Scandinavian Audiology
ISSN: 0105-0397 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/iaud20
The I’ Potential of the Brain-Stem Auditory-Evoked Potential Ernest J. Moore, Jacob J. M. Semela, Brad Rakerd, Randy C. Robb & Ayalur K. Ananthanarayan To cite this article: Ernest J. Moore, Jacob J. M. Semela, Brad Rakerd, Randy C. Robb & Ayalur K. Ananthanarayan (1992) The I’ Potential of the Brain-Stem Auditory-Evoked Potential, Scandinavian Audiology, 21:3, 153-156, DOI: 10.3109/01050399209045996 To link to this article: http://dx.doi.org/10.3109/01050399209045996
Published online: 12 Oct 2009.
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Article views: 2
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Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=iaud20 Download by: [McMaster University]
Date: 19 March 2016, At: 01:05
Scand Audiol 1992; 21: 153-156
THE I’ POTENTIAL OF THE BRAIN-STEM AUDITORY-EVOKED POTENTIAL Ernest J. Moore,’ Jacob J. M. Semela,’ Brad Rakerd,’ Randy C. Robb’ and Ayalur K. Ananthanardyan’
’
Downloaded by [McMaster University] at 01:05 19 March 2016
From the Neuro-Audiologic Laboratory, Department of Audiology & Speech Sciences, Michigan State University, East Lansing. Michigan, and the ‘Deparrment of Audiology & Speech Pathology, University of Tennessee, Knoxville, Tennessee, USA
ABSTRACT
MATERIALS A N D METHODS
The I’ potential of the brain-stem auditory-evoked potential. Moore, E.J., Semela, J.J.M., Rakerd, B., Robb, R.C. and Ananthanarayan, A.K. (The Neuro-Audiologic Laboratory, Department of Audiology & Speech Sciences, Michigan State University, East Lansing, Michigan, and the Department of Audiology & Speech Pathology, University of Tennessee, Knoxville, Tennessee, USA). Scand Audiol 1992; 21: 153~-156.
Clicks were monaurally presented through a shielded earphone (Madsen MSH 300) at a repetition rate of l l.l/s to the right ear of 38 normal-hearing subjects (19 males, 19 females; aged 18.4-29.3 years, mean age 21.6 years). Twenty (52%) of the subjects were testcd on more than one occasion, several were tested using four differcnt ‘evoked-potential’ systems, and one of the authors (AKA) replicated the experimental conditions on an additional five subjects in another laboratory. Stimuli were presented from 50 to 80 dB nHL in 10-dB steps, with levels randomized. A digitally generated rectangular pulse of 180-ps duration using modular components (Modular Instruments, 100/200 series) was used to generate an acoustic gaussian-shaped click of approximately 3.0-ms duration using condensation (CON), rarefaction (RAR) or alternating (ALT) polarity. A gold-cup electrode (Grass) filled with a conducting gel (Medi-trace) was applied at the vertex (Cz, active), a gold clip-on electrode was attached to the ipsilateral earlobe (Ai, reference), and a second gold-cup electrode was applied at the forehead (Fpz, patient ground). Inter-electrode impedance was kept at or below I .O kOhms, and was measured using a battery-operated impedance meter (Grass EZMSA). An amplifier (Grass P51 IK) was differentially AC coupled with a bandwidth of 0.1-3 kHz ( - 3 dB, I pole). The amplified (2 x lo5) EEG activity was averaged (2048 sweeps), digitized (12 bit, > 80 dB CMR) and quantized (10 ps/l 000 data points) using a microcomputer-based system. Amplitude and latency of waveforms were measured using the digital read-out of two interactive cursors.
