msec, respectively, for the 2 earliest peaks. The decrease in the amplitude of specific peaks of the ICX response confirms that some neurons of the extralemniscal system receive both auditory and somatosensory input, and that such somatosensory input is mainly inhibitory on these neurons. We interpret the increase in the amplitude of the early peak to be a result of median nerve stimulation caused by a decreased centrifugal inhibitory influence on superior olivary neurons.

Chronic electrical stimulation of the facial nerve causes signs of facial nucleus hyperactivity

The proximal segment of the facial nerve in rats was stimulated electrically daily for a duration of 2-10 min. After 4-8 weeks of such stimulation, 12 of 18 rats developed abnormal muscle responses that could be demonstrated by recording the electromyographic response from lower face muscles (the mentalis muscle) while the temporal branch of the facial nerve was being stimulated electrically. This abnormal electromyographic response consists of activity that appears in the latency range 6.5-15 ms. In addition, these chronically stimulated rats developed signs of facial synkinesis on the side that had been chronically stimulated. This could be demonstrated by recording electromyographic activity when the blink reflex was being elicited by electrical stimulation of the ophthalmic nerve. Rats in which electrodes had been implanted but which had not been stimulated did not develop any abnormal electromyographic activity. The abnormal electromyographic activity that could be recorded in rats that had been stimulated chronically could not be recorded 4-8 weeks after the stimulation had been terminated. We interpret these results to indicate that chronic electrical stimulation of the facial nerve can render the facial motonucleus hyperactive, and that the signs of this hyperactivity (abnormal muscle response and synkinesis) are similar to those typically seen in patients with hemifacial spasm. We thus presume that these results support the hypothesis that it is the irritation of the facial nerve from a compressing blood vessel that causes the facial nucleus to become hyperactive in patients with hemifacial spasm.

Restoration of useful hearing after microvascular decompression of the cochlear nerve

This report describes a patient with sudden sensorineural hearing loss who was found to have a megadolichoectasia vertebrobasilar system that appeared to be causing compression of the ipsilateral facial and vestibulocochlear nerves. The patient was treated conservatively for 4 months, during which time no hearing returned. He then underwent microvascular decompression of the affected nerves. At surgery, marked compression of the cranial nerves VII-VIII complex and the pons was observed. Postoperatively, the patient experienced a gradual return of useful hearing. We suggest that vascular compression may be a rare, but treatable, cause of sensorineural hearing loss.

Abnormal response from the sternocleidomastoid muscle in patients with spasmodic torticollis: observations during microvascular decompression operations

Electromyographic (EMG) recordings from patients undergoing microvascular decompression (MVD) operations to relieve spasmodic torticollis were studied. When EMG potentials were recorded from the sternocleidomastoid muscle in response to electrical stimulation of the spinal accessory nerve (SAN) at the neck, an abnormal (delayed) response was seen in 9 of 12 patients who had unilateral symptoms. In 5 patients with bilateral symptoms, no such delayed response was seen. We assume that this abnormal muscle response depends on an abnormal cross-transmission. Neural conduction time measurements, using electrical stimulation of the intracranial portion of the SAN, indicated that the location of this cross-transmission was more central than the vascular compression of the SAN. We hypothesize that this location might be in the motonucleus of the SAN. Similarities between these abnormal EMG findings in patients with spasmodic torticollis and those reported earlier in patients with hemifacial spasm (HFS) are presented.

Vascular decompression surgery for severe tinnitus: selection criteria and results

Seventy-four patients were operated on within a period of 10 years to treat incapacitating tinnitus; 72 underwent microvascular decompression (MVD) of the intracranial portion of the auditory nerve, and 2 underwent section of the eighth nerve close to the brain stem. Of those who underwent MVD, 2 had no change in symptoms and later also underwent section of the eighth nerve near the brain stem. Two patients did not return for follow-up. Of the 72 remaining patients, 13 (18.1%) experienced total relief from tinnitus, 16 (22.2%) showed marked improvement, 8 (11.1%) showed slight improvement, 33 (45.8%) had no improvement, and 2 (2.8%) became worse. The patients who experienced total relief and those who showed marked improvement had experienced their tinnitus for an average of 2.9 years and 2.7 years, respectively; those who showed slight improvement and those who had no improvement had experienced their tinnitus for a longer time before the operation (mean, 5.2 and 7.9 years, respectively). Of the 72 patients who were operated on and followed, 32 were women. Of these, 54.8% experienced total relief from tinnitus or marked improvement, while only 29.3% of the men showed such relief or improvement. Selection of the patients for operation was mainly based on patient history and, to some extent, on auditory test results (brainstem auditory evoked potentials [BAEP], acoustic middle ear reflexes, and audiometric data).

