Acta Oto-Laryngologica
ISSN: 0001-6489 (Print) 1651-2251 (Online) Journal homepage: http://www.tandfonline.com/loi/ioto20
Cortical Evoked Potentials Recorded from the Guinea Pig Without Averaging R. A. Walloch To cite this article: R. A. Walloch (1975) Cortical Evoked Potentials Recorded from the Guinea Pig Without Averaging, Acta Oto-Laryngologica, 80:1-6, 7-12, DOI: 10.3109/00016487509121294 To link to this article: http://dx.doi.org/10.3109/00016487509121294
Published online: 08 Jul 2009.
Submit your article to this journal
View related articles
Citing articles: 2 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ioto20 Download by: [McMaster University]
Date: 24 April 2016, At: 23:53
Acta Otolaryngol 80: 7-12, 1975
CORTICAL EVOKED POTENTIALS RECORDED FROM THE GUINEA PIG WITHOUT AVERAGING R. A. Walloch From the Kresge Hearing Research Laboratory, Department of Otolaryngology, University of Oregon Medical School, Portland, Ore., USA
Downloaded by [McMaster University] at 23:53 24 April 2016
(Received July 7, 1974)
Abstract. Potentials evoked by tonal pulses and recorded with a monopolar electrode on the pial surface over the auditory cortex of the guinea pig are presented. These potentials are compared with averaged potentials recorded in previous studies with an electrode on the dura. The potentials recorded by these two techniques have similar waveforms, peak latencies and thresholds. They appear to be generated within the same region of the cerebral cortex. As can be expected, the amplitude of the evoked potentials recorded from the pial surface is larger than that recorded from the dura. Consequently, averaging is not needed to extract the evoked potential once the dura is removed. The thresholds for the evoked cortical potential are similar to behavioral thresholds for the guinea pig at high frequencies; however, evoked potential thresholds are elevated over behavioral thresholds at low frequencies. The removal of the dura and the direct recording of the evoked potential appears most appropriate for acute experiments. The recording of an evoked potential with dura electrodes employing averaging procedures appears most appropriate for chronic studies.
Several types of bioelectric potentials have been recorded from the human scalp (Davis, 1965; Cody & Bickford, 1965; Cody & Klass, 1968; Rapin & Bergman, 1969). These potentials have been especially useful with infants, the mentally retarded and malingerers. Similar potentials have been recorded from experimental animals in an attempt to quickly assess their auditory acuity. In order to test the auditory ability of animals, thresholds for the evoked cortical potential have The data presented in this article were collected in partial fulfillment of the requirements for the doctoral degree in the Department of Medical Psychology, University of Oregon Medical School. The work was in part sponsored by Program Project no. NS 09889.
been obtained for the cat (Hind & Schuknecht, 1954; Kimura et al., 1956; Hattori et al., 1971) as well as for the guinea pig. In the guinea pig, Djalilian & Cody (1973) and Hattori (1970) have us2d averaging techniques to obtain evoked potentials from dura electrodes. In one study (Kern et al., 1969) the vertex response was recorded from the guinea pig. In a number of recent studies (Walloch et al., 1971a, b, 1973a, b, 1974a, b; Cowden & Wdlnch, 1973) I have removed the dura and placed the recording electrode directly on the pial surface. In this manner, the evoked cortical potential could be recorded without averaging. The present paper compares and contrasts the records obtained from electrodes on the pia without averaging with those obtained from electrodes on the dura with averaging. The data contained in the present paper have not been reported except as a brief communication (Walloch, 1971a).
METHOD Eleven guinea pigs of the Topeka stock weighing 400-750 g were used in this study. Each guinea pig had an active Preyer reflex. The guinea pig was deeply anesthetized with diallylbarbituric acid (60 mg/kg) and urethane (240 mg/kg). In other studies (Walloch et al., 1974a, b) sodium pentobarbital (45 mb/kg) was used with equal success. Rarely, a supplemental dose was needed to maintain anesthesia. A tracheotomy was perActa Otolaryngol80
Downloaded by [McMaster University] at 23:53 24 April 2016
8 R. A. Walloch
Fig. 1. Record of the stimulus and evoked responses. Negative is up. (a) Averaged output of the calibrated microphone to a standard tonal pulse, showing that the sound pulse was free of significant on-set transients. (b) Superimposed cortical evoked responses to repeated clicks. (c) Cortical responses evoked by 25 kHz tonal pulses. Top tracing shows lack of evoked activity following repeated stimulus presentations at 21 dB SPL. The
middle tracing show a clear evoked response following repeated stimulation at 31 dB. The bottom line indicates the duration of the tonal pulse. (d) Cortical response t o 1 kHz tonal pulses. The top line shows absence of cortical activity following repeated stimulation at! 25 dB. The middle and bottom tracings show a well developed response at 35 and 45 dB respectively.
