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Effects of contralateral noise on measurement of the psychophysical tuning curve
Hearing Research 142 (2000) 63^70 www.elsevier.com/locate/heares
E¡ects of contralateral noise on measurement of the psychophysical tuning curve Tetsuaki Kawase a
a;
*, Masaki Ogura a , Hiroshi Hidaka a , Naoko Sasaki a , Yoªiti Suzuki b , Tomonori Takasaka a
Department of Otolaryngology, Head and Neck Surgery, Tohoku University Graduate School of Medicine, 1-1 Seiryo-machi, Aoba-ku, Sendai 980-8574, Japan b Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8574, Japan Received 14 August 1999; received in revised form 18 November 1999; accepted 27 December 1999
Abstract The effects of the addition of contralateral noise on the psychophysical tuning curve (PTC) were examined in subjects with normal hearing. The masking threshold of the tail part of the PTC tended to decrease with the addition of contralateral noise, although the threshold reduction was usually less than 5 dB. On the other hand, the effects of contralateral noise were relatively small around the tip of the PTC contour. Focusing on the effects of contralateral noise on the masking threshold at the tail part of the PTC, the effects of changing the time between initiation of masking the tone and the presentation of the masked probe tone on the threshold reduction at the tail part of the PTC were also observed. The results indicate that the reduction of the masking threshold by the addition of contralateral noise tended to be larger when the presentation of the signal tone was delayed after the onset of the masker. Usually, when the signal tone was presented under conditions of the forward masking paradigm, the reduction of the threshold was most remarkable. Results obtained in the present study are discussed based on the known characteristics of the olivocochlear (OC)-efferent fibers activated by contralateral noise. ß 2000 Elsevier Science B.V. All rights reserved. Key words: Contra sound; Olivocochlear e¡erent; Psychophysical tuning curve; Human
1. Introduction Based on observation of auditory nerve responses, it has been reported that a sound presented in the contralateral ear can activate the olivocochlear (OC) e¡erent ¢bers (Fex, 1962 ; Cody and Johnstone, 1982; Liberman, 1988 ; Liberman and Brown, 1986; Brown, 1989) and may possibly a¡ect signal perception under the condition of masking (Kawase and Liberman, 1993 ; Kawase et al., 1993) as well as under the condition of quiet (Buno, 1978 ; Liberman, 1989; Warren and Liberman, 1989), as is the case with the electrical stimulation of OC ¢bers (Nieder and Nieder, 1970; Wiederhold and Kiang, 1970; Gi¡ord and Guinan, 1987; Winslow and
* Corresponding author. Tel.: +81 (22) 7177304; Fax: +81 (22) 7177307; E-mail: [email protected]
Sachs, 1987, 1988; Dolan and Nuttall, 1988 ; Guinan and Gi¡ord, 1988a,b). One of the most basic characteristics of the OC-mediated e¡ects on the response of the auditory nerve ¢ber (ANF) is that its activation suppresses the response of the ANFs, which can be seen under conditions of background quiet. These OC-mediated suppression e¡ects are well observable in the response to the CF (characteristic frequency) tone of the ANFs ; however, they are not remarkable for the response to o¡-CF tones (Guinan and Gi¡ord, 1988b). These di¡erences of OC e¡ects according to the signal frequency can be observed as the e¡ects on the tuning function: e¡erent activation elevates the threshold of the tip of the tuning curve with little e¡ect on the tail of the tuning curve. In contrast to the response in quiet, when observing the ANF response under conditions of background masking, auditory nerve responses to the signal are possibly enhanced by the activation of the OC system. Although this enhancement e¡ect is varia-
0378-5955 / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 5 9 5 5 ( 0 0 ) 0 0 0 1 0 - 1
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ble depending on the masking conditions such as masker level and the frequency relation between the signal and the masker, it can be observed in response to the CF tone as well as to the o¡-CF tone (Kawase et al., 1993). Although the e¡ects of OC e¡erent ¢bers activated by contralateral sound are usually smaller than those observed in the case of electrical stimulation (Galambos, 1956; Wiederhold and Kiang, 1970; Gi¡ord and Guinan, 1987; Guinan and Gi¡ord, 1988a), the paradigm using sound-evoked OC-e¡erent ¢bers, in which the sound is alternately on and o¡ in the contralateral ear, is advantageous in that it can be easily applied to measurements in humans. Actually, many studies using this sound evoked paradigm have suggested that it is possible to observe this OC-mediated