Hearing Research, 45 (1990) 75-86

Elsevier

75

HEXRES 01342

Late components in the compound action potentials (CAP) recorded from the intracranial portion of the human eighth nerve

Aage R. Merller and Hae Dong Jho Department of Neuroiogicai Surgery, University of Pittsburgh School of Medicine, Pittsburg& Pennsylvania, U.S.A.

(Received 9 September 1989; accepted 28 October 1989)

The compound action potential (CAP) that can be recorded from the exposed intracranial portion of the eighth nerve in man to

stimulation with broadband clicks of about 100 dB Pe SPL normally has an initial (small) positivity followed by a sharp negative

peak with a latency of about 3 to 3.5 ms. The negative peak is usually followed by another positive-negative deflection with a latency

of about 4 ms. Usually, no stim~us-related potential can be discerned at latencies longer than 5 ms. However, in a few patients we

found a series of waves that occurred between 4 and 12 ms after the stimulation. The polarity of these waves reversed precisely (180”

phase shift) when the polarity of the sound was reversed. Thus, these waves appeared clearly when the responses to clicks of opposite

polarity were subtracted. These late waves were quasiperiodic with intervals between 1 and 2 ms. The waveform and duration differed

between patients, but were remarkably constant in each patient. The timing of these late peaks was nearly independent of the

stimulus intensity in the range studied (between 105 and 70 dB Pe SPL); in this respect the late waves differ fundamentally from the

initial peaks, which showed a monotone decrease in latency with increasing stimulus intensity over this intensity range. Although the

origin of these late waves is not known, the similarities between these waves and stimulated otoacoustic emissions indicate that the

late waves may be the result of active cochlear processes similar to those that produce the cochlear echo.

Human auditory nerve; Compound action potentials; Active co&ear processes

Whole-nerve action potentials (known as corn- pound action potentials, or CAP), recorded either from the round window of animals or man or from the exposed intracranial portion of the eighth cranial nerve, have been the focus of numerous studies. The CAP recorded from the human ear using the so-called ‘electrocochleography tech- nique,’ in which the recording electrode is placed on the promontorium (Spoor et al., 1976; Egg- ermont, 1978) or in the ear canal (Coats and Dickey, 1970; Elberling, 1976) is dominated by a single negative peak (N1). It is thus similar to the CAP recorded from a distal location on the in- tracranial portion of the human auditory nerve

Correspondence to: A.R. Meller, Department of Neurological

Surgery, Room 9402, Presbyterian University Hospital, 230 Lothrop Street, Pittsburgh, PA 15213, U.S.A.

with a monopolar electrode (MGller et al., 1981; Meller and Jannetta, 1981, 1983; Spire et al., 1982). In small animals a second negative peak (Nz) appears after the first negative peak. The second negative peak (Nz) in the CAP recorded from the auditory nerve in man (Msller and Jan- netta, 1981, 1983) and small animals presumabIy originates in the cochlear nucleus (Mnller, 1983a). The human N2 is smaller, particularly when re- corded from the promontorium or the distal part of the intracranial portion of the human eighth nerve, because it is, anatomically, much farther away from the cochlear nucleus in man than it is in small animals.

In the present study we show that in a few patients there are components in the CAP re- corded from the intracranial portion of the human eighth nerve in response to click sounds that ap- pear with a latency that is longer than that of the N2, and that these potentials behave rather differ- ently from the initial negative peaks in the CAP.

0378-5955/90/$03.50 0 1990 Elsevier Science Publishers B.V. (Biomedical Division)

