Hearing Research, 53 (1991) 153-158 0 1991 Elsevier Science Publishers B.V. 0378-5955/91/$03.50
HEARES
153
01564
Short Communication Normative
N, audiogram
data for the barbiturate-anaesthetised domestic cat
R. Rajan, D.R.F. Irvine and J.F. Case11 Department of Psychology, Monash University, Clayton, Victoria, Australia (Received
16 August
1990; accepted
The N, audiogram or electrocochleogram (Dal10s et al., 1978; Eggermont, 1976; Johnstone et al., 1979) is a quick and convenient means of monitoring the condition of the cochlea. Details of this measure and its use have been presented by others (e.g., Antoli-Candela and Kiang, 1978; Dallos et al., 1978; Durrant et al., 1977; Eggermont, 1976; Goldstein and Kiang, 1958; ‘Harris, 1979; Johnstone et al., 1979; Mitchell, 1976; Ozdamar and Dallos, 1976, 1978; Prijs and Eggermont, 1981). Detailed normative N, audiograms have been presented for some commonly-used animals (e.g., chinchilla and gerbil-DaIlos et al., 1978; guinea pig-Johnstone et al., 1979; Rajan, 1989) but only limited data have been published for the domestic cat, a species as widely used. In the cat, Price (1978) measured N, thresholds at eight frequencies from 0.5-20 kHz, while Gorga and Abbas (1981) measured N, thresholds at 14 frequencies from 0.5-15 kHz. Here we present more extensive N, threshold data for this species with some details of the measurement. We extend the utility of these data by comparing two methods of measuring N, thresholdsresponse averaging, and the visual detection method (Johnstone et al., 1979). The latter is faster but is dependent upon experience. Domestic cats were anaesthetised with Nembutal (40 mg/kg) intra-peritoneally, administered atropine sulphate (120 pg) intra-muscularly, tracheostomized and mounted in a stereotaxic frame.
Correspondence: R. Rajan, Department of Psychology, University, Clayton, Vie. 3168, Australia.
Monash
12 December
1990)
Surgery was carried out to expose the tympanic bullae and place a stainless-steel spring electrode in contact with the round window membrane (Wise and Irvine, 1983). No middle ear surgery ( e.g., removal of the septum) was carried out. A reference electrode was placed in musculature close to the bulla and a ground electrode sub-dermally in the neck. The hole in the bulla was sealed with dental acrylic after inserting a thin-bore long polyethylene tube to allow static middle-ear pressure equalization. The auditory meatus was cut to position a sound delivery tube close to the tympanum. Sinusoidal stimuli generated by a digital synthesis system (after Rhode, 1976) were delivered through a sound delivery tube leading from a Stax SR 44 electrostatic earphone in a specially-designed coupler (Sokolich, 1981). Sound pressure levels (SPLs) were previously determined by calibration, under computer control, of the sound system into one end of a closed-cavity coupler; to the other end was attached a Brtiel and Kjaer (B and K) 0.25” condenser microphone Type 4135 connected to a B and K Measuring Amplifier Type 2606. Calibrations were stored to allow computer setting of SPLS. N, thresholds were measured to gated lo-30 ms tone bursts, with 1-ms linear rise-fall times, presented at 7 or 8/s. Tone bursts commenced at positive zero crossings. Responses from the round window electrode were amplified 1000 X and band-pass filtered (300 Hz-10 kHz) before being displayed on an oscilloscope. The visual detection threshold (VDT) criterion was a just-detectable N, response in the oscilloscope display, correspond-
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TABLE
1
N, VISUAL
DETECTION
THRESHOLDS
IN THE DOMESTIC
CAT
Frequency (kHz) Mean threshold (dB SPL) SD Number of data points
0.50 40.15 2.11 26
0.75 31.13 2.93 26
1.00 35.18 3.75 28
2.00 32.24 3.03 34
3.00 33.17 3.91 35
4.00 32.25 3.73 40
5.00 23.78 3.45 41
6.00 17.67 3.03 42
Frequency (kHz) Mean threshold (dB SPL) SD Number of data points
7.00 14.73 2.58 41
8.00 13.16 2.83 40
9.00 12.19 2.59 41
10.00 11.72 2.71 42
11.00 11.85 2.28 41
12.00 12.21 2.37 42
13.00 12.60 1.55 37
14.00 13.66 2.84 42
Frequency (kHz) Mean threshold (dB SPL) SD Number of data points
15.00 14.51 2.98 42
16.00 15.26 2.65 42
17.00 16.55 3.15 42
18.00 17.08 2.99 42
19.00 18.44 3.35 42
20.00 19.74 3.73 42
22.00 21.34 3.31 42
24.00 23.96 3.93 41
Frequency (kHz) Mean threshold (dB SPL) SD Number of data points
26.00 26.05 3.67 31
28.00 28.98 4.90 30
30.00 31.13 5.11 35
32.00 33.82 6.13 34
34.00 34.73 6.02 21
36.00 36.99 5.22 33
38.00 36.77 3.37 13
40.00 41.28 4.32 25