phenomenon in human subjects as well (Collet et al., 1990; Veuillet et al., 1991 ; Moulin et al., 1993; Williams et al., 1994 ; Kawase and Takasaka, 1995). In the present study, the e¡ects of contralateral sound on auditory sensitivity were examined by measurement of the psychophysical tuning curve (PTC) (Vogten, 1974; Zwicker, 1974; Zwicker and Schorn, 1978). In the measurement of the PTC, the masking thresholds of the maskers, which can just mask the audibility of the low-level probe tone (masked tone), are determined for various frequencies of the masker. The e¡ects of contralateral sound on several combinations of the masker and the masked probe tone could, therefore, be examined. E¡ects of contralateral sound on performance of some psychophysical task may involve binaural interaction in the central auditory system, in addition to peripheral events. Nevertheless, it is important to examine how and to what degree the effects of OC-e¡erent activation can be observed in a psychoacoustical task. The results obtained are discussed based on the known characteristics of the OCe¡ects on nerve physiology and the possible OC-e¡ects involved in the phenomenon are considered. 2. Materials and methods 2.1. Subjects Twelve ears from six healthy subjects (four males and two females, with a mean age of 27.7 years) were observed. No pathological ¢ndings of the tympanic membrane and middle ear were observed by inspection of the tympanic membrane and tympanometric ¢ndings. Standard tonal audiometry carried out in a soundproof room showed no elevation of pure-tone thresholds at the standard test frequencies of 0.25, 0.5, 1, 2, 4 and 8 kHz (within 20 dB HL at all tested frequencies) in any of the subjects.
2.2. Measurements of the acoustic re£ex of the middle ear muscles Contralateral sound stimulation can cause the acoustic re£ex of the middle ear muscles (MEMs). In the present study, the acoustic re£ex threshold (ART) of the MEMs was measured by means of an impedance audiometer (Teledyne Avionics, model TA-2C) with a probe tone of 226 Hz in all the participants. In this device, impedance changes are represented by a compliance value which is shown by an equivalent volume of air in cubic centimeters (cc). Impedance changes induced by contraction of the MEMs were recorded on an X^Y plotter included in this device. Broadband noise, which was used for the sound presented to the contralateral ear to evoke OC activity, was also used as an activator to elicit the AR. 2.3. Measurement of DPOAEs To assess the magnitude of sound-evoked OC e¡ects in each subject, the e¡ects of contralateral sound on the level of DPOAEs (distortion product otoacoustic emissions) at 2f1 3f2 were measured using a measurement system from Etymotic Research (earphone : ER-2; microphone: ER-10B; IBM PC based DSP board: Ariel DSP 16+; software: CUBDIS, v 2.4). Equilevel primaries (L1 = L2 = 55 dB SPL) at a frequency ratio of f2 / f1 = 1.2 were used. As the probe tone at a frequency of 2000 Hz was used for the PTC measurements in the present study, the DPOAEs for the f2 frequency of 2000 Hz were focused on. Broadband noise at 50 dB SPL was used for the sound presented to contralateral ear. The DPOAEs with and 10 DPOAEs without contralateral noise were measured and the average level di¡erences between the two conditions were calculated. 2.4. E¡ects of contralateral noise on PTC To examine the e¡ects of the addition of contralateral noise on the PTC contour, masking thresholds of the masker (1/6 octave band noise) to mask the probe tone (short tone burst) were obtained with the forward masking method as well as by the simultaneous masking method (Vogten, 1974 ; Zwicker, 1974; Zwicker and Schorn, 1978). A schematic paradigm for the presentation of the test signals is shown in Fig. 1. A 2-kHz tone burst (duration 20 ms, rise^fall time 5 ms), 1/6 octave band noise (duration 495 ms, rise-fall time 5 ms) centered at several frequencies (500, 750, 1000, 1500, 1800, 2000 and 3000) and broad band noise (duration 495 ms, rise-fall time 5 ms) were used as a probe (masked) tone, a masker and a noise presented at the contralateral ear (contralateral noise), respectively. A test probe tone at the level of 35 dB SPL was used. As shown in Fig. 1,
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Fig. 1. Schematic paradigm of the tone presentation used in the present study. Contralateral noise was simultaneously presented with the masker. On the other hand, the probe tone to be masked was presented 50 (a), 250 (b) or 450 (c) ms after the onset of the presentation of the masker in the simultaneous masking paradigm, while in the forward masking paradigm, the masked probe tone was presented 5 ms after the masker presentation (d) (see text for further details).