formed and the animal was mechanically ventilated. In the present experiment the guinea pig's rectal temperature was continuously monitored (Yellow Springs Instrument 46 TUC) and maintained at 38°C. When the body temperature falls below 35"C, the evoked cortical potential disappears. The evoked potential returns upon warming the animal. Above 40"C, the evoked potential deteriorates and in this case the deterioration is not reversible. Both pinnae were amputated to facilitate placement of the animal in a stereotaxic unit (David Kopf). The scalp and skull flap were removed to expose the entire left hemisphere. To remove the dura, an old tungsten micro-
electrode was used, though any sharp thin probe would suffice. The dura was impaled medially from the auditory area and lifted away from the cortex. An incision was made beneath the microelectrode with Vannas scissors. This incision was expanded anterolaterally. The dura was reflected anteriad. Some bleeding occurred on occasion while the dura was being removed. When this occurred, it was best to proceed quickly with the reflection of the dura. With the dura reflected, the bleeding could then be controlled with the use of gelfoam. Most often the bleeding came from the soft tissue surrounding the cortical exposure. Only rarely did the bleeding stem from the cortex itself. After the dura was reflected, warm mineral oil
Actn Otolnryngol80
Downloaded by [McMaster University] at 23:53 24 April 2016
Cortical evoked potentials without averaging 9
was allowed to flow across the exposed cortex. However, the mineral oil was not found to be needed in subs2quent studies. A silver ball electrode (0.18 mm) was moved across the temporal area of the guinea pig in 1 mm steps. The impedance of the electrode was about 2 kilohms. At each cortical position studied, the threshold for an auditory evoked potential was obtained at 18 frequencies ranging from 0.1 to 40 kHz. In order to obtain these evoked potentials, sound was delivered through the hollow ear bar contralateral to the exposed cortex. A General Radio wave analyzer (1900A), Monsanto digital counter (103A), Simpson voltmeter, GrasonStadler electronic switch (892E\, Grason-Stadler interval timer (471-I), General Radio decade attenuator (1450), McIntosh power amplifier (M-210-B) and a specially designed power attenuator shaped the electrical signal which drove the Western Electric 555 speaker. The speaker was connected to the hollow ear bar as a closed sound system. The rate of stimulation was once every 2 seconds. Each tonal pulse was 50 msec in duration and had a rise time of 10 msec. At the end of each day’s work a 1/4 inch Bruel & Kjaer calibrated microphone was substituted for the animal and the sound at the end of the hollow ear bar was thus measured (Fig. l a ) . The evoked cortical potentials were initially amplified using a Keithley differential amplifier (103). The filters were set at 100 and 3 000 Hz. The calibration of this amplifier has been previously described (Meikle & Copeland, 1973). After the initial amplification, the potentials were displayed on a Tektronix storage oscilloscope (564B). RESULTS Acoustically evoked potentials were recorded from a restricted area within the temporal region of the cerebral cortex. Other investigators have identified this region as the auditory cortex (Odkvist et al., 1973; Ziegler, 1964; Kayser & Legouix, 1963). The first wave was always surface positive and it had a peak latency of 10-15 msec. A surface negative wave with a peak
t
I
I
I
I
0.5
I , , ,
I
I
I
5
I
I
,,I
10
I
L
kHz
Frequency
Fig. 2. Threshold curves for the evoked responses recorded from four cortical positions in a single guinea pig. All four positions were in the temporal region and within a few millimeters of each other. See text for a fuller discussion of these data.
latency of 20-40 msec generally followed the initial positive wave. A second surface positive wave was observed rarely and only at very high sound intensities. No later waves were observed. In Fig. 1b, an evoked potential to a click is shown. Clicks were used in a few pilot animals to determine the location of the auditory cortex. The waveform of the click evoked potential is highly similar to that evoked by tonal pulses. No attempt was made to obtain the threshold of these click evoked potentials; however, they were very useful in determining the latencies of the evoked potentials. In Fig. 1c, the cortical responses to a 25 kHz tone are shown. The top tracing shows a lack of an evoked response to repeated stimulation at 21 dB re: 0.0002 dynes/cm2.The middle tracing shows a 650 ,uV (p-p) respons:: to repeated stimulation at 31 dB. In Fig. Id, the cortical responses to a 1 kHz tone are shown. In the top tracing, no evoked activity is apparent following repeated stimulation at 23 dB. In the middle trace, a 1 mV potential was evoked by repeated stimulation at 33 dB. These thresholds were stable over 5 and 6 hour recording sessions. In Fig. 2, the thresholds for the evoked potential recorded from four positions on the auditory cortex of a single guinea pig are shown. All Acta OtoIaryngol80
10 R.A . Walloch loot
I
0.5
5
I
10
k Ht
Downloaded by [McMaster University] at 23:53 24 April 2016
Frequency
Fig. 3. Thresholds for the evoked response from 11 guinea pigs. The entire left hemisphere of each guinea pig was explored for evoked cortical activity. At each frequency, the threshold represents the lowest intensity tonal pulse that evoked cortical activity regardless of the position of the recording electrode.