We have consistently recorded a positive wave which precedes wave I , and is called 1’, within the human brain-stem auditory-evoked potential. It is postulated that I’ represents initial neural activity of the auditory nerve, which presumably has as its origin auditory nerve dendrites. Thus, 1’ may represent a summed far-field dendritic potential from currents ofexcitatory postsynaptic potentials. We report latency and amplitude values of 1’. Key words: BAEP, EPSP, EPSP-like, far-field, waves 1’, I, I1
INTRODUCTION The various positive peaks of the brain-stem auditoryevoked potential (BAEP) were first labeled sequentially as waves I-VII by Jewettct al. (1970). Since thcn, wave I of the BAEP has been correlated with N 1 of the auditory compound action potential (CAP) and has as its origin the distal part of the auditory nerve (AntoliCandela & Kiang, 1978; Hashimoto et al, I98 1; Maller & Jannetta, 1981; Fullerton et al., 1987). The excitatory postsynaptic potential (EPSP), which should then precede N I , has as its origin the afferent dendritic fibers of the auditory nerve, but has not been completely characterized for the auditory system and is not generally seen in BAEP recordings. Recently, however, several laboratories have reported finding a potential preceding wave I, variously referred to as I’ (Hughes & Fino, 1980, 1985; Hughes et al., 1981), I” (Benito et al., 1984), or BI (Before wave I ) (Moore & Semela, 1985). In this report we address qualitatively and quantitatively several aspects of the I’ potential.
RESULTS Figure 1 depicts the human BAEP with waves designated according to Jewett et al. (1 970) and Hughes & Fino (1 980). In Fig. 1 the I’ potential precedes wave I with an onset latency of approximately 0.830 ms for either rarefaction, condensation or alternating clicks. It should be noted that waveforms were obtained while varying intensity, as well as phase of the signal. We have observed I’ consistently, both across and within subjects for several years. Peak latency and peak-to-peak amplitude (most positive to most negative deflection) for waves 1’, I and Scand Audiol 21
Downloaded by [McMaster University] at 01:05 19 March 2016
154 E. J . Moore et al.
Fig. 1. Scalp-recorded brain-stem auditory-evoked potentials (BAEP) a t intensity levels o f 80, 70 and 60 d b nHL (re: normal thresholds to 180-ps clicks presented a t I I . I/s to 10 normalhearing adults, mean age 22.6 years, hearing within 10 dB o f 0 dB HL, re: ANSI 1969, 0.25-8 KHz), using RARefaction. CONdensation and ALTernating clicks, respectively.
I1 are depicted in Fig. 2 as a function of signal level. The I' latency decreased as intensity increased, and its function approximated that of waves I and 11. The time separation between wave peaks may suggest spatial and/or temporal conduction time along the cochlea/auditory nerve complex. Note the small variability of each of the waves with no overlap for & 1 SD. Given that we have not been successful in identifying I' below 40 dB nHL with consistency, it follows
that it is a supra-threshold response. We were successful in identifying I' at 70 dB in 100% of subjects, 63% at 60 dB and 42% at 50 dB nHL. We also quantified amplitude for 12 ears (those which showed a systematic increase in magnitude for each increase in intensity). I' increased monotonically as intensity increased, with a magnitude of approximately 45 nV at 50 dB nHL and 100 nV at 70 dB nHL. The amplitude functions of all waves were monotonic; I' amplitude
Fig. 2. ( A ) Input-output latencyintensity functions for waves 1', I and I1 for 38 ears, and ( B ) amplitude-intensity functions for I2 ears for alternating clicks at 50, 60 and 70 dB nHL. The mean and I SD are depicted, except for wave I a t 50 d B nHL which was too small to be seen using these ordinate values.
+
Scand Audio121
Pof’BAEP showed a gradual initial increase from 50 to 60 dB nHL and then exhibited a shallower increase in magnitude from 60 to 70 dB nHL.