Microvascular decompression of the eighth nerve in patients with disabling positional vertigo: selection criteria and operative results in 207 patients

Two-hundred seven patients who were operated on consecutively between January 1983 and December 1990 to relieve disabling positional vertigo (DPV) using the microvascular decompression (MVD) procedure were studied. Selection of the patients for MVD operations was based on both case history and the results of otoneurological tests. Of the 177 patients with unilateral symptoms, 8 were excluded because of previous vestibular nerve section, and 6 did not return for follow-up; of the remaining 163 patients, 129 (79%) were free of symptoms or markedly improved following MVD, and none became worse. Thirty patients had symptoms and signs of bilateral DPV, and of these 1 was excluded because of previous vestibular nerve section and 3 because of multiple operations. Of the remaining 26 patients, 20 (77%) were free of symptoms or markedly improved following MVD. Eleven of these patients had more than 2 operations. The follow-up time was an average of 38 months, ranging from 3 months to 10 years. The cure rate (about 80%) of MVD for DPV is similar to that reported for MVD for trigeminal neuralgia and hemifacial spasm. The cure rate of MVD for DPV was not related to gender or to the duration of the symptoms. Following a total of 254 operations that these 207 patients underwent, 4 patients (1.6%) lost hearing and 4 (1.6%) suffered marked hearing loss. Three patients suffered temporary deficits of other cranial nerves. There were no other complications to these operations.

Some forms of tinnitus may involve the extralemniscal auditory pathway

It has previously been shown that the click-evoked responses recorded from the intracranial portion of the eighth nerve in patients with incapacitating tinnitus are not abnormal, nor is the latency of peak III of the click-evoked brainstem auditory-evoked potentials significantly altered; however, the latency of peak V is slightly (but significantly) shortened in comparison to that of patients with the same degree of hearing loss but no tinnitus. In this study the hypothesis that the extralemniscal auditory system is involved in the generation of tinnitus is tested. We made use of the fact that neurons of the extralemniscal auditory system also receive input from the somatosensory system, and that stimulation of the somatosensory system can influence the processing of auditory information in the extralemniscal system. In 4 of 26 patients with mild-to-severe tinnitus whose median nerve was stimulated electrically, the tinnitus increased noticeably during stimulation, in 6 the intensity of the tinnitus decreased noticeably, and in the remaining 16 there was no noticeable change in the tinnitus. In some of the patients the character of the tinnitus changed in a complex way. There were no significant differences in hearing thresholds in these three groups of patients. Electrical stimulation of the median nerve in 12 individuals with normal hearing who did not have tinnitus either had no effect on the loudness of sounds or it caused a slight increase in the loudness.

The excitation site of the trigeminal nerve to transcranial magnetic stimulation varies and lies proximal or distal to the foramen ovale: an intraoperative electrophysiological study in man

The excitation site of the trigeminal nerve using transcranial magnetic stimulation (magStim) was analyzed in 5 patients in whom the trigeminal nerve was surgically exposed in the posterior fossa during microvascular decompression of the facial nerve for hemifacial spasm. The trigeminal nerve was stimulated (1) magnetically immediately prior to craniotomy, and (2) electrically near the root exit zone (elREZ) of the nerve from the brainstem. Mean latency differences (delta) of masseter compound muscle action potentials (CMAPs) (delta elREZ minus magStim) were 0.7 (range: +0.3 to +1.3) ms (P less than or equal to 0.05, Wilcoxon-test). From these results, an analysis of anatomical data, and using a trigeminal nerve conduction velocity (NCV) of 50 m/s as reported in the literature, the following conclusions were drawn: the excitation site to magStim (1) is variable among individuals, (2) is located 3.4 (1.6-6.5) cm distal to the trigeminal REZ, and (3) which corresponds to segments of the nerve that are located either within or outside the cerebrospinal fluid (CSF), either proximal or distal to the foramen ovale. These findings are in contrast to those we obtained in a previous study of the facial nerve in which the excitation site was found to be constant among subjects and restricted to the location on the nerve where it exists the high conductivity CSF to enter the high-resistance petrous bone.