the contralateral noise was simultaneously presented with the masker. The timing of the presentation of the probe tone to be masked was variable, i.e. it was presented 50, 250 or 450 ms after the onset of the presentation of the masker in the simultaneous masking paradigm. On the other hand, in the forward masking paradigm, the masked probe tone was presented 5 ms after the end of the masker presentation. The masking thresholds were determined by the randomized maximum likelihood sequential procedure (RMLSP) (Suzuki et al., 1996), an adaptive method for the estimation of auditory sensitivity. In this method, the sound level of the next stimulus is determined randomly within a certain range centered at the sound level that yields the maximum likelihood calculated from the subject's responses, in contrast with the `original' maximum likelihood sequential procedure (MLSP), in which the next stimulus is simply determined as the highest level of likelihood (Hall, 1968; He et al., 1998). The generation of the stimulus and the sampling of the listener's responses, as well as the execution of the RMLSP method, were conducted by means of an IBMcompatible computer. Tones were produced by a programmable generator (TDT WG2, DD1) and an attenuator (TDT PA4 in series) and were presented after mixing (TDT SM3). A task was given to judge whether the stimulus of the probe tone was audible or not. After the response by the subject (actually this procedure was conducted using a response button), the next set of stimuli were presented after a delay of 200 ms. To determine the next level of masker, the logistic
function was assumed to be a psychometric function for calculating the likelihood in the run. Our psychometric function (PF) was in the form of PF X ; M; S; 1= 1 expf M3X =Sg
1
with X: the level of the stimulus in decibels, M: the mean of the logistic (the point where the logistic function has a value of 0.5) and S: the standard deviation corresponding to the slope of psychometric functions. The stimuli covered a range of 40 dB ( þ 20 dB) in 1-dB steps. The `threshold' was de¢ned as the track point on the psychometric function corresponding to a correctness level of 50% (Levitt, 1971) using the method of maximum likelihood (Ogura et al., 1989) after 50 or 100 trials. The sound pressure level of the masker was not elevated more than the level at which subjects felt it to be uncomfortably loud (usually 95^100 dB SPL). In the case where the masking thresholds could not be determined due to the above-mentioned limitation of the sound pressure level, determination of the masking threshold for that condition was abandoned. These masking threshold measurements to obtain PTCs were examined with and without the presentation of contralateral noise and the e¡ects of the addition of noise in the contralateral ear were evaluated. Measurement of masking thresholds with and without contralateral sound was alternatively done at least 5 times for each condition and the average values were used for data analysis. Broadband noise at a level of 50 dB SPL was used for the contralateral noise. The threshold
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changes caused by the addition of the contralateral noise were statistically analyzed by the t-test for each condition. All parts of the present study were performed in accordance with the guidelines of the Declaration of Helsinki. 3. Results Representative examples of the e¡ects of contralateral noise on the PTC (frequency of the probe tone = 2.0 kHz) are shown in Fig. 2. Typically, in the tail part of the PTC contour, masking thresholds of the test masker decreased signi¢cantly, while on the other hand, the addition of contralateral noise had only a slight e¡ect around the tip frequency. Threshold reductions caused by the addition of contralateral noise are plotted as a function of the frequency of the masker in Fig. 3. A signi¢cant reduction of masking thresholds was constantly observed for the frequency at the tail part. On the other hand, although the e¡ects of contralateral sound on the frequencies around the tip (1800 and 2000 Hz) were not signi¢cant
in most cases, in some cases, the average masking threshold tended to be reduced, while in others, it tended to be elevated. Although these ¢ndings were observed in the PTCs obtained with both forward and simultaneous masking paradigms, the threshold depressions in the tail part obtained with the forward masking paradigm were often larger than those obtained with the simultaneous one. The e¡ects of changing the timing relation between the presentation of the masker and that of the masked probe tone on the masking threshold at the tail part of the PTC with and without the presentation of contralateral noise (probe tone, 2 kHz ; masker, 1 kHz narrow band noise ; background continuous masker, 2 kHz narrow band noise) were examined in seven ears of four subjects, in which relatively large threshold reductions were observed in the tail part of the PTC (Fig. 4). The reduction of the masking threshold by the addition of contralateral noise tended to be larger when the presentation of the signal tone was delayed after the onset of the masker. Usually, when the signal tone was presented at 500 ms after the onset of the presentation of the masker, i.e. a forward masking situation, the reduction of the threshold was most remarkable.