ing to an N, amplitude of about l-2 /.LV(see also Fig. 1C and Fl). Mean N, VDTs from 0.5-40 kHz are listed in Table I. The mean N, VDTs are compared in Fig. 1A to thresholds measured for single high-spontaneousrate VIIIth nerve fibres with CFs over the same frequency range in the barbiturate-anaesthetized cat (Liberman and Kiang, 1978). From 0.5- = 5 kHz mean N, VDTs were less sensitive (by = 2025 dB from 0.5-4 kHz) than mean single unit thresholds. As discussed below, this may reflect differences in the acoustic impedance of the middle ear according to whether the bulla was sealed (the present study) or open (the single unit study). From 6 kHz on, mean N, VDTs lay within 1 SD of
mean unit thresholds. Although mean N, VDTs at > = 22 kHz were lower than mean frequencies unit thresholds, they were never as low as the lowest single unit thresholds at these frequencies. The mean N, VDTs measured to fixed startingphase tone bursts in this study are compared to thresholds measured using random starting-phase tone bursts and averaging of responses in two previous studies in the anaesthetised cat (Gorga and Abbas, 1981; Price, 1978) in Fig. 1B. In all three studies the N, threshold criterion was a just-noticeable N, response, either in the averaged display (the earlier studies) or in the direct oscilloscope display (the present study). Over the low frequency range from 0.5-4 kHz, the mean VDTs
Fig. 1. Normative N, threshold data in the domestic cat. (A) Mean N, visual detection thresholds (VDTs) compared to single unit thresholds in the barbiturate-anaesthetized cat. Mean unit thresholds (crosses f 1 SD) are from high-spontaneous-rate units while the full line indicates the lowest thresholds recorded for single units in the cat (from Liberman and Kiang, 1978). Liberman and Riang (1978) binned their data for CFs from 0.1-40 kHz into 20 logarithmically-spaced bins. Only the bins relevant to the comparison are illustrated. (B) Mean N, VDTs (filled circles f 1 SD bars) compared to N, thresholds measured with averaging of responses in two previous studies in the cat (Price, 1978; Gorga and Abbas, 1981). The mean thresholds measured by Price (1978) are shown as circles with - 1 SD bars extending down while the mean thresholds measured by Gorga and Abbas (1981) are shown by the crosses with + 1 SD bars extending upwards. In all three studies the criterion for N, threshold was a just-detectable response. (C) Comparison of N, VDTs to thresholds measured with averaging of responses to tone bursts with random starting phase or fixed (zero-crossing) starting phase, in the one animal. The criterion for N, threshold was always a just-detectable response. (D) Effect of bulla condition on VDTs in the one animal. (E) Effect of varying stimulus rise-times on VDTs in the one animal. (F) Comparison of N, VDTs to thresholds measured with averaging of responses to tone bursts with fixed starting phase, with criterion threshold amplitudes as specified in the legend. Fl illustrates the data while F2 illustrates the difference between the VDTs and thresholds obtained at each amplitude criterion with averaging.
156
are consistently higher than the data of Price (1978) and slightly higher than those recorded by Gorga and Abbas (1981). Over the common midfrequency region from = 5-20 kHz, the mean VDTs lie intermediate between the values in the earlier studies. N, thresholds at frequencies > 20 kHz were not measured in the earlier studies. Other than the low-frequency difference noted above, the general similarity between thresholds in the three studies suggests that the SPL calibrations in this study (from calibration of the sound system into a closed-cavity-coupler) can be validly compared to the SPL calibrations made in-situ close to the tympanum by Price (1978) and by Gorga and Abbas (1981). A similar argument can be advanced on the basis of the general similarity (other than at frequencies < 5 kHz) between the mean N, VDTs in this study and the mean single unit thresholds measured by Liberman and Kiang (1978). One possible reasons for the low-frequency difference is that, in the present study, the large cochlear microphonic (CM) recorded with fixed starting-phase tone bursts interfered with clear identification of the N, closer to threshold, resulting in too-conservative estimates of N, thresholds. To determine if this factor accounted for the difference, in one animal N, thresholds were determined visually to fixed starting-phase tone bursts and then by averaging responses to random starting-phase tone bursts to eliminate the CM. The threshold criterion with averaging was also a just-detectable N, response, in the average of 40 sweeps. As shown in Fig. lC, thresholds measured in this manner were considerably elevated over the low frequency range compared to the VDTs. In a second animal similar effects were seen though the differences were not as large. In the guinea pig the stimulus starting-phase has been reported (Dallos and Cheatham, 1976) to be an important factor in eliciting the N, at frequencies < 2 kHz, at which tone bursts with random starting-phase resulted in smaller N, amplitudes. This and other effects (such as the inherent time smear with random-phase tone bursts; e.g., Dallos and Cheatham, 1976) should result in elevated thresholds at low frequencies compared to thresholds measured with fixed starting-phase tone bursts. This was confirmed when, in the same