Downloaded by [McMaster University] at 01:05 19 March 2016
DISCUSSION Our data are consistent with those of Hughes & Fino (1980) who first suggested that I’ may represent farfield summed electric currents from dendritic potentials of the auditory nerve in the form of EPSPs. It is of interest that EPSPs of auditory-system origin have been recorded from a number of laboratory animals, namely, turtle, bullfrog, skate, fish and guinea pig (Furukawa & Ishii, 1967; Flock & Russell, 1973; Crawford & Fettiplace, 1980; Palmer & Russell, 1986; Siege1 & Dallos, 1986). It is therefore reasonable to expect to be able to detect these small voltages from the human scalp using principles of digital electronic averaging. While Hughes et al. attribute their initial success in recording this potential to the use of a pizeoelectric earphone, we have obtained consistent results using a shielded dynamic earphone, which also eliminates initial stimulus artifact. We attribute our success also to the use of low-noise gold electrodes, impedance values of electrodes of d 1.5 kOhms, short electrode leads of 45.7 cm, low-noise (< 10 nV) preamplification, a dwell-time of 10 p s and total data points of 1 000, the high sampling rate of 100 kHz and averaging 2 048 responses. The role of EPSPs in auditory nerve functioning is not well understood. It has been suggested that the EPSP may be the generator potential that initiates the action potential, the former having as its origin afferent dendrites, while the latter has as its origin axons of the auditory nerve (Dallos & Cheatham, 1974; Dallos, 1984). It follows that the EPSP should then precede wave I of the BAEP, or N1 of the CAP, since peaks I and N1 have been shown to originate from axons of the auditory nerve (Antoli-Candela & Kiang, 1978; Hashimoto et al, 1981; Merller & Jannetta, 1981). Whether I’ is exclusively of neural origin is perhaps still debatable. For example, it could be argued that I’ is generated by the vestibular system (Wit et al., 1984) and not the cochlea. While we d o not have empirical data to refute such a claim, there is suggestive evidence against it, specifically since I’ was obtained with highfrequency stimulation with levels as low as 50 dB nHL. These low-intensity levels are not likely to evoke a vestibular response. It could also be argued that I’ is
155
the so-called ‘transient’ summating potential (SP) (Chatrian et al., 1982), since both SP and I’ occur within the same time domain. However. using various high-pass filter settings and an ear-canal electrode, Semela (1991) has found the wave which is described as SP in most electrocochleographic recordings (a shoulder on the rising phase of wave N I ) becomes dissociated from N 1 as the high-pass filter is decreased from 300 Hz (- 3 dB) to 10 Hz. Thus, I’ and SP may be the same phenomenon (Dallos & Cheatham, 1974). We have also recorded an 1’-like potential (called ‘NoPo’), followed by a ‘slow negative wave’ (‘EPSP-like’) in the cat (Klinke et al., 1988; Moore et al., 1988), and our findings have been confirmed recently (Dolan et al., 1989). We have also ruled out CM as the source of I’ based on a non-inverting polarity for both potentials to condensation or rarefaction clicks, and the failure of I’ to diminish while using clicks of alternating phase. The temporal character of a peak latency for I’ of 830 ps provokes the question of whether o r not it is of a sufficient delay to be generated postsynaptically, i.e. beyond the hair cell-dendritic junction. While we have not made direct measurements, the combined time delays calculated by Chatrian et al. (1985) seem applicable to our data and we argue that they are reasonable. Chatrian and his colleagues provide approximate time delays for the following: rise time of recording equipment, 150 p s ; time period between onset ofearphone electrical wave to acoustic wave, 100 ps; travel time of acoustic wave through the external auditory meatus to the tympanic membrane, 70 ps: from tympanic membrane to basilar membrane, 110 ps; basilar membrane to hair cell receptor, 110 ps; and we added an additional delay from the hair cell receptor to the postsynaptic region of 100-200 p s , for a total time period of approximately 640-740 p s , values well within the 830-ps time-of-occurrence for our wave I’ latency. CONCLUSIONS Until further empirical investigations are completed, it would be premature to suggest that I’ may be used routinely for clinical o r experimental purposes. If further experiments with animals and humans reveal that the origin of I’ is the summed far-field currents of auditory nerve dendrites, this may add additional value to the BAEP technique. That is, I’ may prove to be a rather sensitive neural index, presumably since I’ Scand Audio1 21