four positions were within 4 mm of each other. Two of the positions (a and b) were sensitive to high frequency tones, and tonal puls-s below 1.5 kHz were totally ineffective in evoking a potential. One position (c) was sensitive to low frequency stimuli. One position ( d ) was sensitive to
both low and high frequency stimuli. Consequently, in order to obtain the lowest threshold for the evoked cortical potential at all frequencies without averaging, a number of cortical positions must be sampled. In Fig. 3, the thresholds for the auditory evoked potential obtained from 11 guinea pigs are shown. The entire left hemisphere was sampled in 1 mm steps. At each frequency, the lowest sound intensity that could evoke a cortical potential regardless of the recording position is shown. Consequently, each threshold curve represents potentials recorded from a number of cortical positions. Fig. 4 compares the mean of the threshold curves shown in Fig. 3 with other common measures of the auditory acuity in guinea pigs. The thresholds obtained by computing averaged evoked potentials recorded from the dura of guinea pigs (Djalilian & Cody, 1973; Hattori & Shoyama, 1970) are very similar to those obtained in the present study. These evoked potential thresholds were similar to the behavioral threshold of guinea pigs (Miller & Murray, 1966) at high frequencies, but these evoked potential thresholds were elevated over behavioral thresholds at the low frequencies. COMMENT
.--,, I
0.5
I
5 Frequency
, < * , I
10
1
kHz
Fig. 4. Comparison of the mean of the threshold curves plotted in Fig. 3 with other measures of auditory acuity in the guinea pig. (a) Mean of the threshold curves obtained in the present study. (b) Thresholds for the averaged evoked potentials reported by Hattori (1970) from the dura overlying the auditory cortex of the guinea pig. (c) Thresholds for the averaged evoked potentials recorded by Djalilian (1973) from the dura overlying the auditory cortex of the guinea pig. ( d ) Behavioral threshold obtained by Miller (1966) in the guinea pig. (e) Vertex potential threshold obtained by Kern (1969) from dura electrodes in the guinea pig.
Acta Otolxyngol80
The thresholds reported in the present study are very similar to those reported by Hattori & Shoyama (1970) and to thosc reported by Djalilian & Cody (1973) (Fig. 4). Hattori & Djalilian used an averaging computer to record evoke potentials from electrodes on the dura over the auditory cortex. The potentials recorded in all three studies had, 1) similar waveforms, 2) latencies, 3) thresholds, and 4) spacial distributions on the cerebral hemisphere. Consequently the potentials recorded in the three studies appear to have the same origin. For certain acute expzrimental problems, the removal of the dura and the direct recording of the evoked cortical potential appears most appropriate. The large amplitude of the evoked potential (Fig. l c , 6) at threshold makes aver-
Downloaded by [McMaster University] at 23:53 24 April 2016
Cortical evoked potentials without averaging 11 aging unnecessary. Consequently, thresholds can be obtained much more rapidly with direct recording techniques than with averaging procedures. While 50 stimulus presentations were used to obtain an averaged evoked potential in Djalilian’s (1973) and Hattori’s (1970) studies, only 2 stimulus presentations were needed in the present study. On the other hand, for certain chronic experimental problems, the recording of averaged evoked potentials from an electrode on the dura appears most appropriate. The opening in the skull can be easily closed and the guinea pig may be allowed to recover from the anesthesia. The choice of the position on the cerebral cortex from which to record the evoked potentials greatly influences the thresholds obtained. The position used by either Hattori (1970) or Djalilian (1973) appear appropriate for chronic studies. For acute studies in which the dura has been removed, the cortex can be quickly searched for an appropriate position. At each cortical position, the threshold for a potential evoked by 1 kHz and 10 kHz tonal pulses can be quickly ascertained. If the 1 kHz threshold is below 45 dB and the 10 kHz threshold is below 35 dB, the cortical position is suitable for the experiment. From such a position a complete threshold curve would appear similar to Fig. 2d. The behavioral threshold for pure tone stimuli have been obtained several times for the guinea pig (Horton, 1933; Anderson & Wedenberg, 1965; Miller & Murray, 1966; Heffner et al., 1971). The thresholds depicted in Fig. 4 d are representative of other behavioral determinations. The thresholds for the evoked potentials obtained in the present study are similar to the behavioral thresholds at the high frequencies. The evoked potential thresholds are elevated over the behavioral thresholds at the low frequencies. The majority of the positions on the auditory cortex are most sensitive to high frequency stimuli. While 63 % of the positions had best frequencies over 5 kHz, only 37 % had best frequencies below 3 kHz (Walloch, 1971c). Perhaps this can explain the relative insensitivity of the evoked potential to low frequency stimuli.
On the other hand, differencesin 1) sound measurement procedures, 2) the rise time, and 3) duration of the tones cannot be discounted as contributing to the differences in the evoked potential and the behavioral thresholds. The vertex response of the guinea pig (Kern et al., 1969) is highly elevated over the other measures of auditory acuity shown in Fig. 4. This contrasts with the human vertex potential which is similar to the thresholds obtained by conventional pure tone audiometry (Cody & Bickford, 1965; Cody & Klass, 1968; Rapin & Bergman, 1969). The reason for this difference between the guinea pig and human vertex potential is not apparent. The evoked potential from the auditory cortex has been used to study the effects upon the auditory system of intense sound stimulation (Hattori, 1970, 1971) ethacrynic acid (Walloch, 1974a) and mechanical lesions of the cochlea (Kimura et al., 1956; Hind & Schuknecht, 1954). The evoked potential has also been used to investigate various placements of electrodes for the electrical stimulation of the ear (Walloch et al., 1973a, 19743). In the future, we can expect that the evoked potential will be used to study an ever-increasing scope of problems.