Transcranial magnetic stimulation of the facial nerve: intraoperative study on the effect of stimulus parameters on the excitation site in man

Magnetic stimulation (magStim) of the intracranial facial nerve is performed in clinical and research settings, but the activation site is a matter of controversy. Latencies of nasalis muscle responses to magStim were, therefore, compared with those obtained by direct electrical stimulation of the facial nerve (a) at the root exit zone (REZ); (b) at the porus of the facial canal; and (c) in the stylomastoid fossa during microvascular decompression operations in the cerebellopontine angle (CPA). Measurements of latencies of the nasalis muscle response, obtained while the stimulating coil was placed over the parieto-occipital area of the scalp, indicated that it was the labryinthine segment of the facial canal, 5 to 16 mm distal to the CPA, that was activated. This would be in agreement with studies of physical models reported in the literature that showed (a) the strength of the electrical current generated by a magnetic field is particularly high close to a nerve foramen; and (b) excitation to magStim is most likely to occur where the induced electrical field changes rapidly over distance, i.e., at anatomical boundaries between media of high and low specific resistance. These characteristics are found at the end of the labyrinthine segment of the facial canal, where the facial nerve leaves the low-resistance cerebrospinal fluid and enters the high-resistance petrous bone. The site of neural excitation is robust and unaffected by stimulus intensity and current direction within a wide range, or by large changes in location of the coil.(ABSTRACT TRUNCATED AT 250 WORDS)

Compound action potentials recorded from the exposed eighth nerve in patients with intractable tinnitus

Compound action potentials (CAP) were recorded directly from the exposed intracranial portion of the eighth nerve in 19 patients undergoing microvascular decompression (MVD) of the eighth nerve for intractable tinnitus. The waveform of the CAPs recorded in patients with tinnitus varied from normal to highly abnormal, but only in 1 patient were there distinct abnormalities in the waveform of the CAP that could not be attributed to the patient's hearing loss. The mean values of the latencies of the N1 and N2 peaks in the CAPs recorded from the exposed eighth nerve in patients with tinnitus and high-frequency hearing loss were virtually indistinguishable from the latencies obtained in patients with similar hearing loss but no tinnitus. There was no statistically significant difference between the latency of peak III in the brainstem auditory evoked potentials (BAEPs) in these two groups of patients, but the latency of peak V was slightly shorter (statistically significant) in the patients with tinnitus than it was in the patients without tinnitus.

Neuromonitoring in operations in the skull base

Methods to monitor the integrity of cranial motor nerves during operations on skull base tumors have been developed over the past decade. These methods can help the surgeon to identify cranial motor nerves that are located in the surgical field but which may not be visible directly. Methods have also been developed that allow monitoring of the function of sensory systems such as the auditory system and it has been shown that brainstem auditory evoked potentials can provide important information about the integrity of the auditory nervous system. It has been shown in several studies that such neuromonitoring, when performed during operations in the skull base, can help reduce the incidence of permanent neurological deficits that may occur with the removal of large tumors. We have also shown that such monitoring can be performed routinely without interfering noticeably with the actual surgical procedures.

Effect of high-frequency hearing loss on compound action potentials recorded from the intracranial portion of the human eighth nerve