156
E. J. Moore et al.
m a y be more indicative o f cochlear integrity than waves I-V o f the BAEP due t o its presumably more peripheral origin. W i t h additional refinement o f techniques, p e r h a p s I’ m i g h t also be used to indicate t h e integrity o f neurotransmitter kinetics at the hair cella u d i t o r y nerve synapse (Klinke et al., 1988). Also, in cases o f diseased states o f the a u d i t o r y nerve, where wave I of the BAEP cannot be detected, b u t I’ is present, it m a y prove t o be more indicative o f the functional s t a t e of t h e distal a u d i t o r y nerve, e.g., with candidates f o r a cochlear i m p l a n t w h o s e disease state m a y be due to involvement o f the a x o n a l portion o f the a u d i t o r y nerve. On t h e o t h e r hand, however, one would expect a priori t h a t identification o f I’ m a y be even m o r e difficult in pathological ears and thus limit its clinical usefulness.
Downloaded by [McMaster University] at 01:05 19 March 2016
ACKNOWLEDGMENTS The authors are indebted to George Gamble for technical assistance. Drs S . Allen Counter and John R. Hughes provided editorial suggestions. Supported in part by an All University Research Initiation Grant, funds from the Office of the Provost, MSU; and NSF BNS 87-13304. Informed consent was obtained from all subjects after the nature and possible consequences of the investigation had been fully explained.
REFERENCES Antoli-Candela F, Kiang, N. Unit activity underlying the N1 potential. In: Naunton R, Fernandez C, eds. Evoked electrical activity in the auditory system. New York: Academic Press, 1978; 165-91. Benito M, Falco P, Lauro A. Brain-stem auditory evoked potentials: Description of the “0” wave of possible electrocochlear origin. Electroencephalogr Clin Neurophysiol 1984; 58: IOOP. Chatrian G , Wirch A, Edwards K , Turella G, Kaufman M, Snyder J. Cochlear summating potential to broadband clicks detected from the human external auditory meatus: A study of subjects with normal hearing for age. Ear Hear 1985; 6: 130-8. Chatrian G , Wirch A, Lettich E, Turella G, Synder J. Click evoked human electrocochleogram. Noninvasive recording method, origin and physiologic significance. Am J EEG Techno1 1982; 22: I 5 1-74. Crawford A, Fettiplace R . The frequency selectivity of auditory nerve fibers and hair cells in the cochlea of the turtle. J Physiol 1980; 306: 79-125. Dallos P. Peripheral mechanisms of hearing. In: DarianSmith I. ed. Handbook of physiology, Section I: The nervous system, Vol 111. Sensory processes, part 2. 1984: 595-637. Dallos P, Cheatham M. Generator potentials: Are they identifiable from gross cochlear recordings? J Acoust SOC Am 1974; 56: SIO. DoIan D , Xi L, Nuttall A. Characterization of a n EPSP-like potential recorded remotely from the round window. J Acoust SOCAm 1989; 86: 2167-71. Srand Audio1 21
Flock A, Russell I. The postsynaptic action of efferent fibers in the lateral line organ of the Burbot Lota Iota. J Physiol 1973; 235: 591-605. Fullerton B, Levine R, Wosford-Dunn H, Kiang N. Comparison of cat and human brain-stem auditory evoked potentials. Electroencephalogr Clin Neurophysiol 1987; 66: 547-70. Furukawa T , Ishii Y. Neurophysiological studies on hearing in goldfish. J Neurophysiol 1967; 30: 1377-1403. Hashimoto I, Ishiyama Y, Yoshimoto T , Nemoto S . Brainstem auditory evoked potentials recorded directly from human brain-stem and thalamus. Brain 1981; 104: 841-59. Hughes J, Fino J. Usefulness of piezoelectric earphones in recording the brain stem auditory evoked potentials: A new early deflection. Electroencephalogr Clin Neurophysiol 1980; 48: 357-60. Hughes J, Fino J. Review of generators of the