Methods

Recordings were made from the intracranial portion of the eighth nerve in patients undergoing microvascular decompression (MVD) operations for intractable tin&us, disabling positional vertigo (DPV), trigeminal neuralgia (TN), or hemifacial spasm (HFS). Preoperatively, all patients under- went complete audiological evaluation, including

pure tone audiograms, determination of speech discrimination scores, recording of acoustic mid- dle ear reflexes and BAEP using 2-kHz tonebursts, and, occasionally, click stimuli. The tympanic

membrane was inspected and cerumen was re- moved, if present, by an experienced otologist. Patients with disabling positional vertigo (DPV) in addition underwent complete otoneurological

evaluation. All patients had routine computerized

axial tomography (CAT) or nuclear magnetic reso- nance (NMR) scans. All patients included in this

study had normal otological findings with normal middle ears judged by otoscopic examinations and normal tympanograms. The mentioned imaging tests showed no pathologies or abnormalities in

any of the patients. The anesthesia and operative technique (Jan-

netta, 1981a,b), as well as the methods of record- ing the CAP from the exposed intracranial portion

of the eighth nerve, were the same as were used earlier (see Moller and Jannetta, 1983; Moller, 1988; Moller and Jho, 1989) and, therefore, they will be described only briefly here.

The patients who were operated upon to relieve

TN and DPV were anesthetized using a balanced technique with a strong narcotic (fentanyl) and a muscle relaxant (pancuronium or vecuronium), to-

A IOOps CLICK

I I I 0 1 2 3 ms

dS

I I I I IIIIIII I I I llllll I

100 1000 10000 Hz

Fig. 1. Waveform of condensation (solid lines) and rarefaction (dashed lines) clicks (A) and spectrum (B), as measured in a 2.5-cc slightly acoustically damped cavity using a Bruel and Kjaer l/4-inch microphone.

2.5 c

2.0

I I I I I

65 75 85 95 105

dB PeSPL

Fig. 2. (A) Examples of typical CAP recorded from the exposed intracranial portion of the eighth nerve in response to broadband rarefaction (solid lines) and condensation (dashed lines) clicks presented at different sound intensities (indicated by legend numbers). These results were obtained in a patient who was operated upon to relieve trigeminal neuralgia. The patient had normal hearing before and after the operation and BAEP did not change noticeably during the operation. Negativity is shown as an upward deflection in this and in subsequent graphs. (B) Latency of the positive (lower curves) and negative (upper curves) peaks in the

recordings shown in (A).

gether with nitrous oxide. Patients operated upon to relieve HFS underwent surgery without the use of muscle relaxants; they were maintained on iso- flurane and nitrous oxide, sometimes supple- mented with a small amount of narcotic. None of these anesthesia techniques is known to affect

short-latency auditory evoked potentials noticea- bly.

The stimulus sound was generated by applying lOO+s rectangular waves of either polarity (sup- plied by a Grass Instrument Co., Type SlO CTCMA audiostimulator) to miniature stereo ear-

78

Fig. 3. Recordings similar to those shown in Fig. 2(A) but obtained in a patient in whom there were late waves in the CAP. The

patient was operated upon to relieve trigeminal neuralgia. (A) Recorded from a distal location on the exposed eighth nerve. (B)

Recordings similar to those in (A) but from a more proximal location.

phones (Radio Shack, Realistic) (see Moller, 1988) that were secured in the patients’ ears by plastic adhesive tape. The sound generated by these ear- phones, as measured in a 2.5-cc slightly damped cavity, is shown in Fig. 1.

Needle electrodes (Grass Instrument Co., Type E2 subdermal electrodes) were placed on the vertex and both earlobes for recording brainstem audi- tory evoked potentials (BAEP), with a ground electrode over the sternum. A fine, Teflon-in- sulated, multistrand, silver wire with a cotton wick sutured to its uninsulated tip (Moller and Jan- netta, 1983) was placed on the exposed eighth nerve for monopolar recording of the auditory nerve CAP. The reference electrode was a subder- ma1 needle electrode placed either on the opposite earlobe or on the ipsilateral shoulder.

The potentials were amplified using differential amplifiers with filter settings at ~-HZ highpass (6 dB/octave rolloff) and 3-kHz lowpass (24 dB/oc- tave rolloff) (Grass Instrument Co., Model 12). The recorded potentials were averaged using an LSI 11/73 processor, which also stored the data on computer disks for later processing. Potentials from the exposed eighth nerve were usually re- corded from the time that the eighth nerve was exposed until the end of the intracranial part of the operation. BAEP to the same stimuli were recorded during the entire procedure, from the time the patient was anesthetized until closure of the wound.