ZUSAMMENFASSUNG Aktionspotentiale der Horrinde wurden bei Meerschweinchen durch Tonimpulse hervorgerufen und mittels monopolarer Elektrode an der pia mater abgeleitet. Sie wurden mit Durchschnittsaktionspotentialen verglichen, die in friiheren Untersuchungen von der dura mater abgeleitet worden waren. Diese mit verschiedener Methodik erhaltenen Potentiale ahneln einander hinsichtlich Verlaufsform, maximaler Latenzzeit und Schwellenwert. Sie scheinen in derselben Gegend der Hirnrinde zu entstehen. Wie zu erwarten, ist die Amplitude der von der pia mater abgeleiteten Aktionspotentiale grosser als die von der dura mater. Daher ist es nicht notwendig, Aktionspotentiale zu ermitteln, wenn einmal die dura mater beseitigt ist. Hervorgerufene Rindenpotentiale haben beim Meerschweinchen bei hoher Reizfrequenz ahnliche Schwellenwerte wie Verhaltungsreaktionen. Jedoch liegen bei niedrigen Frequenzen die Schwellenwerte von hervorgerufenen Potentialen hoher als die Schwellenwerte von Verhaltungsreaktionen. Beseitigung der dura mater und direktes Registrieren des Aktionspotentials scheinen am besten fur akute Experimente geeignet zu sein, wahrend Acta OtolaryngolSO
12 R. A . Walloch Registrieren von Durchschnittsaktionspotentialen mit dura-Elektroden sich am besten fur chronische Experimente zu eignen scheint.
Downloaded by [McMaster University] at 23:53 24 April 2016
REFERENCES Anderson, H. & Wedenberg, E. 1965. A new method for hearing tests in the guinea pig. Acta Otolaryngol (Stockh) 60, 375. Cody, D. T. R. & Bickford, R. G. 1965. Cortical audiometry: An objective method of evaluating auditory acuity in man. Mayo Clin Proc 40, 273. Cody, D. T. R. & Klass, D. W. 1968. Cortical audiometry: Potential pitfalls in testing. Arch Otoluryngol88, 396. Cowden, D. & Walloch, R. 1973. Placement of electrodes for activation of VIII nerve. J Acoust Soc A m 53, 326. Davis, H. 1965. Slow cortical responses evoked by acoustic stimuli. Acta Otolaryngol (Stockh) 59, 179. Djalilian, M. & Cody, D.T.R. 1973. Averaged cortical responses evoked by pure tones in the chinchilla and the guinea pig. Arch Otolaryngol98, 196. Hattori, H., Shida, E. & Umeda, H. 1971. Computer audiometry in cats. Arch Otolaryngol93, 147. Hattori, H. & Shoyama, T. 1970. Cortical audiometry in guinea pigs. J Auditory Res 10, 269. Heffner, R., Heffner, H. & Masterton, €3.1971. Behavioral measurements of absolute and frequency-difference thresholds in guinea pig. J Acoust Soc A m 49, 1888. Hind, J . E. & Schuknecht, H. F. 1954. A cortical test of auditory function in experimentally deafened cats. J Acoust Soc Am 26, 89. Horton, G. P. 1933. A quantitative study of hearing in the guinea pig (Cauia cobayu). J Comp Physiol Psychol 15, 59. Kayser, D. & Legouix, J. P. 1963. Projections tonotopiques sur le cortex auditif du Cobaye. C R Soc Biol (Paris) 157, 2161. Kern, E. B., Cody, D. T. R. &Bickford, R. G. 1969.Vertexresponse thresholds to pure tones in guinea pigs. Arch Otolaryngol90, 315. Kimura, R. S., Schuknecht, H. F. & Sutton, S. 1956. Effects of cochlear lesions on the threshold responses
Actn Otolaryngol80
of the auditory cortex in chronic experiments. J Comp Physiol Psychol49, 96. Meikle, M. B. & Copeland, A. B. 1973. Calibration of differential amplifiers used in biological recording. Arch Otolaryngol98, 66. Miller, J. D. & Murray, F. S. 1966. Guinea pig’s immobility response to sound: Threshold and habituation, J Comp Physiol Psychol 61, 227. Odkvist, L. M., Rubin, A. M., Schwarz, D. W. F. & Fredrickson, J. M. 1973. Vestibular and auditory cortical projection in the guinea pig (Cauiu porcellus). Exp Brain Res 18, 279. Rapin, I. & Bergman, M. 1969. Auditory evoked responses in uncertain diagnosis. Arch Otolaryngol 90, 307. Walloch, R. A. 1 9 7 1 ~ .Tonotopic organization of the guinea pig’s auditory cortex. J Acoust Soc Am 50, 119. Walloch, R. A. & Brummett, R. E. 1971b. Intensity function of the cochlea and cortex. J Acoust Soc A m 49, 122. Walloch, R., DeWeese, D., Brummett, R. & Vernon, J. 1973a. Electrical stimulation of the inner ear. Ann Otol Rhino1 Luryngol82, 1. Walloch, R. A. 19736. Intensity function for cortical potential evoked by electrical stimulation of the VIII nerve. J Acoust Soc A m 53, 362. Walloch, R. A., Anthony, J. & Gibbons, N. 1974. Depression of the evoked potential by ethacrynic acid. Laryngoscope 84, 256. Walloch, R. A. & Cowden, D. A., 1974b. Placement of electrodes for excitation of the VIII nerve. Arch Otolaryngol 100, 19. Walloch, R. A. 1971c. A study of the auditory cortex in the guinea pig with implications for the development of a n electrical prosthesis for the hard of hearing. Dissertation, Univ, of Ore. Med. School Library. Ziegler, H. P. 1964. Cortical sensory and motor areas of the guinea pig (Cauiu porcellus). Arch Ital Biol 102, 587.