brainstem auditory evoked potentials: Contribution of an experimental study. J Clin Neurophysiol 1985; 2: 355-81. Hughes J, Fino J, Gagnon L. The importance of phase of stimulus and the reference recording electrode in brain stem auditory evoked potentials. Electroencephalogr Clin Neurophysiol 1981; 51: 61 1-23. Jewett D, Romano M, Williston J. Human auditory evoked potentials: Possible brain-stem components detected on the scalp. Science 1970; 167: 1517--8. Klinke R, Caird D, Lowenheim H, Moore E. Beeinfluss ein intracochleares exzitatorisches postsynaptisches Potential die Hirnstammpotentiale? Arch Otorhinolaryngol 1988; (SUPPI.11): 159-60. M d e r A, Jannetta P. Compound action potentials recorded intracranially from the auditory nerve in man. Exp Neurol 1981; 74: 862-74. Moore E, Caird D, Klinke R , Lowenheim H. Effects of intracochlear tetrodotoxin and transmitter blockers on auditory evoked potentials in the cat. St Petersburg Beach, Florida, USA: Assoc Res Otolaryngol Abst, 1988. Moore E, Semela J. The “ B Y potential of the auditory brain stem response: A neural response or systematic artifact? Wiirzburg, Germany: XXII Workshop on Inner Ear Biology, 1985. Palmer A, Russell I. Phase-locking in the cochlear nerve of the guinea-pig and its relation to the receptor potential of inner hair cells. Hear Res 1986; 24: 1L15. Semela J. An investigation of Wave I’ of the brain stem auditory evoked potential. Michigan State University, East Lansing, MI: Unpublished Doctoral Dissertation, 1991. Siege1 J, Dallos P. Spike activity recorded from the organ of C o d . Hear Res 1986; 22: 245- 8. Wit H, Bleeker J, Mulder H. Responses of pigeon vestibular nerve fibers to sound and vibration with audio frequencies. J Acoust Soc Am 1984; 75: 202-8. Receioed M a y 2IAccepled September 26, 1991
Address for offprints: Ernest J. Moore, Ph.D. Department of Audiology & Speech Sciences Michigan State University 378 Comm. Arts & Sciences Bldg. East Lansing MI 48824-1212 USA
ISSN: 0105-0397 (Print) (Online) Journal homepage: http://www.tandfonline.com/loi/iaud20
The I’ Potential of the Brain-Stem Auditory-Evoked Potential Ernest J. Moore, Jacob J. M. Semela, Brad Rakerd, Randy C. Robb & Ayalur K. Ananthanarayan To cite this article: Ernest J. Moore, Jacob J. M. Semela, Brad Rakerd, Randy C. Robb & Ayalur K. Ananthanarayan (1992) The I’ Potential of the Brain-Stem Auditory-Evoked Potential, Scandinavian Audiology, 21:3, 153-156, DOI: 10.3109/01050399209045996 To link to this article: http://dx.doi.org/10.3109/01050399209045996
Published online: 12 Oct 2009.
Submit your article to this journal
Article views: 2
View related articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=iaud20 Download by: [McMaster University]
Date: 19 March 2016, At: 01:05
Scand Audiol 1992; 21: 153-156
THE I’ POTENTIAL OF THE BRAIN-STEM AUDITORY-EVOKED POTENTIAL Ernest J. Moore,’ Jacob J. M. Semela,’ Brad Rakerd,’ Randy C. Robb’ and Ayalur K. Ananthanardyan’
’
Downloaded by [McMaster University] at 01:05 19 March 2016
From the Neuro-Audiologic Laboratory, Department of Audiology & Speech Sciences, Michigan State University, East Lansing. Michigan, and the ‘Deparrment of Audiology & Speech Pathology, University of Tennessee, Knoxville, Tennessee, USA
ABSTRACT
MATERIALS A N D METHODS
The I’ potential of the brain-stem auditory-evoked potential. Moore, E.J., Semela, J.J.M., Rakerd, B., Robb, R.C. and Ananthanarayan, A.K. (The Neuro-Audiologic Laboratory, Department of Audiology & Speech Sciences, Michigan State University, East Lansing, Michigan, and the Department of Audiology & Speech Pathology, University of Tennessee, Knoxville, Tennessee, USA). Scand Audiol 1992; 21: 153~-156.