animal (Fig. lC), N, thresholds were also determined by averaging responses to tone bursts with a fixed starting-phase and with the same threshold criterion of a just-detectable N,, in the average of 40 sweeps. Low-frequency N, thresholds measured in this way were slightly lower than the VDTs and much lower than those determined by averaging responses to tone bursts with random starting phase, although with the attendant problem of a large CM. Thus, the averaging procedure used by Price (1978) and Gorga and Abbas (1981) can not account for the difference in low-frequency N, thresholds noted above. A second possibility lies in the fact that, in the earlier studies, N, thresholds were measured with the bulla open, which in the cat decreases the acoustic impedance and consequently increases transmission at frequencies < = 4 kHz (e.g., Guinan and Peake, 1966; Moller, 1963: Tonndorf and Khanna, 1966). To clarify this. N, VDTs were determined with the bulla open and then with the bulla sealed in the usual manner, in one animal (Fig. 1D). Low frequency VDTs were lower in the open-bulla condition than in the closed-bulla condition, in a frequency-dependent manner consistent with the data obtained in the cat by Guinan and Peake (1966), Msller (1963) and Tonndorf and Khanna (1966). Very similar effects were obtained in another animal. If these corrections are applied to the data in Fig. lB, there are no substantial differences between low frequency thresholds in this study and those recorded by Gorga and Abbas (1981) or between thresholds at frequencies from 2 kHz on in this study and those recorded by Price (1978). Despite the corrections. thresholds at frequencies < 2 kHz in this study and that by Gorga and Abbas (1981) are still higher than those measured by Price (1978). We can suggest no explanation for these differences. However, generally, it is clear that the N, VDTs are similar to thresholds measured with averaging, with the same N, detection criterion. It also appears that generally the three sets of data are comparable despite test conditions (e.g., levels of anaesthesia) presumably being somewhat different, suggesting the generality of our data. (In this regard, we have also obtained similar N, VDTs in ketamine-anaesthetized cats.) Sealing the bulla is reported to result in a
157
steady deterioration in cochlear responses, possibly due to build up of ‘negative’ middle ear pressure (Guinan and Peake, 1966). We have found very good stability of N, thresholds during experiments lasting 40 hours or more, indicating that the pressure equalization tube sealed into the bulla opening provides sufficient aeration of the middle ear cavity. The effect of varying stimulus rate and tone burst duration on VDTs was tested with rates of 3, 5 and 7/s, and durations of 10, 30 ,and 50 ms. None of the nine duration-rate combinations produced significant or systematic variations in thresholds. The results are therefore not illustrated. The effect of varying the tone burst rise-time on VDTs was examined from 0.5 to 20 kHz in another animal in which 30-ms tone bursts were presented at 7/s. Rise-times of 0.1, 0.5, 1.0, 1.5 and 2.0 ms were used. Rise-times of 0.1-1.0 ms did not produce different N, VDTs (Fig. 1E). With slower rise-times VDTs were elevated, possibly due to greater desynchronization of afferent responses (Goldstein and Kiang, 1958; Mitchell. 1976a; Ozdamar and Dallos, 1978) with greatest elevations at the slowest rise-time. Thus, a 1-ms rise-time appears to provide a good compromise between avoiding the frequency splatter that occurs with fast rise-times and achieving the synchronization of afferent responses required to elicit an N, response. Although the greater frequency splatter with shorter rise-times did not affect N, thresholds measured with the low-amplitude criterion used here, it may affect thresholds measured with higher-amplitude criteria, since amplitude growth patterns may not be the same at all frequencies. In this context, tone bursts with very fast rise-times may present a problem in measuring N, thresholds in cochleas with losses in sensitivity since the greater frequency splatter could mask the true extent of threshold losses. In such cochlear regions, the effects of frequency splatter with different rise-times may also differ from the effects detailed here. As noted above, the N, VDTs were generally similar to thresholds measured with response averaging in previous studies with the same N, detection criterion. Use of the visual detection method does require considerable experience and this can