Fig. 2. Typical example of the e¡ects of contralateral noise on the masked PTC (frequency of the probe tone = 2.0 kHz). PTCs measured by means of simultaneous and forward masking paradigm are represented in the top left (A) and top right (B), respectively. The probe tone was presented 450 ms after the onset of the presentation of the masker in the simultaneous masking paradigm. In the bottom panel, the masking threshold changes caused by the addition of contralateral noise are plotted as a function of frequency of the masker. Asterisks indicate the points at which signi¢cant changes were observed in the masking threshold when the contralateral noise was presented. (P 6 0.05, t-test).
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Fig. 4. The e¡ects of changing the timing relation between the presentation of masker and that of the masked probe tone on the masking threshold at the tail part of the PTC with and without the presentation of contralateral noise (probe tone, 2 kHz; masker, 1 kHz narrow band noise). The X-axis and the Y-axis indicate the timing of the presentation of the probe tone (relative time to the onset of the presentation of masker) and the reduction masking threshold, respectively. The data points represented by the ¢lled circles in the ¢gure indicate that the threshold change caused by the addition of contralateral sound was signi¢cant (P 6 0.05, t-test); on the other hand, those shown by open circles indicate that the change was not signi¢cant.
Fig. 3. A threshold reduction caused by the addition of contralateral noise are plotted as a function of the frequency of the masker. The probe tones were presented 450 and 500 ms after the onset of the presentation of the masker in the simultaneous masking paradigm and forward masking paradigm, respectively. A reduction of masking thresholds was constantly observed for the frequency at the tail part. The data points represented by the ¢lled circles in the ¢gure indicate that the threshold change caused by the addition of contralateral sound was signi¢cant (P 6 0.05, t-test); on the other hand, those shown by open circles indicate that the change was not signi¢cant.
In Fig. 5, the magnitude of the reduction of the masking thresholds (in the condition of forward masking) by the addition of contralateral noise is compared with that of depression in DP measurement. A signi¢cantly positive relation between the two factors is observable. The measurements of acoustic re£ex revealed that there was no middle ear muscle contraction in response to the contralateral presentation of the BBN at the level of 50 dB SPL in any subjects in the present study.
Fig. 5. The magnitude of the reduction of the masking thresholds (average threshold reductions for the masker at frequencies of 750 and 1000 Hz in the condition of forward masking) caused by the addition of contralateral noise are compared with those obtained from the contralateral sound e¡ects on DPOAEs (f2 = 2 kHz). A signi¢cantly positive relationship between the two factors seems to be observable.
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Fig. 6. Schematic drawing to consider the results obtained from the masking threshold measurements with the forward masking paradigm from the viewpoint of known OC e¡ects. (For further details, see text.)