R. A. Walloch, Ph.D. Dept. of Otolaryngology Uniuersity of Oregon Medical School Portland, Ore. 97201, USA
ISSN: 0001-6489 (Print) 1651-2251 (Online) Journal homepage: http://www.tandfonline.com/loi/ioto20
Cortical Evoked Potentials Recorded from the Guinea Pig Without Averaging R. A. Walloch To cite this article: R. A. Walloch (1975) Cortical Evoked Potentials Recorded from the Guinea Pig Without Averaging, Acta Oto-Laryngologica, 80:1-6, 7-12, DOI: 10.3109/00016487509121294 To link to this article: http://dx.doi.org/10.3109/00016487509121294
Published online: 08 Jul 2009.
Submit your article to this journal
View related articles
Citing articles: 2 View citing articles
Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=ioto20 Download by: [McMaster University]
Date: 24 April 2016, At: 23:53
Acta Otolaryngol 80: 7-12, 1975
CORTICAL EVOKED POTENTIALS RECORDED FROM THE GUINEA PIG WITHOUT AVERAGING R. A. Walloch From the Kresge Hearing Research Laboratory, Department of Otolaryngology, University of Oregon Medical School, Portland, Ore., USA
Downloaded by [McMaster University] at 23:53 24 April 2016
(Received July 7, 1974)
Abstract. Potentials evoked by tonal pulses and recorded with a monopolar electrode on the pial surface over the auditory cortex of the guinea pig are presented. These potentials are compared with averaged potentials recorded in previous studies with an electrode on the dura. The potentials recorded by these two techniques have similar waveforms, peak latencies and thresholds. They appear to be generated within the same region of the cerebral cortex. As can be expected, the amplitude of the evoked potentials recorded from the pial surface is larger than that recorded from the dura. Consequently, averaging is not needed to extract the evoked potential once the dura is removed. The thresholds for the evoked cortical potential are similar to behavioral thresholds for the guinea pig at high frequencies; however, evoked potential thresholds are elevated over behavioral thresholds at low frequencies. The removal of the dura and the direct recording of the evoked potential appears most appropriate for acute experiments. The recording of an evoked potential with dura electrodes employing averaging procedures appears most appropriate for chronic studies.
Several types of bioelectric potentials have been recorded from the human scalp (Davis, 1965; Cody & Bickford, 1965; Cody & Klass, 1968; Rapin & Bergman, 1969). These potentials have been especially useful with infants, the mentally retarded and malingerers. Similar potentials have been recorded from experimental animals in an attempt to quickly assess their auditory acuity. In order to test the auditory ability of animals, thresholds for the evoked cortical potential have The data presented in this article were collected in partial fulfillment of the requirements for the doctoral degree in the Department of Medical Psychology, University of Oregon Medical School. The work was in part sponsored by Program Project no. NS 09889.
been obtained for the cat (Hind & Schuknecht, 1954; Kimura et al., 1956; Hattori et al., 1971) as well as for the guinea pig. In the guinea pig, Djalilian & Cody (1973) and Hattori (1970) have us2d averaging techniques to obtain evoked potentials from dura electrodes. In one study (Kern et al., 1969) the vertex response was recorded from the guinea pig. In a number of recent studies (Walloch et al., 1971a, b, 1973a, b, 1974a, b; Cowden & Wdlnch, 1973) I have removed the dura and placed the recording electrode directly on the pial surface. In this manner, the evoked cortical potential could be recorded without averaging. The present paper compares and contrasts the records obtained from electrodes on the pia without averaging with those obtained from electrodes on the dura with averaging. The data contained in the present paper have not been reported except as a brief communication (Walloch, 1971a).
METHOD Eleven guinea pigs of the Topeka stock weighing 400-750 g were used in this study. Each guinea pig had an active Preyer reflex. The guinea pig was deeply anesthetized with diallylbarbituric acid (60 mg/kg) and urethane (240 mg/kg). In other studies (Walloch et al., 1974a, b) sodium pentobarbital (45 mb/kg) was used with equal success. Rarely, a supplemental dose was needed to maintain anesthesia. A tracheotomy was perActa Otolaryngol80
Downloaded by [McMaster University] at 23:53 24 April 2016
8 R. A. Walloch
Fig. 1. Record of the stimulus and evoked responses. Negative is up. (a) Averaged output of the calibrated microphone to a standard tonal pulse, showing that the sound pulse was free of significant on-set transients. (b) Superimposed cortical evoked responses to repeated clicks. (c) Cortical responses evoked by 25 kHz tonal pulses. Top tracing shows lack of evoked activity following repeated stimulus presentations at 21 dB SPL. The
middle tracing show a clear evoked response following repeated stimulation at 31 dB. The bottom line indicates the duration of the tonal pulse. (d) Cortical response t o 1 kHz tonal pulses. The top line shows absence of cortical activity following repeated stimulation at! 25 dB. The middle and bottom tracings show a well developed response at 35 and 45 dB respectively.