Clicks were monaurally presented through a shielded earphone (Madsen MSH 300) at a repetition rate of l l.l/s to the right ear of 38 normal-hearing subjects (19 males, 19 females; aged 18.4-29.3 years, mean age 21.6 years). Twenty (52%) of the subjects were testcd on more than one occasion, several were tested using four differcnt ‘evoked-potential’ systems, and one of the authors (AKA) replicated the experimental conditions on an additional five subjects in another laboratory. Stimuli were presented from 50 to 80 dB nHL in 10-dB steps, with levels randomized. A digitally generated rectangular pulse of 180-ps duration using modular components (Modular Instruments, 100/200 series) was used to generate an acoustic gaussian-shaped click of approximately 3.0-ms duration using condensation (CON), rarefaction (RAR) or alternating (ALT) polarity. A gold-cup electrode (Grass) filled with a conducting gel (Medi-trace) was applied at the vertex (Cz, active), a gold clip-on electrode was attached to the ipsilateral earlobe (Ai, reference), and a second gold-cup electrode was applied at the forehead (Fpz, patient ground). Inter-electrode impedance was kept at or below I .O kOhms, and was measured using a battery-operated impedance meter (Grass EZMSA). An amplifier (Grass P51 IK) was differentially AC coupled with a bandwidth of 0.1-3 kHz ( - 3 dB, I pole). The amplified (2 x lo5) EEG activity was averaged (2048 sweeps), digitized (12 bit, > 80 dB CMR) and quantized (10 ps/l 000 data points) using a microcomputer-based system. Amplitude and latency of waveforms were measured using the digital read-out of two interactive cursors.
We have consistently recorded a positive wave which precedes wave I , and is called 1’, within the human brain-stem auditory-evoked potential. It is postulated that I’ represents initial neural activity of the auditory nerve, which presumably has as its origin auditory nerve dendrites. Thus, 1’ may represent a summed far-field dendritic potential from currents ofexcitatory postsynaptic potentials. We report latency and amplitude values of 1’. Key words: BAEP, EPSP, EPSP-like, far-field, waves 1’, I, I1
INTRODUCTION The various positive peaks of the brain-stem auditoryevoked potential (BAEP) were first labeled sequentially as waves I-VII by Jewettct al. (1970). Since thcn, wave I of the BAEP has been correlated with N 1 of the auditory compound action potential (CAP) and has as its origin the distal part of the auditory nerve (AntoliCandela & Kiang, 1978; Hashimoto et al, I98 1; Maller & Jannetta, 1981; Fullerton et al., 1987). The excitatory postsynaptic potential (EPSP), which should then precede N I , has as its origin the afferent dendritic fibers of the auditory nerve, but has not been completely characterized for the auditory system and is not generally seen in BAEP recordings. Recently, however, several laboratories have reported finding a potential preceding wave I, variously referred to as I’ (Hughes & Fino, 1980, 1985; Hughes et al., 1981), I” (Benito et al., 1984), or BI (Before wave I ) (Moore & Semela, 1985). In this report we address qualitatively and quantitatively several aspects of the I’ potential.