be circumvented by using averaging routines. To compare directly the two methods, in one animal N, thresholds were determined visually by one experimenter and then, independently, using averaging techniques, by another experimenter. Responses to 100 tone bursts of fixed starting phase were averaged at each of a number of frequencies using N, threshold-amplitude criteria of 5, 8, 10 and 20 pV. As shown in Fig. lF, increasing the threshold-amplitude criterion resulted in progressively elevated thresholds. With a 5-pV criterion, thresholds were, on average, about 5 dB higher than the VDTs across the entire frequency range. With an 8-PV criterion, thresholds were about 8-9 dB higher than the VDT, with a lo-FV criterion about 11 dB higher and with a 20-PV criterion about 17.5 dB higher. In summary, this report provides a more extensive normative data base for the N, audiogram for the domestic cat than was previously available. Although the visual detection technique used here is simple and reproducible both for a single observer and between experienced observers (Johnstone et al., 1979), its use is dependent upon experience. We have generalized the applicability of these data by comparing the VDTs to the thresholds determined by averaging responses with threshold-amplitude criteria set at easily-detectable levels. Acknowledgements This work was supported by grants from the Australian National Health and Medical Research Council. We thank R. Williams for drawing the figures and V. Kohout for the photographs. References Antoli-Candela, Jr.. F. and Kiang, N.Y.-S. (1978) Unit activity underlying the N, potential. In: R.F. Naunton and C. Fernandez (Eds.), Evoked Electrical Activity in the Auditory Nervous System. Academic Press, New York, pp. 165-191. Dallas, P. and Cheatham. M.A. (1976) Compound action potential (AP) tuning curves. J. Acoust. Sot. Am. 59. 591591. Dallas. P.. Ozdamar, 0. and Ryan. A. (1978) Behavioural, compound action potential. and single unit thresholds: relationship in normal and abnormal ears. J. Acoust. Sot. Am. 64, 151-157.
158 Durrant, J.D., Burns, A. and Ronis. M.L. (1978) Electrocochleographic studies in animals. Adv. Oto-Rhino-Laryngol. 22, 14-23. Eggermont. J.J. (1976) Electrocochleography. In: W.D. Keidel and W.D. Neff (Eds.). Handbook of Sensory Physiology. Vol. V: Auditory System. Springer-Verlag, Berlin, pp. 625705. Goldstein. M.H. and Kiang, N.Y.-S. (1958) Synchrony of neural activity in electric responses evoked by transient acoustic stimuli, J. Acoust. Sot. Am. 30, 107-114. Gorga, M.P. and Abbas. P.J. (1981) AP measurements of short-term adaptation in normal and in acoustically traumatized ears. J. Acoust. Sot. Am. 70, 1310-1321. Guinan, Jr., J.J. and Peake, W.T. (1966) Middle-ear characteristics of anaesthetized cats. J. Acoust. Sot. Am. 41, 12371261. Harris. D.M. (1979) Action potential suppression. tuning curves and thresholds: comparison with single fibre data. Hear. Res. 1. 133-154. Johnstone. J.R., Alder. V.A.. Johnstone, B.M., Robertson. D. and Yates. G.K. (1979) Cochlear action potential threshold and single unit thresholds. J. Acoust. Sot. Am. 65. 254-257. Liherman, M.C. and Kiang. N.Y.-S. (1978) Acoustic trauma in cats. Acta Oto-Laryngol. Suppl. 258. l-63. Mitchell, C. (1976) Frequency specificity of the Nl potential from the cochlear nerve under various stimulus conditions. J. Aud. Res. 16, 247-255. Moller. A.R. (1963) Transfer function of the middle ear. J. Acoust. Sot. Am. 35. 1526-2534.
Ozdamar. 0. and Dallas. P. (1976) Input-output functions of cochlear whole-nerve action potentials: interpretation in terms of one population of neurons. J. Acoust. Sot. Am. 59. 143-147. Ozdamar, 0. and Dallas. P. (1978) Synchronous responses of the primary auditory fibers to the onset of tone bursts and their relation to compound action potentials. Brain Res. 155, 169-175. Price, G.R. (1978) Action potentials in the cat at low sound intensities: thresholds, latencies and rates of change. J. Acoust. Sot. Am. 64. 1400-1405. Prijs. V.F. and Eggermont. J.J. (1981) Narrow-band analysis of compound action potentials for several stimulus conditions in the guinea pig. Hear. Res. 4. 23-41. Rajan, R. (1989) Tonic activity of the crossed olivocochlear bundle in guinea pigs with idiopathic losses in auditory sensitivity. Hear. Res. 39. 299-308. Rhode, W.S. (1976) A digital system for auditory neurophysiological research. In: P. Brown (Ed.), Computer Technology in Neuroscience. Hemisphere. Washington DC, pp. 543567. Sokolich. W.G. (1981) Closed sound delivery system. United States Patent 4.251.686. Tonndorf. J. and Khanna, S.M. (1966) Some properties of sound transmission in the middle and outer ears of cats. J. Acoust. Sot. Am. 41. 513-521. Wise. L.Z. and Irvine, D.R.F. (1983) Auditory response properties of neurons in deep layers of cat superior colliculus. J. Neurophysiol. 49, 674-685.