formed and the animal was mechanically ventilated. In the present experiment the guinea pig's rectal temperature was continuously monitored (Yellow Springs Instrument 46 TUC) and maintained at 38°C. When the body temperature falls below 35"C, the evoked cortical potential disappears. The evoked potential returns upon warming the animal. Above 40"C, the evoked potential deteriorates and in this case the deterioration is not reversible. Both pinnae were amputated to facilitate placement of the animal in a stereotaxic unit (David Kopf). The scalp and skull flap were removed to expose the entire left hemisphere. To remove the dura, an old tungsten micro-
electrode was used, though any sharp thin probe would suffice. The dura was impaled medially from the auditory area and lifted away from the cortex. An incision was made beneath the microelectrode with Vannas scissors. This incision was expanded anterolaterally. The dura was reflected anteriad. Some bleeding occurred on occasion while the dura was being removed. When this occurred, it was best to proceed quickly with the reflection of the dura. With the dura reflected, the bleeding could then be controlled with the use of gelfoam. Most often the bleeding came from the soft tissue surrounding the cortical exposure. Only rarely did the bleeding stem from the cortex itself. After the dura was reflected, warm mineral oil
Actn Otolnryngol80
Downloaded by [McMaster University] at 23:53 24 April 2016
Cortical evoked potentials without averaging 9
was allowed to flow across the exposed cortex. However, the mineral oil was not found to be needed in subs2quent studies. A silver ball electrode (0.18 mm) was moved across the temporal area of the guinea pig in 1 mm steps. The impedance of the electrode was about 2 kilohms. At each cortical position studied, the threshold for an auditory evoked potential was obtained at 18 frequencies ranging from 0.1 to 40 kHz. In order to obtain these evoked potentials, sound was delivered through the hollow ear bar contralateral to the exposed cortex. A General Radio wave analyzer (1900A), Monsanto digital counter (103A), Simpson voltmeter, GrasonStadler electronic switch (892E\, Grason-Stadler interval timer (471-I), General Radio decade attenuator (1450), McIntosh power amplifier (M-210-B) and a specially designed power attenuator shaped the electrical signal which drove the Western Electric 555 speaker. The speaker was connected to the hollow ear bar as a closed sound system. The rate of stimulation was once every 2 seconds. Each tonal pulse was 50 msec in duration and had a rise time of 10 msec. At the end of each day’s work a 1/4 inch Bruel & Kjaer calibrated microphone was substituted for the animal and the sound at the end of the hollow ear bar was thus measured (Fig. l a ) . The evoked cortical potentials were initially amplified using a Keithley differential amplifier (103). The filters were set at 100 and 3 000 Hz. The calibration of this amplifier has been previously described (Meikle & Copeland, 1973). After the initial amplification, the potentials were displayed on a Tektronix storage oscilloscope (564B). RESULTS Acoustically evoked potentials were recorded from a restricted area within the temporal region of the cerebral cortex. Other investigators have identified this region as the auditory cortex (Odkvist et al., 1973; Ziegler, 1964; Kayser & Legouix, 1963). The first wave was always surface positive and it had a peak latency of 10-15 msec. A surface negative wave with a peak
t
I
I
I
I
0.5
I , , ,
I
I
I
5
I
I
,,I
10
I
L
kHz
Frequency
Fig. 2. Threshold curves for the evoked responses recorded from four cortical positions in a single guinea pig. All four positions were in the temporal region and within a few millimeters of each other. See text for a fuller discussion of these data.
latency of 20-40 msec generally followed the initial positive wave. A second surface positive wave was observed rarely and only at very high sound intensities. No later waves were observed. In Fig. 1b, an evoked potential to a click is shown. Clicks were used in a few pilot animals to determine the location of the auditory cortex. The waveform of the click evoked potential is highly similar to that evoked by tonal pulses. No attempt was made to obtain the threshold of these click evoked potentials; however, they were very useful in determining the latencies of the evoked potentials. In Fig. 1c, the cortical responses to a 25 kHz tone are shown. The top tracing shows a lack of an evoked response to repeated stimulation at 21 dB re: 0.0002 dynes/cm2.The middle tracing shows a 650 ,uV (p-p) respons:: to repeated stimulation at 31 dB. In Fig. Id, the cortical responses to a 1 kHz tone are shown. In the top tracing, no evoked activity is apparent following repeated stimulation at 23 dB. In the middle trace, a 1 mV potential was evoked by repeated stimulation at 33 dB. These thresholds were stable over 5 and 6 hour recording sessions. In Fig. 2, the thresholds for the evoked potential recorded from four positions on the auditory cortex of a single guinea pig are shown. All Acta OtoIaryngol80
10 R.A . Walloch loot
I
0.5
5
I
10
k Ht
Downloaded by [McMaster University] at 23:53 24 April 2016
Frequency
Fig. 3. Thresholds for the evoked response from 11 guinea pigs. The entire left hemisphere of each guinea pig was explored for evoked cortical activity. At each frequency, the threshold represents the lowest intensity tonal pulse that evoked cortical activity regardless of the position of the recording electrode.