RESULTS Figure 1 depicts the human BAEP with waves designated according to Jewett et al. (1 970) and Hughes & Fino (1 980). In Fig. 1 the I’ potential precedes wave I with an onset latency of approximately 0.830 ms for either rarefaction, condensation or alternating clicks. It should be noted that waveforms were obtained while varying intensity, as well as phase of the signal. We have observed I’ consistently, both across and within subjects for several years. Peak latency and peak-to-peak amplitude (most positive to most negative deflection) for waves 1’, I and Scand Audiol 21
Downloaded by [McMaster University] at 01:05 19 March 2016
154 E. J . Moore et al.
Fig. 1. Scalp-recorded brain-stem auditory-evoked potentials (BAEP) a t intensity levels o f 80, 70 and 60 d b nHL (re: normal thresholds to 180-ps clicks presented a t I I . I/s to 10 normalhearing adults, mean age 22.6 years, hearing within 10 dB o f 0 dB HL, re: ANSI 1969, 0.25-8 KHz), using RARefaction. CONdensation and ALTernating clicks, respectively.
I1 are depicted in Fig. 2 as a function of signal level. The I' latency decreased as intensity increased, and its function approximated that of waves I and 11. The time separation between wave peaks may suggest spatial and/or temporal conduction time along the cochlea/auditory nerve complex. Note the small variability of each of the waves with no overlap for & 1 SD. Given that we have not been successful in identifying I' below 40 dB nHL with consistency, it follows
that it is a supra-threshold response. We were successful in identifying I' at 70 dB in 100% of subjects, 63% at 60 dB and 42% at 50 dB nHL. We also quantified amplitude for 12 ears (those which showed a systematic increase in magnitude for each increase in intensity). I' increased monotonically as intensity increased, with a magnitude of approximately 45 nV at 50 dB nHL and 100 nV at 70 dB nHL. The amplitude functions of all waves were monotonic; I' amplitude
Fig. 2. ( A ) Input-output latencyintensity functions for waves 1', I and I1 for 38 ears, and ( B ) amplitude-intensity functions for I2 ears for alternating clicks at 50, 60 and 70 dB nHL. The mean and I SD are depicted, except for wave I a t 50 d B nHL which was too small to be seen using these ordinate values.
+
Scand Audio121
Pof’BAEP showed a gradual initial increase from 50 to 60 dB nHL and then exhibited a shallower increase in magnitude from 60 to 70 dB nHL.
Downloaded by [McMaster University] at 01:05 19 March 2016
DISCUSSION Our data are consistent with those of Hughes & Fino (1980) who first suggested that I’ may represent farfield summed electric currents from dendritic potentials of the auditory nerve in the form of EPSPs. It is of interest that EPSPs of auditory-system origin have been recorded from a number of laboratory animals, namely, turtle, bullfrog, skate, fish and guinea pig (Furukawa & Ishii, 1967; Flock & Russell, 1973; Crawford & Fettiplace, 1980; Palmer & Russell, 1986; Siege1 & Dallos, 1986). It is therefore reasonable to expect to be able to detect these small voltages from the human scalp using principles of digital electronic averaging. While Hughes et al. attribute their initial success in recording this potential to the use of a pizeoelectric earphone, we have obtained consistent results using a shielded dynamic earphone, which also eliminates initial stimulus artifact. We attribute our success also to the use of low-noise gold electrodes, impedance values of electrodes of d 1.5 kOhms, short electrode leads of 45.7 cm, low-noise (< 10 nV) preamplification, a dwell-time of 10 p s and total data points of 1 000, the high sampling rate of 100 kHz and averaging 2 048 responses. The role of EPSPs in auditory nerve functioning is not well understood. It has been suggested that the EPSP may be the generator potential that initiates the action potential, the former having as its origin afferent dendrites, while the latter has as its origin axons of the auditory nerve (Dallos & Cheatham, 1974; Dallos, 1984). It follows that the EPSP should then precede wave I of the BAEP, or N1 of the CAP, since peaks I and N1 have been shown to originate from axons of the auditory nerve (Antoli-Candela & Kiang, 1978; Hashimoto et al, 1981; Merller & Jannetta, 1981). Whether I’ is exclusively of neural origin is perhaps still debatable. For example, it could be argued that I’ is generated by the vestibular system (Wit et al., 1984) and not the cochlea. While we d o not have empirical data to refute such a claim, there is suggestive evidence against it, specifically since I’ was obtained with highfrequency stimulation with levels as low as 50 dB nHL. These low-intensity levels are not likely to evoke a vestibular response. It could also be argued that I’ is