4. Discussion In the measurement of PTC, in which the masking threshold to mask the probe tone presented at a low level is obtained, it is thought that the auditory nerve ¢bers whose CFs are near those of the probe tone frequency may play an important role. That is, as long as the level of probe tone is low, only the ANFs whose CFs are around the frequency of the probe tone respond to the probe tone. Therefore, in a sense, the masking threshold of the masker used to mask the audibility of the probe tone in PTC measurement would strongly re£ect the threshold of the masker to mask the excitation of these focused ANFs which respond to the low level CF tone. In Fig. 6, hypothetical schematic drawings explaining the results obtained from the present study based on the known OC-e¡ects are shown. When considering the OC-mediated e¡erent e¡ects elicited by contralateral noise on the PTC measurements, the e¡ects on the measurement of masking thresholds were a¡ected not only via the masker but also via the masked probe tone, i.e. the e¡ects on the masking threshold might re£ect the interaction of both e¡ects. Therefore, as mentioned above, if it can be assumed that the e¡ects on the particular ANFs whose CFs are near the frequency of the probe tone play an important role in PTC measurement (the measurement masking thresholds for low level probe tone), the magnitude of the OC-mediated e¡ects on such measurement re£ects the di¡erent OC e¡ects according to the frequency of the sound. Considering the well-known characteristic of the different OC e¡ects on the responses of ANFs which
varies according to the frequency of the tone, namely, the remarkable e¡erent mediated e¡ects observable not for an o¡-CF tone but rather for the CF tone of the recorded ANF (Guinan and Gi¡ord, 1988b; Warren and Liberman, 1989), contralateral e¡ects would be small when using the o¡-CF tone. On the other hand, OC-mediated contralateral e¡ects were larger for the tone of the same frequency as the probe tone. Therefore, in the case where the o¡-CF tone was used as a masker, the e¡ects of OC-mediated suppression mainly occurred for the probe tone, the masker having little e¡ect (top panel in Fig. 6). This imbalance may cause the masking threshold reduction of the o¡-CF masker by the addition of contralateral noise. On the other hand, when the frequency of the masker is the same as that of the probe tone, OC-mediated suppression e¡ects occur for the masker as well as for the probe tone (bottom panel of Fig. 6). These same directional e¡ects seem to result in little net change in respect of the measurement of masking threshold. It is known that sound evoked OC e¡ects usually grow after the beginning of the presentation of contralateral sound; according to studies in which ANFs were investigated, it takes more than several hundred milliseconds to obtain almost maximum e¡ects (Warren and Liberman, 1989 ; Kawase et al., 1993). The results obtained in the present study, in which the reduction of the masking threshold by the addition of contralateral noise tended to be larger as the presentation of the signal tone was delayed after the onset of the masker, may be explained by the time course of the sound evoked OC e¡ects. In addition, as for the relatively larger e¡ects of con-
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tralateral noise on the masking threshold depression at the tail part of the PTC measured by the forward masking method, the following evidence on the interaction between the OC-mediated suppression and the two-tone suppression may be relevant. According to a report by Winslow and Sachs (1988), the OC-mediated suppressive e¡ects tend to be small when CF tones are suppressed by the o¡-CF suppressor. It is thought that the suppression mechanism is not involved in the forward masking condition but rather in simultaneous masking. Therefore, the di¡erence of the involvement of the suppression mechanism in masking threshold measurement between the two methods may a¡ect the magnitude of the contralateral noise e¡ects on the reduction of the masking threshold. Thus, it appears that the psychoacoustical phenomena obtained in the present study can be explained by the OC e¡ects. In addition, the signi¢cantly positive relation between the e¡ects of contralateral noise on the masking threshold in the tail part of the PTC and those on the level of the DPOAE (Fig. 5) support the hypothesis that the OC-e¡ects are involved in these phenomena. In general, when considering the e¡ects of contralateral noise on the auditory events in the opposite ear, several factors other than OC-mediated e¡ects are possible. Even as a peripheral factor, middle ear muscle contraction elicited by contralateral sound and cross talk of the contralateral sound might be possible. As for the former possibility, if the middle ear muscle re£ex is involved, contraction of muscle would possibly attenuate the sound energy of the low frequency masker. This possibly causes the elevation of the masking thresholds (i.e. e¡ects opposite the phenomenon observed in the present study). Moreover, the acoustic re£ex thresholds of the subjects who participated in the present study were higher than the sound pressure level of the BBN used as contralateral sound. Therefore, involvement of the acoustic re£ex seems to be less possible in the present phenomenon. As for the cross talk e¡ects of the contralateral sound on the opposite ear, it may be possible to decrease the masking thresholds by masking the probe tone directly by the cross talk phenomenon. It appears to be hard to exclude this possibility completely ; however, if these e¡ects are involved in the phenomenon obtained in the present study, the depression e¡ects of contralateral sound application should have been obtained in the tip frequency region as well. The actual signi¢cance of the tail depression of the PTC in the presence of the low level CF tone is not clear. However, considering the important role of the excitation of the ANF for the o¡-CF tone, depression of the tail may play some role in relation to the spread of excitation such as loudness sensation. As is well known, although background broadband masking can
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elevate the thresholds of the signal, the loudness sensation grows more rapidly than normal. As a result, loudness of the high level sound becomes equal to that under conditions of quiet. If the spread of excitation is a key factor in this situation, OC-mediated e¡ects may contribute to this phenomenon. As suggested by the ANF study, in the background masking condition, the tuning function of the masked ANF shows an upward shift not only at the tip but also at the tail. This may reduce the spread of excitation. Therefore, OCmediated tail depression in this situation seems to facilitate the e¡ective spread of excitation. Considerable evidence with regard to the OC-e¡erent system has been revealed. However, the practical role of this system is not yet fully understood. Thus, psychoacoustical approaches to this system, such as the present endeavor, are felt to be important. Acknowledgements The authors wish to thank Professor M.C. Liberman for helpful comments on an earlier version of the manuscript. This study was supported by a grant from the Ministry of Education, Science and Culture (Grant-in Aid for Scienti¢c Research (B) 11557123 and (C) 11671668).