four positions were within 4 mm of each other. Two of the positions (a and b) were sensitive to high frequency tones, and tonal puls-s below 1.5 kHz were totally ineffective in evoking a potential. One position (c) was sensitive to low frequency stimuli. One position ( d ) was sensitive to
both low and high frequency stimuli. Consequently, in order to obtain the lowest threshold for the evoked cortical potential at all frequencies without averaging, a number of cortical positions must be sampled. In Fig. 3, the thresholds for the auditory evoked potential obtained from 11 guinea pigs are shown. The entire left hemisphere was sampled in 1 mm steps. At each frequency, the lowest sound intensity that could evoke a cortical potential regardless of the recording position is shown. Consequently, each threshold curve represents potentials recorded from a number of cortical positions. Fig. 4 compares the mean of the threshold curves shown in Fig. 3 with other common measures of the auditory acuity in guinea pigs. The thresholds obtained by computing averaged evoked potentials recorded from the dura of guinea pigs (Djalilian & Cody, 1973; Hattori & Shoyama, 1970) are very similar to those obtained in the present study. These evoked potential thresholds were similar to the behavioral threshold of guinea pigs (Miller & Murray, 1966) at high frequencies, but these evoked potential thresholds were elevated over behavioral thresholds at the low frequencies. COMMENT
.--,, I
0.5
I
5 Frequency
, < * , I
10
1
kHz
Fig. 4. Comparison of the mean of the threshold curves plotted in Fig. 3 with other measures of auditory acuity in the guinea pig. (a) Mean of the threshold curves obtained in the present study. (b) Thresholds for the averaged evoked potentials reported by Hattori (1970) from the dura overlying the auditory cortex of the guinea pig. (c) Thresholds for the averaged evoked potentials recorded by Djalilian (1973) from the dura overlying the auditory cortex of the guinea pig. ( d ) Behavioral threshold obtained by Miller (1966) in the guinea pig. (e) Vertex potential threshold obtained by Kern (1969) from dura electrodes in the guinea pig.
Acta Otolxyngol80
The thresholds reported in the present study are very similar to those reported by Hattori & Shoyama (1970) and to thosc reported by Djalilian & Cody (1973) (Fig. 4). Hattori & Djalilian used an averaging computer to record evoke potentials from electrodes on the dura over the auditory cortex. The potentials recorded in all three studies had, 1) similar waveforms, 2) latencies, 3) thresholds, and 4) spacial distributions on the cerebral hemisphere. Consequently the potentials recorded in the three studies appear to have the same origin. For certain acute expzrimental problems, the removal of the dura and the direct recording of the evoked cortical potential appears most appropriate. The large amplitude of the evoked potential (Fig. l c , 6) at threshold makes aver-
Downloaded by [McMaster University] at 23:53 24 April 2016
Cortical evoked potentials without averaging 11 aging unnecessary. Consequently, thresholds can be obtained much more rapidly with direct recording techniques than with averaging procedures. While 50 stimulus presentations were used to obtain an averaged evoked potential in Djalilian’s (1973) and Hattori’s (1970) studies, only 2 stimulus presentations were needed in the present study. On the other hand, for certain chronic experimental problems, the recording of averaged evoked potentials from an electrode on the dura appears most appropriate. The opening in the skull can be easily closed and the guinea pig may be allowed to recover from the anesthesia. The choice of the position on the cerebral cortex from which to record the evoked potentials greatly influences the thresholds obtained. The position used by either Hattori (1970) or Djalilian (1973) appear appropriate for chronic studies. For acute studies in which the dura has been removed, the cortex can be quickly searched for an appropriate position. At each cortical position, the threshold for a potential evoked by 1 kHz and 10 kHz tonal pulses can be quickly ascertained. If the 1 kHz threshold is below 45 dB and the 10 kHz threshold is below 35 dB, the cortical position is suitable for the experiment. From such a position a complete threshold curve would appear similar to Fig. 2d. The behavioral threshold for pure tone stimuli have been obtained several times for the guinea pig (Horton, 1933; Anderson & Wedenberg, 1965; Miller & Murray, 1966; Heffner et al., 1971). The thresholds depicted in Fig. 4 d are representative of other behavioral determinations. The thresholds for the evoked potentials obtained in the present study are similar to the behavioral thresholds at the high frequencies. The evoked potential thresholds are elevated over the behavioral thresholds at the low frequencies. The majority of the positions on the auditory cortex are most sensitive to high frequency stimuli. While 63 % of the positions had best frequencies over 5 kHz, only 37 % had best frequencies below 3 kHz (Walloch, 1971c). Perhaps this can explain the relative insensitivity of the evoked potential to low frequency stimuli.
On the other hand, differencesin 1) sound measurement procedures, 2) the rise time, and 3) duration of the tones cannot be discounted as contributing to the differences in the evoked potential and the behavioral thresholds. The vertex response of the guinea pig (Kern et al., 1969) is highly elevated over the other measures of auditory acuity shown in Fig. 4. This contrasts with the human vertex potential which is similar to the thresholds obtained by conventional pure tone audiometry (Cody & Bickford, 1965; Cody & Klass, 1968; Rapin & Bergman, 1969). The reason for this difference between the guinea pig and human vertex potential is not apparent. The evoked potential from the auditory cortex has been used to study the effects upon the auditory system of intense sound stimulation (Hattori, 1970, 1971) ethacrynic acid (Walloch, 1974a) and mechanical lesions of the cochlea (Kimura et al., 1956; Hind & Schuknecht, 1954). The evoked potential has also been used to investigate various placements of electrodes for the electrical stimulation of the ear (Walloch et al., 1973a, 19743). In the future, we can expect that the evoked potential will be used to study an ever-increasing scope of problems.