To detect changes in auditory function as a result of surgical manipulation, BAEP obtained after the patients had been anesthetized but before

the operations were begun were used as a baseline. BAEP recorded during the operation were then compared to each patient’s baseline.

The recordings on which this study is based were obtained in the course of intraoperative monitoring of BAEP that is done routinely in

patients undergoing such neurological operations to minimize the risks of hearing loss due to mani- pulations of the eighth nerve. The monitoring procedures have been approved by the Biomedical Institutional Review Board of the University of Pittsburgh School of Medicine, and the patients gave informed consent to the procedure.

Results

Typically, compound action potentials (CAP) recorded from the most distal point on the in-

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79

tracranial portion of the uninjured eighth nerve in

response to stimulation with broadband clicks in patients with normal hearing have an initial small positive deflection, followed by a large negative peak with a latency of 3 to 3.5 ms (Fig. 2A). (A large difference between the response to rarefac-

tion and condensation clicks, as seen at 95 dB, is present occasionally and it was reproducible in this patient.) When recordings are made at loca-

tions that are closer to the brainstem, the negative peak is often followed by a small positive deflec- tion, and often by another negative deflection of smaller amplitude than the first one.

In patients with normal hearing in whom dis-

section of the eighth nerve did not result in any detectable changes in the farfield potentials (brainstem auditory evoked potentials, BAEP), the latency of the initial positive peak and that of the

Fig. 4. Same data as shown in Fig. 3(B). (A) Sum of the responses to condensation and rarefaction clicks. (B) Difference between the responses to condensation and rarefaction clicks.

80

L I II I I I I I I I I U-d I I I I I I 0 2 4 6 8 10 (2 14 16 18 20 ms 0 2 4 6 * 10 12 14 16 18 20 ins

Fig. 5. Recordings from the same patient whose results are shown in Figs. 3-4, obtained at three different repetition rates. (A)

Compound action potentials recorded in response to condensation and rarefaction clicks. (S) Difference between the recordings shown in (A).

main negative peak in the CAP recorded from the intracranial portion of the eighth nerve increased monotonically when the stimulus intensity was decreased. Conversely, the polarity of the click stimuli in these cases had little effect on the waveform or latency of the CAP (Fig. 2).

A rather different pattern of responses was seen in another patient (Fig. 3). While the waveform of the initial portion of the potentials shown in Fig. 3 is similar to the waveform of the potentials shown in Fig. 2, in the patient whose results are shown in Fig. 3 a series of waves followed the initial compo- nents of the CAP. The initial portion of the CAP changed little as a result of reversing the click polarity, but the polarity of these late waves re- versed when the stimulus polarity was reversed. When the stimulus intensity was decreased these waves were still discernible.

These later components of the potentials re- corded from the exposed eighth nerve cancelled when the responses to clicks of opposite polarity

were added (Fig. 4A), but they were seen clearly when the responses to opposite click polarity were subtracted (Fig. 4B). These later waves can be seen to have the form of oscillations, with several waves being recognizable over a period ranging from 4 to 10 ms after the presentation of the stimulus. The timing of these waves (Fig. 4B) showed no noticeable dependence on the stimulus intensity in the range that was studied, in contrast to the increase in latencies of the early compo- nents of the CAP that occurred when stimulus level was decreased.

Increasing the rate of stimulus presentation from 19 to 49 or 99 pulses per second (pps) (Fig. 5) resulted in a decrease in amplitude of the initial negative peak in the CAP as well as in the ampli- tude of the late potentials (Fig. 5). The latency of the initial negative peak in the CAP increased slightly as a result of increasing the repetition rate of the stimulus, while there was no noticeable difference in the timing of the late potentials.