References Brown, M.C., 1989. Morphology and response properties of single olivocochlear ¢bers in the guinea pig. Hear. Res. 40, 93^110. Buno, W., 1978. Auditory nerve activity in£uenced by contralateral ear sound stimulation. Exp. Neurol. 59, 62^74. Cody, A.R., Johnstone, B.M., 1982. Acoustically evoked activity of single e¡erent neurons in the guinea pig cochlea. J. Acoust. Soc. Am. 72, 280^282. Collet, L., Kemp, D.T., Veuillet, E., Duclaux, R., Moulin, A., Morgon, A., 1990. E¡ect of contralateral auditory stimuli on active cochlear micromechanical properties in human subjects. Hear. Res. 43, 251^262. Dolan, D.F., Nuttall, A.L., 1988. Masked cochlear whole-nerve response intensity functions altered by electrical stimulation of the crossed olivocochlear bundle. J. Acoust. Soc. Am. 83, 1081^1086. Fex, J., 1962. Auditory activity in centrifugal and centripetal cochlear ¢bers in cat. Acta Physiol. Scand. 55 (Suppl. 189), 2^68. Galambos, R., 1956. Suppression of auditory-nerve activity by stimulation of e¡erent ¢bers to the cochlea. J. Neurophysiol. 19, 424^ 437. Gi¡ord, M.L., Guinan, J.J., 1987. E¡ects of electrical stimulation of medial olivocochlear neurons on ipsilateral and contralateral cochlear responses. Hear. Res. 29, 179^194. Guinan, J.J., Gi¡ord, M.L., 1988a. E¡ects of electrical stimulation of e¡erent olivocochlear neurons on cat auditory nerve ¢bers. I. Rate-level functions. Hear. Res. 33, 97^114. Guinan, J.J., Gi¡ord, M.L., 1988b. E¡ects of electrical stimulation of e¡erent olivocochlear neurons on cat auditory nerve ¢bers. III. Tuning curve and thresholds at CF. Hear. Res. 37, 29^46.
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Hall, J.L., 1968. Maximum-likelihood sequential procedure for estimation of psychometric functions. J. Acoust. Soc. Am. 44, 370. He, N., Dubno, L.R., Mills, J.H., 1998. Frequency and intensity discrimination measured in a maximum-likelihood procedure from young and aged normal-hearing subjects. J. Acoust. Soc. Am. 103, 553^565. Kawase, T., Liberman, M.C., 1993. Anti-masking e¡ects of the olivocochlear re£ex, I: enhancement of compound action potentials to masked tones. J. Neurophysiol. 70, 2519^2532. Kawase, T., Delgutte, B., Liberman, M.C., 1993. Anti-masking e¡ects of the olivocochlear re£ex, II: enhancement of auditory-nerve responses to masked tones. J. Neurophysiol. 70, 2533^2549. Kawase, T., Takasaka, T., 1995. The e¡ects of contralateral noise on masked compound action potential in humans. Hear. Res. 91, 1^6. Levitt, H., 1971. Transformed up-down procedures in psychoacoustics. J. Acoust. Soc. Am. 49, 467^477. Liberman, M.C., 1988. Response properties of cochlear e¡erent neurons: Monaural vs. binaural stimulation and the e¡ect of noise. J. Neurophysiol. 60, 1779^1798. Liberman, M.C., 1989. Rapid assessment of sound-evoked olivocochlear feedback: suppression of compound action potential by contralateral sound. Hear. Res. 38, 47^56. Liberman, M.C., Brown, M.C., 1986. Physiology and anatomy of single olivocochlear neurons in the cat. Hear. Res. 24, 17^36. Moulin, A., Collet, L., Duclaux, R., 1993. Contralateral auditory stimulation alters acoustic distortion products in humans. Hear. Res. 65, 193^210. Nieder, P., Nieder, I., 1970. Antimasking e¡ects of crossed olivocochlear bundle stimulation with loud clicks in guinea pig. Exp. Neurol. 28, 179^188. Ogura, Y., Sone, T., Suzuki, Y., 1989. On the errors in estimates when the method of maximum likelihood was applied to the results of psychoacoustical experiment obtained by the constant method (in Japanese). J. Acoust. Soc. Jpn. 45, 441^445.