155
the so-called ‘transient’ summating potential (SP) (Chatrian et al., 1982), since both SP and I’ occur within the same time domain. However. using various high-pass filter settings and an ear-canal electrode, Semela (1991) has found the wave which is described as SP in most electrocochleographic recordings (a shoulder on the rising phase of wave N I ) becomes dissociated from N 1 as the high-pass filter is decreased from 300 Hz (- 3 dB) to 10 Hz. Thus, I’ and SP may be the same phenomenon (Dallos & Cheatham, 1974). We have also recorded an 1’-like potential (called ‘NoPo’), followed by a ‘slow negative wave’ (‘EPSP-like’) in the cat (Klinke et al., 1988; Moore et al., 1988), and our findings have been confirmed recently (Dolan et al., 1989). We have also ruled out CM as the source of I’ based on a non-inverting polarity for both potentials to condensation or rarefaction clicks, and the failure of I’ to diminish while using clicks of alternating phase. The temporal character of a peak latency for I’ of 830 ps provokes the question of whether o r not it is of a sufficient delay to be generated postsynaptically, i.e. beyond the hair cell-dendritic junction. While we have not made direct measurements, the combined time delays calculated by Chatrian et al. (1985) seem applicable to our data and we argue that they are reasonable. Chatrian and his colleagues provide approximate time delays for the following: rise time of recording equipment, 150 p s ; time period between onset ofearphone electrical wave to acoustic wave, 100 ps; travel time of acoustic wave through the external auditory meatus to the tympanic membrane, 70 ps: from tympanic membrane to basilar membrane, 110 ps; basilar membrane to hair cell receptor, 110 ps; and we added an additional delay from the hair cell receptor to the postsynaptic region of 100-200 p s , for a total time period of approximately 640-740 p s , values well within the 830-ps time-of-occurrence for our wave I’ latency. CONCLUSIONS Until further empirical investigations are completed, it would be premature to suggest that I’ may be used routinely for clinical o r experimental purposes. If further experiments with animals and humans reveal that the origin of I’ is the summed far-field currents of auditory nerve dendrites, this may add additional value to the BAEP technique. That is, I’ may prove to be a rather sensitive neural index, presumably since I’ Scand Audio1 21
156
E. J. Moore et al.
m a y be more indicative o f cochlear integrity than waves I-V o f the BAEP due t o its presumably more peripheral origin. W i t h additional refinement o f techniques, p e r h a p s I’ m i g h t also be used to indicate t h e integrity o f neurotransmitter kinetics at the hair cella u d i t o r y nerve synapse (Klinke et al., 1988). Also, in cases o f diseased states o f the a u d i t o r y nerve, where wave I of the BAEP cannot be detected, b u t I’ is present, it m a y prove t o be more indicative o f the functional s t a t e of t h e distal a u d i t o r y nerve, e.g., with candidates f o r a cochlear i m p l a n t w h o s e disease state m a y be due to involvement o f the a x o n a l portion o f the a u d i t o r y nerve. On t h e o t h e r hand, however, one would expect a priori t h a t identification o f I’ m a y be even m o r e difficult in pathological ears and thus limit its clinical usefulness.
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ACKNOWLEDGMENTS The authors are indebted to George Gamble for technical assistance. Drs S . Allen Counter and John R. Hughes provided editorial suggestions. Supported in part by an All University Research Initiation Grant, funds from the Office of the Provost, MSU; and NSF BNS 87-13304. Informed consent was obtained from all subjects after the nature and possible consequences of the investigation had been fully explained.
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Address for offprints: Ernest J. Moore, Ph.D. Department of Audiology & Speech Sciences Michigan State University 378 Comm. Arts & Sciences Bldg. East Lansing MI 48824-1212 USA