81

PREOP A

CAP

INTRAOP

SKIN CLOSURE

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0 1 2 3 4 5 s 7 s 9 10 Ins

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0

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120

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Dx: TRIGEMINAL NEURALGIA

Fig. 6. (A) BAEP obtained before and during the operation on the patient whose results are shown in Fig. 3, together with CAP recorded from the eighth nerve. Vertex negativity is shown as an upward deflection and the BAEP were subjected to digital filtering

(Meller, 1983~). Solid lines: the responses to rarefaction clicks; dashed lines: the responses to condensation clicks. (B) Pure tone audiograms obtained before and 6 days after the operation.

82

120

Fig. 7. Sums and differences of the responses to rarefaction

and condensation clicks recorded from locations on the eighth

nerve near the porus acusticus (solid lines) and near the

brainstem (dashed lines), in the same patient whose results are

illustrated in Figs. 2-6. The difference between the responses

to rarefaction and condensation clicks was highpass filtered

using a zero-phase digital filter.

During the operation, the BAEP of the patient whose results are shown in Figs. 3-5 changed slightly (Fig. 6A). The patient had a slight hearing loss preoperatively (Fig.6B), which was unchanged after the operation.

It is important to ensure that the long-latency components we are describing result from propa- gated neural activity in the auditory nerve and are not a result of volume-conducted potentials that originate from sources at a distance from the recording site. Comparison of the responses re- corded from two different locations on the eighth nerve (Fig. 7) shows that the latencies of the early components of the CAP increase when the record- ing electrode is moved from a distal to a more proximal position, and that these late potentials are shifted about the same amount.

The waveforms, durations, and latencies of these late potentials varied considerably from patient to patient, as seen from Fig. 8, which shows results similar to those in Fig. 3 but from two other patients who had late waves. It is seen that the amplitudes of the waves are smaller in the patient illustrated in Fig. 8 than in the patient illustrated

in Fig. 3. Examples of the individual variability are further illustrated in Fig. 9, which shows CAP from 5 other patients.

Of a total of 77 patients operated upon con- secutively to relieve vascular compression of cranial nerves V, VII, or VIII over a period of 2

years and 8 months in whom we recorded in- traoperatively from the exposed nerve, 18 showed late potentials such as those illustrated in Figs.

2-8.

Discussion

We have shown in this study that the response from the exposed eighth nerve in a few individuals contains late components consisting of a series of waves that appear between about 4 and 12 ms after the stimulus. This is considerably later than

the commonly seen components, which typically are dominated by a negative deflection with a latency of 3-3.5 ms. The polarity of these late waves is reversed when click polarity is reversed, and their timing is nearly independent of the stimulus intensity in the range 75-105 dB PeSPL.

The fact that the latency of these late compo- nents shifts when the recording electrode is moved along the exposed eighth nerve indicates that these potentials are a result of propagated neural activ- ity in the auditory nerve. This, in turn, seems to rule out the possibility that they are generated by any stationary source such as the cochlea or the cochlear nucleus, or that they are picked up by the

reference electrode. Considerable experimental evidence has been

accumulated to show that the neural discharges of single auditory nerve fibers in response to broad- band transient sounds are phase-locked to the damped oscillations of the basilar membrane (Pfeiffer and Kim, 1972; Kiang et al., 1965) in nerve fibers with characteristic frequencies up to 5 or 6 kHz. It is generally assumed that the gross potentials that can be recorded from the auditory nerve are a convolution between the summed dis- tribution of discharges in all the nerve fibers from which the recording is made and the waveform of a single neural discharge (Goldstein, 1960). The finding that the late components in the response from the human auditory nerve reverse in polarity when the polarity of the stimulus sound is reversed

83

DIFFERENCE

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65 d8

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Fig. 8. (A) Compound action potentials recorded in response to condensation (dashed lines) and rarefaction (solid fines) clicks. (B) The differences between the responses to condensation and rarefaction clicks. The results are from another patient who showed late waves in the response from the eighth nerve. The patient was operated upon to relieve t~ge~n~ neuralgia. The audiogram of the

patient is also shown (C).

84

:

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I!

-/ ,,’

.-.