ZUSAMMENFASSUNG Aktionspotentiale der Horrinde wurden bei Meerschweinchen durch Tonimpulse hervorgerufen und mittels monopolarer Elektrode an der pia mater abgeleitet. Sie wurden mit Durchschnittsaktionspotentialen verglichen, die in friiheren Untersuchungen von der dura mater abgeleitet worden waren. Diese mit verschiedener Methodik erhaltenen Potentiale ahneln einander hinsichtlich Verlaufsform, maximaler Latenzzeit und Schwellenwert. Sie scheinen in derselben Gegend der Hirnrinde zu entstehen. Wie zu erwarten, ist die Amplitude der von der pia mater abgeleiteten Aktionspotentiale grosser als die von der dura mater. Daher ist es nicht notwendig, Aktionspotentiale zu ermitteln, wenn einmal die dura mater beseitigt ist. Hervorgerufene Rindenpotentiale haben beim Meerschweinchen bei hoher Reizfrequenz ahnliche Schwellenwerte wie Verhaltungsreaktionen. Jedoch liegen bei niedrigen Frequenzen die Schwellenwerte von hervorgerufenen Potentialen hoher als die Schwellenwerte von Verhaltungsreaktionen. Beseitigung der dura mater und direktes Registrieren des Aktionspotentials scheinen am besten fur akute Experimente geeignet zu sein, wahrend Acta OtolaryngolSO
12 R. A . Walloch Registrieren von Durchschnittsaktionspotentialen mit dura-Elektroden sich am besten fur chronische Experimente zu eignen scheint.
Downloaded by [McMaster University] at 23:53 24 April 2016
REFERENCES Anderson, H. & Wedenberg, E. 1965. A new method for hearing tests in the guinea pig. Acta Otolaryngol (Stockh) 60, 375. Cody, D. T. R. & Bickford, R. G. 1965. Cortical audiometry: An objective method of evaluating auditory acuity in man. Mayo Clin Proc 40, 273. Cody, D. T. R. & Klass, D. W. 1968. Cortical audiometry: Potential pitfalls in testing. Arch Otoluryngol88, 396. Cowden, D. & Walloch, R. 1973. Placement of electrodes for activation of VIII nerve. J Acoust Soc A m 53, 326. Davis, H. 1965. Slow cortical responses evoked by acoustic stimuli. Acta Otolaryngol (Stockh) 59, 179. Djalilian, M. & Cody, D.T.R. 1973. Averaged cortical responses evoked by pure tones in the chinchilla and the guinea pig. Arch Otolaryngol98, 196. Hattori, H., Shida, E. & Umeda, H. 1971. Computer audiometry in cats. Arch Otolaryngol93, 147. Hattori, H. & Shoyama, T. 1970. Cortical audiometry in guinea pigs. J Auditory Res 10, 269. Heffner, R., Heffner, H. & Masterton, €3.1971. Behavioral measurements of absolute and frequency-difference thresholds in guinea pig. J Acoust Soc A m 49, 1888. Hind, J . E. & Schuknecht, H. F. 1954. A cortical test of auditory function in experimentally deafened cats. J Acoust Soc Am 26, 89. Horton, G. P. 1933. A quantitative study of hearing in the guinea pig (Cauia cobayu). J Comp Physiol Psychol 15, 59. Kayser, D. & Legouix, J. P. 1963. Projections tonotopiques sur le cortex auditif du Cobaye. C R Soc Biol (Paris) 157, 2161. Kern, E. B., Cody, D. T. R. &Bickford, R. G. 1969.Vertexresponse thresholds to pure tones in guinea pigs. Arch Otolaryngol90, 315. Kimura, R. S., Schuknecht, H. F. & Sutton, S. 1956. Effects of cochlear lesions on the threshold responses
Actn Otolaryngol80
of the auditory cortex in chronic experiments. J Comp Physiol Psychol49, 96. Meikle, M. B. & Copeland, A. B. 1973. Calibration of differential amplifiers used in biological recording. Arch Otolaryngol98, 66. Miller, J. D. & Murray, F. S. 1966. Guinea pig’s immobility response to sound: Threshold and habituation, J Comp Physiol Psychol 61, 227. Odkvist, L. M., Rubin, A. M., Schwarz, D. W. F. & Fredrickson, J. M. 1973. Vestibular and auditory cortical projection in the guinea pig (Cauiu porcellus). Exp Brain Res 18, 279. Rapin, I. & Bergman, M. 1969. Auditory evoked responses in uncertain diagnosis. Arch Otolaryngol 90, 307. Walloch, R. A. 1 9 7 1 ~ .Tonotopic organization of the guinea pig’s auditory cortex. J Acoust Soc Am 50, 119. Walloch, R. A. & Brummett, R. E. 1971b. Intensity function of the cochlea and cortex. J Acoust Soc A m 49, 122. Walloch, R., DeWeese, D., Brummett, R. & Vernon, J. 1973a. Electrical stimulation of the inner ear. Ann Otol Rhino1 Luryngol82, 1. Walloch, R. A. 19736. Intensity function for cortical potential evoked by electrical stimulation of the VIII nerve. J Acoust Soc A m 53, 362. Walloch, R. A., Anthony, J. & Gibbons, N. 1974. Depression of the evoked potential by ethacrynic acid. Laryngoscope 84, 256. Walloch, R. A. & Cowden, D. A., 1974b. Placement of electrodes for excitation of the VIII nerve. Arch Otolaryngol 100, 19. Walloch, R. A. 1971c. A study of the auditory cortex in the guinea pig with implications for the development of a n electrical prosthesis for the hard of hearing. Dissertation, Univ, of Ore. Med. School Library. Ziegler, H. P. 1964. Cortical sensory and motor areas of the guinea pig (Cauiu porcellus). Arch Ital Biol 102, 587.
R. A. Walloch, Ph.D. Dept. of Otolaryngology Uniuersity of Oregon Medical School Portland, Ore. 97201, USA