HEARES
153
01564
Short Communication Normative
N, audiogram
data for the barbiturate-anaesthetised domestic cat
R. Rajan, D.R.F. Irvine and J.F. Case11 Department of Psychology, Monash University, Clayton, Victoria, Australia (Received
16 August
1990; accepted
The N, audiogram or electrocochleogram (Dal10s et al., 1978; Eggermont, 1976; Johnstone et al., 1979) is a quick and convenient means of monitoring the condition of the cochlea. Details of this measure and its use have been presented by others (e.g., Antoli-Candela and Kiang, 1978; Dallos et al., 1978; Durrant et al., 1977; Eggermont, 1976; Goldstein and Kiang, 1958; ‘Harris, 1979; Johnstone et al., 1979; Mitchell, 1976; Ozdamar and Dallos, 1976, 1978; Prijs and Eggermont, 1981). Detailed normative N, audiograms have been presented for some commonly-used animals (e.g., chinchilla and gerbil-DaIlos et al., 1978; guinea pig-Johnstone et al., 1979; Rajan, 1989) but only limited data have been published for the domestic cat, a species as widely used. In the cat, Price (1978) measured N, thresholds at eight frequencies from 0.5-20 kHz, while Gorga and Abbas (1981) measured N, thresholds at 14 frequencies from 0.5-15 kHz. Here we present more extensive N, threshold data for this species with some details of the measurement. We extend the utility of these data by comparing two methods of measuring N, thresholdsresponse averaging, and the visual detection method (Johnstone et al., 1979). The latter is faster but is dependent upon experience. Domestic cats were anaesthetised with Nembutal (40 mg/kg) intra-peritoneally, administered atropine sulphate (120 pg) intra-muscularly, tracheostomized and mounted in a stereotaxic frame.
Correspondence: R. Rajan, Department of Psychology, University, Clayton, Vie. 3168, Australia.
Monash
12 December
1990)
Surgery was carried out to expose the tympanic bullae and place a stainless-steel spring electrode in contact with the round window membrane (Wise and Irvine, 1983). No middle ear surgery ( e.g., removal of the septum) was carried out. A reference electrode was placed in musculature close to the bulla and a ground electrode sub-dermally in the neck. The hole in the bulla was sealed with dental acrylic after inserting a thin-bore long polyethylene tube to allow static middle-ear pressure equalization. The auditory meatus was cut to position a sound delivery tube close to the tympanum. Sinusoidal stimuli generated by a digital synthesis system (after Rhode, 1976) were delivered through a sound delivery tube leading from a Stax SR 44 electrostatic earphone in a specially-designed coupler (Sokolich, 1981). Sound pressure levels (SPLs) were previously determined by calibration, under computer control, of the sound system into one end of a closed-cavity coupler; to the other end was attached a Brtiel and Kjaer (B and K) 0.25” condenser microphone Type 4135 connected to a B and K Measuring Amplifier Type 2606. Calibrations were stored to allow computer setting of SPLS. N, thresholds were measured to gated lo-30 ms tone bursts, with 1-ms linear rise-fall times, presented at 7 or 8/s. Tone bursts commenced at positive zero crossings. Responses from the round window electrode were amplified 1000 X and band-pass filtered (300 Hz-10 kHz) before being displayed on an oscilloscope. The visual detection threshold (VDT) criterion was a just-detectable N, response in the oscilloscope display, correspond-
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TABLE
1
N, VISUAL
DETECTION
THRESHOLDS
IN THE DOMESTIC
CAT
Frequency (kHz) Mean threshold (dB SPL) SD Number of data points
0.50 40.15 2.11 26
0.75 31.13 2.93 26
1.00 35.18 3.75 28
2.00 32.24 3.03 34
3.00 33.17 3.91 35
4.00 32.25 3.73 40
5.00 23.78 3.45 41
6.00 17.67 3.03 42
Frequency (kHz) Mean threshold (dB SPL) SD Number of data points
7.00 14.73 2.58 41
8.00 13.16 2.83 40
9.00 12.19 2.59 41
10.00 11.72 2.71 42
11.00 11.85 2.28 41
12.00 12.21 2.37 42
13.00 12.60 1.55 37
14.00 13.66 2.84 42
Frequency (kHz) Mean threshold (dB SPL) SD Number of data points
15.00 14.51 2.98 42
16.00 15.26 2.65 42
17.00 16.55 3.15 42
18.00 17.08 2.99 42
19.00 18.44 3.35 42
20.00 19.74 3.73 42
22.00 21.34 3.31 42
24.00 23.96 3.93 41
Frequency (kHz) Mean threshold (dB SPL) SD Number of data points
26.00 26.05 3.67 31
28.00 28.98 4.90 30
30.00 31.13 5.11 35
32.00 33.82 6.13 34
34.00 34.73 6.02 21
36.00 36.99 5.22 33
38.00 36.77 3.37 13
40.00 41.28 4.32 25
ing to an N, amplitude of about l-2 /.LV(see also Fig. 1C and Fl). Mean N, VDTs from 0.5-40 kHz are listed in Table I. The mean N, VDTs are compared in Fig. 1A to thresholds measured for single high-spontaneousrate VIIIth nerve fibres with CFs over the same frequency range in the barbiturate-anaesthetized cat (Liberman and Kiang, 1978). From 0.5- = 5 kHz mean N, VDTs were less sensitive (by = 2025 dB from 0.5-4 kHz) than mean single unit thresholds. As discussed below, this may reflect differences in the acoustic impedance of the middle ear according to whether the bulla was sealed (the present study) or open (the single unit study). From 6 kHz on, mean N, VDTs lay within 1 SD of