Suzuki, Y., Takeshima, H., Fujii, H., Sone, T., 1996. Equal loudness level measurement by maximum likelihood method sequential procedure. ISO/TC 43/WG1. Veuillet, E., Collet, L., Duclaux, R., 1991. E¡ect of contralateral acoustic stimulation on active cochlear micromechanical properties in human subjects: dependence on stimulus variables. J. Neurophysiol. 65, 724^735. Vogten, L.L.M., 1974. Pure tone masking: a new result from a new method. In: Zwicker, E., Terhardt, E. (Eds.), Facts and Models in Hearing. Springer-Verlag, Berlin, pp. 142^155. Warren, E.H., Liberman, M.C., 1989. E¡ects of contralateral sound on auditory-nerve responses. I. Contributions of cochlear e¡erents. Hear. Res. 37, 89^104. Wiederhold, M.L., Kiang, N.Y.S., 1970. E¡ects of electrical stimulation of the crossed olivocochlear bundle on single auditory nerve ¢bers in cat. J. Acoust. Soc. Am. 48, 950^965. Williams, E.A., Brookers, G.B., Prasher, D.K., 1994. E¡ects of olivocochlear bundle section on otoacoustic emissions in humans: Efferent e¡ects in comparison with control subjects. Acta Otolaryngol. 114, 121^129. Winslow, R.L., Sachs, M.B., 1987. E¡ect of electrical stimulation of the crossed olivocochlear bundle on auditory nerve response to tone in noise. J. Neurophysiol. 57, 1002^1021. Winslow, R.L., Sachs, M.B., 1988. Single-tone intensity discrimination based on auditory nerve rate responses in backgrounds of quiet, noise and with stimulation of crossed olivocochlear bundle. Hear. Res. 35, 165^190. Zwicker, E., 1974. On a psychoacoustical equivalent of tuning curves. In: Zwicker, E., Terhardt, E. (Eds.), Facts and Models in Hearing. Springer, Berlin, pp. 132^141. Zwicker, E., Zwicker, K., 1978. Psychoacoustical tuning curves in audiology. Audiology 17, 120^140.
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E¡ects of contralateral noise on measurement of the psychophysical tuning curve Tetsuaki Kawase a
a;
*, Masaki Ogura a , Hiroshi Hidaka a , Naoko Sasaki a , Yoªiti Suzuki b , Tomonori Takasaka a
Department of Otolaryngology, Head and Neck Surgery, Tohoku University Graduate School of Medicine, 1-1 Seiryo-machi, Aoba-ku, Sendai 980-8574, Japan b Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8574, Japan Received 14 August 1999; received in revised form 18 November 1999; accepted 27 December 1999
Abstract The effects of the addition of contralateral noise on the psychophysical tuning curve (PTC) were examined in subjects with normal hearing. The masking threshold of the tail part of the PTC tended to decrease with the addition of contralateral noise, although the threshold reduction was usually less than 5 dB. On the other hand, the effects of contralateral noise were relatively small around the tip of the PTC contour. Focusing on the effects of contralateral noise on the masking threshold at the tail part of the PTC, the effects of changing the time between initiation of masking the tone and the presentation of the masked probe tone on the threshold reduction at the tail part of the PTC were also observed. The results indicate that the reduction of the masking threshold by the addition of contralateral noise tended to be larger when the presentation of the signal tone was delayed after the onset of the masker. Usually, when the signal tone was presented under conditions of the forward masking paradigm, the reduction of the threshold was most remarkable. Results obtained in the present study are discussed based on the known characteristics of the olivocochlear (OC)-efferent fibers activated by contralateral noise. ß 2000 Elsevier Science B.V. All rights reserved. Key words: Contra sound; Olivocochlear e¡erent; Psychophysical tuning curve; Human
1. Introduction Based on observation of auditory nerve responses, it has been reported that a sound presented in the contralateral ear can activate the olivocochlear (OC) e¡erent ¢bers (Fex, 1962 ; Cody and Johnstone, 1982; Liberman, 1988 ; Liberman and Brown, 1986; Brown, 1989) and may possibly a¡ect signal perception under the condition of masking (Kawase and Liberman, 1993 ; Kawase et al., 1993) as well as under the condition of quiet (Buno, 1978 ; Liberman, 1989; Warren and Liberman, 1989), as is the case with the electrical stimulation of OC ¢bers (Nieder and Nieder, 1970; Wiederhold and Kiang, 1970; Gi¡ord and Guinan, 1987; Winslow and