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--.

j2PV

,---

Fig. 9. Compound action potentials recorded in response to condensation and rarefaction clicks obtained in five patients (A-E) who displayed late waves. The audiograms of these patients are also shown.

85

thus indicates that the late components are the results of neural activity that is phase-locked to a low-frequency oscillation of the basilar membrane.

If we assume that click stimuli excite the basilar membrane relatively uniformly and that sensitivity is relatively uniform along the basilar membrane, then the gross response would not bear any re- semblance to any specific oscillation because it will be a sum of damped oscillations of different

frequencies. Therefore, only the initial portion of the damped oscillations of the basal portion of the basilar membrane will result in synchronization of neural activity in many nerve fibers. This probably explains why, in the majority of the patients

studied, the response from the auditory nerve is a single positive-negative wave resembling the re-

sponse from a long nerve to a brief excitation (Lorente de No, 1947).

Sharp discontinuities in the mechanical proper- ties of the basilar membrane could result in over-

representation of the damped oscillations pro- duced by fibers that are tuned to frequencies within a narrow range. This conceivably could leave enough residual oscillations in the summed neural activity to be noticeable in the CAP, which could be one explanation for the late components seen late in the CAP. However, even though these late potentials might originate from a point on the basilar membrane that is tuned to a frequency that is equal to that of the oscillations (around 500-1000 kHz), the observed latencies seem too long to explain the travel time on the basilar membrane to that point on the basis of available data on travel times in the normal human cochlea (von BekkCsy, 1960). These data are from human cadaver ears. Nonlinearities that are known to exist in the normal cochlea may, however, result in a much longer travel time. Results from human electrocochleography show latency times of 4 ms in the 1-kHz region (Eggermont, 1976). When recorded from the intracranial portion of the audi- tory nerve, about 1 ms of neural conduction time should be added, which means that values of about 5 ms could be expected. This seems to be in agreement with the results obtained in the present study.

The late components in the CAP, described in the present paper, bear some resemblances to evoked otoacoustic emissions. Both late compo-

nents in the CAP and otoacoustic emissions have relatively long latencies and a quasiperiodic nature (Kemp, 1978, 1979, 1980). Otoacoustic emissions have been explained on the basis of inhomogenei- ties of the basilar membrane, and they are gener- ally assumed to be due to co&ear nonlinearities (see Kemp, 1979). An important difference be- tween otoacoustic emissions and late components in the CAP, however, is that the former seem to be

constant phenomena that can be demonstrated in nearly all people, while we have only seen the late components in the CAP in a small percentage of the patients we have studied. Also, late compo- nents in the CAP have been seen in patients with

hearing loss, while it has been shown that

otoacoustic emission is generally not detectable when the hearing loss exceeds 15 dB (Rutten, 1980; Kemp et al., 1986). This indicates that either the phenomenon described in the present paper is not identical to otoacoustic emission or that re- cording of CAP is a more sensitive method to detect this phenomenon.

The finding that the latency of the late compo- nents does not change noticeably when the stimu- lus intensity is altered is in agreement with known properties of phase-locked activity in single audi- tory neurons in response to transients (Kiang et al., 1965) or continuous noise (Moller, 1977, 1983b). The small decrease in latency with increas-

ing stimulus intensity that has been shown to exist in the results from single nerve fibers may be explained by the shift in maximal excitation of the

basilar membrane towards the base of the cochlea when the stimulus intensity is increased (Moller, 1977, 1983b), but this may not have been detecta- ble in all of our recordings from the human audi- tory nerve.

The fact that these late potentials are only seen in a small proportion of the patients that were studied indicates that it is not a normal phenome- non. However, we have studied too few patients to date to show definitely that these long-latency components are uniquely associated with patholo- gies in the ear or the auditory nerve.

Acknowledgements

This study was supported by a grant from the National Institutes of Health (Grant No. 1 ROl-

86

NS213’78). The author is grateful to Peter Jan- netta, M.D., for making the patients under his care available for this study, and to Margareta B. Msller, M.D., Dr. Med Sci., for audiolo~cal and otological evaluation of the patients in this study.

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