mean unit thresholds. Although mean N, VDTs at > = 22 kHz were lower than mean frequencies unit thresholds, they were never as low as the lowest single unit thresholds at these frequencies. The mean N, VDTs measured to fixed startingphase tone bursts in this study are compared to thresholds measured using random starting-phase tone bursts and averaging of responses in two previous studies in the anaesthetised cat (Gorga and Abbas, 1981; Price, 1978) in Fig. 1B. In all three studies the N, threshold criterion was a just-noticeable N, response, either in the averaged display (the earlier studies) or in the direct oscilloscope display (the present study). Over the low frequency range from 0.5-4 kHz, the mean VDTs
Fig. 1. Normative N, threshold data in the domestic cat. (A) Mean N, visual detection thresholds (VDTs) compared to single unit thresholds in the barbiturate-anaesthetized cat. Mean unit thresholds (crosses f 1 SD) are from high-spontaneous-rate units while the full line indicates the lowest thresholds recorded for single units in the cat (from Liberman and Kiang, 1978). Liberman and Riang (1978) binned their data for CFs from 0.1-40 kHz into 20 logarithmically-spaced bins. Only the bins relevant to the comparison are illustrated. (B) Mean N, VDTs (filled circles f 1 SD bars) compared to N, thresholds measured with averaging of responses in two previous studies in the cat (Price, 1978; Gorga and Abbas, 1981). The mean thresholds measured by Price (1978) are shown as circles with - 1 SD bars extending down while the mean thresholds measured by Gorga and Abbas (1981) are shown by the crosses with + 1 SD bars extending upwards. In all three studies the criterion for N, threshold was a just-detectable response. (C) Comparison of N, VDTs to thresholds measured with averaging of responses to tone bursts with random starting phase or fixed (zero-crossing) starting phase, in the one animal. The criterion for N, threshold was always a just-detectable response. (D) Effect of bulla condition on VDTs in the one animal. (E) Effect of varying stimulus rise-times on VDTs in the one animal. (F) Comparison of N, VDTs to thresholds measured with averaging of responses to tone bursts with fixed starting phase, with criterion threshold amplitudes as specified in the legend. Fl illustrates the data while F2 illustrates the difference between the VDTs and thresholds obtained at each amplitude criterion with averaging.
156
are consistently higher than the data of Price (1978) and slightly higher than those recorded by Gorga and Abbas (1981). Over the common midfrequency region from = 5-20 kHz, the mean VDTs lie intermediate between the values in the earlier studies. N, thresholds at frequencies > 20 kHz were not measured in the earlier studies. Other than the low-frequency difference noted above, the general similarity between thresholds in the three studies suggests that the SPL calibrations in this study (from calibration of the sound system into a closed-cavity-coupler) can be validly compared to the SPL calibrations made in-situ close to the tympanum by Price (1978) and by Gorga and Abbas (1981). A similar argument can be advanced on the basis of the general similarity (other than at frequencies < 5 kHz) between the mean N, VDTs in this study and the mean single unit thresholds measured by Liberman and Kiang (1978). One possible reasons for the low-frequency difference is that, in the present study, the large cochlear microphonic (CM) recorded with fixed starting-phase tone bursts interfered with clear identification of the N, closer to threshold, resulting in too-conservative estimates of N, thresholds. To determine if this factor accounted for the difference, in one animal N, thresholds were determined visually to fixed starting-phase tone bursts and then by averaging responses to random starting-phase tone bursts to eliminate the CM. The threshold criterion with averaging was also a just-detectable N, response, in the average of 40 sweeps. As shown in Fig. lC, thresholds measured in this manner were considerably elevated over the low frequency range compared to the VDTs. In a second animal similar effects were seen though the differences were not as large. In the guinea pig the stimulus starting-phase has been reported (Dallos and Cheatham, 1976) to be an important factor in eliciting the N, at frequencies < 2 kHz, at which tone bursts with random starting-phase resulted in smaller N, amplitudes. This and other effects (such as the inherent time smear with random-phase tone bursts; e.g., Dallos and Cheatham, 1976) should result in elevated thresholds at low frequencies compared to thresholds measured with fixed starting-phase tone bursts. This was confirmed when, in the same