Compound action potentials (CAP) were recorded from the exposed intracranial portion of the eighth nerve to stimulation with click sounds in patients with sensorineural high-frequency hearing loss who underwent microvascular decompression (MVD) operations to treat trigeminal neuralgia (TN). In patients with normal hearing the CAP recorded in that way is characterized by a negative peak, preceded by a small positivity and followed by a positivity and sometimes a second negative peak. In patients with high-frequency hearing loss the CAP also usually had an initial sharp negative peak in response to clicks of high intensity (105 to 110 dB Pe SPL), similar to findings in patients with normal hearing, but in patients with high-frequency hearing loss the initial negative peak was often followed by a slow negative deflection. The latency of the initial negative peak in the CAP in patients with high-frequency hearing loss was longer than the latency of this peak in patients with normal hearing, but the difference in latencies of this peak to condensation and rarefaction clicks was small. When the stimulus intensity was lowered the amplitude of the initial peak decreased, and the CAP became dominated by a broad negative peak with a latency of 6 to 8 ms. In 11 of 15 patients with severe high-frequency hearing loss, a series of quasi-periodic waves was superimposed on the CAP. The frequency of these waves varied between 500 and 1200 Hz, and the waves could be detected between 6 and 16 ms after presentation of the click stimulus. These waves were usually present in the response to stimuli in the intensity range from 75 to 110 dB Pe SPL. Only 4 of 17 patients with normal hearing had similar waves.

Comparison of somatosensory evoked potentials recorded from the scalp and dorsal column nuclei to upper and lower limb stimulation in the rat

Responses from the dorsal column nuclei (DCN) in the rat to stimulation of the upper limbs (median nerve) and lower limbs (sciatic nerve) showed a difference in the wave forms of the two responses. These results support results of earlier studies in the cat, monkey, and man that showed that only slow-conducting cutaneous afferents from the lower limbs travel in the dorsal column, while all afferents from the upper limbs travel in the dorsal column and synapse in the DCN. A comparison between the response from the DCN and that from the vertex to stimulation of the upper limbs showed correspondence between short-latency peaks, while no clear earlier waves could be discerned in the response from the vertex to stimulation of the lower limbs. Even when the dorsal column was transected on one side, the correspondence between the early peaks in the scalp and the DCN responses to stimulation of the upper limbs was maintained. The effect of the dorsal column lesion on the response recorded from the surface of the DCN to stimulation of the sciatic nerve was mainly a reduction in the number of peaks. Transection at the midbrain level resulted in elimination of the long-latency response in the scalp recording, but the initial negative peak was maintained, which corresponded to the initial negative peak of the DCN response to stimulation of the upper limbs.

Auditory nerve compound action potentials and brain stem auditory evoked potentials in patients with various degrees of hearing loss

Click-evoked compound action potentials recorded in normal-hearing patients through a monopolar electrode placed on the intracranial portion of the eighth nerve were compared with the responses recorded in patients with high-frequency hearing loss or with high- and low-frequency hearing losses. That multiple peaks appear in the compound action potential in patients with hearing loss implies that click sounds elicit successive and separated volleys of neural excitation in the ascending auditory pathway, whereas click sounds in patients with normal hearing mainly give rise to a single volley of neural activity. This difference in the pattern of auditory nerve activity might explain why there are often multiple peaks in the brain stem auditory evoked potentials in patients with hearing loss and that the peaks are often less well-defined than peaks in patients with normal hearing.

Transcranial magnetic stimulation excites the labyrinthine segment of the facial nerve: an intraoperative electrophysiological study in man

The site where transcranial magnetic stimulation (magStim) depolarizes the facial nerve was investigated in 6 patients who underwent surgery of the cerebellopontine angle (CPA). The facial nerve was stimulated (1) magnetically prior to craniotomy, (2) electrically near the brainstem (elREZ), (3) at the exit from the CPA into the facial canal (elPorus), and (4) in the stylomastoid fossa (elStylo). The range of latency differences (delta) of compound muscle action potentials (CMAPs) recorded from the ipsilateral mentalis muscle were as follows: delta elREZ-magStim: +0.5 to +1.1 ms (P less than or equal to 0.03, Wilcoxon test); delta elPorus-magStim: +0.2 to +0.5 ms (P less than or equal to 0.03); delta elStylo-magStim: +0.8 to +1.0 ms (P less than or equal to 0.03). On the basis of anatomical data and a facial nerve conduction velocity of 33-46 m/s in these patients, it was concluded that transcranial magnetic stimuli depolarized the facial nerve at a location 10-15 mm distal to its entrance into the facial canal. This corresponds to the end of the labyrinthine segment of the facial nerve, i.e. the transit zone where the nerve ceases to be surrounded by cerebrospinal fluid (CSF) with its high electrical conductivity and enters the high-resistance tissue of the petrous bone.