animal (Fig. lC), N, thresholds were also determined by averaging responses to tone bursts with a fixed starting-phase and with the same threshold criterion of a just-detectable N,, in the average of 40 sweeps. Low-frequency N, thresholds measured in this way were slightly lower than the VDTs and much lower than those determined by averaging responses to tone bursts with random starting phase, although with the attendant problem of a large CM. Thus, the averaging procedure used by Price (1978) and Gorga and Abbas (1981) can not account for the difference in low-frequency N, thresholds noted above. A second possibility lies in the fact that, in the earlier studies, N, thresholds were measured with the bulla open, which in the cat decreases the acoustic impedance and consequently increases transmission at frequencies < = 4 kHz (e.g., Guinan and Peake, 1966; Moller, 1963: Tonndorf and Khanna, 1966). To clarify this. N, VDTs were determined with the bulla open and then with the bulla sealed in the usual manner, in one animal (Fig. 1D). Low frequency VDTs were lower in the open-bulla condition than in the closed-bulla condition, in a frequency-dependent manner consistent with the data obtained in the cat by Guinan and Peake (1966), Msller (1963) and Tonndorf and Khanna (1966). Very similar effects were obtained in another animal. If these corrections are applied to the data in Fig. lB, there are no substantial differences between low frequency thresholds in this study and those recorded by Gorga and Abbas (1981) or between thresholds at frequencies from 2 kHz on in this study and those recorded by Price (1978). Despite the corrections. thresholds at frequencies < 2 kHz in this study and that by Gorga and Abbas (1981) are still higher than those measured by Price (1978). We can suggest no explanation for these differences. However, generally, it is clear that the N, VDTs are similar to thresholds measured with averaging, with the same N, detection criterion. It also appears that generally the three sets of data are comparable despite test conditions (e.g., levels of anaesthesia) presumably being somewhat different, suggesting the generality of our data. (In this regard, we have also obtained similar N, VDTs in ketamine-anaesthetized cats.) Sealing the bulla is reported to result in a
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steady deterioration in cochlear responses, possibly due to build up of ‘negative’ middle ear pressure (Guinan and Peake, 1966). We have found very good stability of N, thresholds during experiments lasting 40 hours or more, indicating that the pressure equalization tube sealed into the bulla opening provides sufficient aeration of the middle ear cavity. The effect of varying stimulus rate and tone burst duration on VDTs was tested with rates of 3, 5 and 7/s, and durations of 10, 30 ,and 50 ms. None of the nine duration-rate combinations produced significant or systematic variations in thresholds. The results are therefore not illustrated. The effect of varying the tone burst rise-time on VDTs was examined from 0.5 to 20 kHz in another animal in which 30-ms tone bursts were presented at 7/s. Rise-times of 0.1, 0.5, 1.0, 1.5 and 2.0 ms were used. Rise-times of 0.1-1.0 ms did not produce different N, VDTs (Fig. 1E). With slower rise-times VDTs were elevated, possibly due to greater desynchronization of afferent responses (Goldstein and Kiang, 1958; Mitchell. 1976a; Ozdamar and Dallos, 1978) with greatest elevations at the slowest rise-time. Thus, a 1-ms rise-time appears to provide a good compromise between avoiding the frequency splatter that occurs with fast rise-times and achieving the synchronization of afferent responses required to elicit an N, response. Although the greater frequency splatter with shorter rise-times did not affect N, thresholds measured with the low-amplitude criterion used here, it may affect thresholds measured with higher-amplitude criteria, since amplitude growth patterns may not be the same at all frequencies. In this context, tone bursts with very fast rise-times may present a problem in measuring N, thresholds in cochleas with losses in sensitivity since the greater frequency splatter could mask the true extent of threshold losses. In such cochlear regions, the effects of frequency splatter with different rise-times may also differ from the effects detailed here. As noted above, the N, VDTs were generally similar to thresholds measured with response averaging in previous studies with the same N, detection criterion. Use of the visual detection method does require considerable experience and this can
be circumvented by using averaging routines. To compare directly the two methods, in one animal N, thresholds were determined visually by one experimenter and then, independently, using averaging techniques, by another experimenter. Responses to 100 tone bursts of fixed starting phase were averaged at each of a number of frequencies using N, threshold-amplitude criteria of 5, 8, 10 and 20 pV. As shown in Fig. lF, increasing the threshold-amplitude criterion resulted in progressively elevated thresholds. With a 5-pV criterion, thresholds were, on average, about 5 dB higher than the VDTs across the entire frequency range. With an 8-PV criterion, thresholds were about 8-9 dB higher than the VDT, with a lo-FV criterion about 11 dB higher and with a 20-PV criterion about 17.5 dB higher. In summary, this report provides a more extensive normative data base for the N, audiogram for the domestic cat than was previously available. Although the visual detection technique used here is simple and reproducible both for a single observer and between experienced observers (Johnstone et al., 1979), its use is dependent upon experience. We have generalized the applicability of these data by comparing the VDTs to the thresholds determined by averaging responses with threshold-amplitude criteria set at easily-detectable levels. Acknowledgements This work was supported by grants from the Australian National Health and Medical Research Council. We thank R. Williams for drawing the figures and V. Kohout for the photographs. References Antoli-Candela, Jr.. F. and Kiang, N.Y.-S. (1978) Unit activity underlying the N, potential. In: R.F. Naunton and C. Fernandez (Eds.), Evoked Electrical Activity in the Auditory Nervous System. Academic Press, New York, pp. 165-191. Dallas, P. and Cheatham. M.A. (1976) Compound action potential (AP) tuning curves. J. Acoust. Sot. Am. 59. 591591. Dallas. P.. Ozdamar, 0. and Ryan. A. (1978) Behavioural, compound action potential. and single unit thresholds: relationship in normal and abnormal ears. J. Acoust. Sot. Am. 64, 151-157.
158 Durrant, J.D., Burns, A. and Ronis. M.L. (1978) Electrocochleographic studies in animals. Adv. Oto-Rhino-Laryngol. 22, 14-23. Eggermont. J.J. (1976) Electrocochleography. In: W.D. Keidel and W.D. Neff (Eds.). Handbook of Sensory Physiology. Vol. V: Auditory System. Springer-Verlag, Berlin, pp. 625705. Goldstein. M.H. and Kiang, N.Y.-S. (1958) Synchrony of neural activity in electric responses evoked by transient acoustic stimuli, J. Acoust. Sot. Am. 30, 107-114. Gorga, M.P. and Abbas. P.J. (1981) AP measurements of short-term adaptation in normal and in acoustically traumatized ears. J. Acoust. Sot. Am. 70, 1310-1321. Guinan, Jr., J.J. and Peake, W.T. (1966) Middle-ear characteristics of anaesthetized cats. J. Acoust. Sot. Am. 41, 12371261. Harris. D.M. (1979) Action potential suppression. tuning curves and thresholds: comparison with single fibre data. Hear. Res. 1. 133-154. Johnstone. J.R., Alder. V.A.. Johnstone, B.M., Robertson. D. and Yates. G.K. (1979) Cochlear action potential threshold and single unit thresholds. J. Acoust. Sot. Am. 65. 254-257. Liherman, M.C. and Kiang. N.Y.-S. (1978) Acoustic trauma in cats. Acta Oto-Laryngol. Suppl. 258. l-63. Mitchell, C. (1976) Frequency specificity of the Nl potential from the cochlear nerve under various stimulus conditions. J. Aud. Res. 16, 247-255. Moller. A.R. (1963) Transfer function of the middle ear. J. Acoust. Sot. Am. 35. 1526-2534.
Ozdamar. 0. and Dallas. P. (1976) Input-output functions of cochlear whole-nerve action potentials: interpretation in terms of one population of neurons. J. Acoust. Sot. Am. 59. 143-147. Ozdamar, 0. and Dallas. P. (1978) Synchronous responses of the primary auditory fibers to the onset of tone bursts and their relation to compound action potentials. Brain Res. 155, 169-175. Price, G.R. (1978) Action potentials in the cat at low sound intensities: thresholds, latencies and rates of change. J. Acoust. Sot. Am. 64. 1400-1405. Prijs. V.F. and Eggermont. J.J. (1981) Narrow-band analysis of compound action potentials for several stimulus conditions in the guinea pig. Hear. Res. 4. 23-41. Rajan, R. (1989) Tonic activity of the crossed olivocochlear bundle in guinea pigs with idiopathic losses in auditory sensitivity. Hear. Res. 39. 299-308. Rhode, W.S. (1976) A digital system for auditory neurophysiological research. In: P. Brown (Ed.), Computer Technology in Neuroscience. Hemisphere. Washington DC, pp. 543567. Sokolich. W.G. (1981) Closed sound delivery system. United States Patent 4.251.686. Tonndorf. J. and Khanna, S.M. (1966) Some properties of sound transmission in the middle and outer ears of cats. J. Acoust. Sot. Am. 41. 513-521. Wise. L.Z. and Irvine, D.R.F. (1983) Auditory response properties of neurons in deep layers of cat superior colliculus. J. Neurophysiol. 49, 674-685.