* Corresponding author. Tel.: +81 (22) 7177304; Fax: +81 (22) 7177307; E-mail: [email protected]
Sachs, 1987, 1988; Dolan and Nuttall, 1988 ; Guinan and Gi¡ord, 1988a,b). One of the most basic characteristics of the OC-mediated e¡ects on the response of the auditory nerve ¢ber (ANF) is that its activation suppresses the response of the ANFs, which can be seen under conditions of background quiet. These OC-mediated suppression e¡ects are well observable in the response to the CF (characteristic frequency) tone of the ANFs ; however, they are not remarkable for the response to o¡-CF tones (Guinan and Gi¡ord, 1988b). These di¡erences of OC e¡ects according to the signal frequency can be observed as the e¡ects on the tuning function: e¡erent activation elevates the threshold of the tip of the tuning curve with little e¡ect on the tail of the tuning curve. In contrast to the response in quiet, when observing the ANF response under conditions of background masking, auditory nerve responses to the signal are possibly enhanced by the activation of the OC system. Although this enhancement e¡ect is varia-
0378-5955 / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 5 9 5 5 ( 0 0 ) 0 0 0 1 0 - 1
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ble depending on the masking conditions such as masker level and the frequency relation between the signal and the masker, it can be observed in response to the CF tone as well as to the o¡-CF tone (Kawase et al., 1993). Although the e¡ects of OC e¡erent ¢bers activated by contralateral sound are usually smaller than those observed in the case of electrical stimulation (Galambos, 1956; Wiederhold and Kiang, 1970; Gi¡ord and Guinan, 1987; Guinan and Gi¡ord, 1988a), the paradigm using sound-evoked OC-e¡erent ¢bers, in which the sound is alternately on and o¡ in the contralateral ear, is advantageous in that it can be easily applied to measurements in humans. Actually, many studies using this sound evoked paradigm have suggested that it is possible to observe this OC-mediated phenomenon in human subjects as well (Collet et al., 1990; Veuillet et al., 1991 ; Moulin et al., 1993; Williams et al., 1994 ; Kawase and Takasaka, 1995). In the present study, the e¡ects of contralateral sound on auditory sensitivity were examined by measurement of the psychophysical tuning curve (PTC) (Vogten, 1974; Zwicker, 1974; Zwicker and Schorn, 1978). In the measurement of the PTC, the masking thresholds of the maskers, which can just mask the audibility of the low-level probe tone (masked tone), are determined for various frequencies of the masker. The e¡ects of contralateral sound on several combinations of the masker and the masked probe tone could, therefore, be examined. E¡ects of contralateral sound on performance of some psychophysical task may involve binaural interaction in the central auditory system, in addition to peripheral events. Nevertheless, it is important to examine how and to what degree the effects of OC-e¡erent activation can be observed in a psychoacoustical task. The results obtained are discussed based on the known characteristics of the OCe¡ects on nerve physiology and the possible OC-e¡ects involved in the phenomenon are considered. 2. Materials and methods 2.1. Subjects Twelve ears from six healthy subjects (four males and two females, with a mean age of 27.7 years) were observed. No pathological ¢ndings of the tympanic membrane and middle ear were observed by inspection of the tympanic membrane and tympanometric ¢ndings. Standard tonal audiometry carried out in a soundproof room showed no elevation of pure-tone thresholds at the standard test frequencies of 0.25, 0.5, 1, 2, 4 and 8 kHz (within 20 dB HL at all tested frequencies) in any of the subjects.
2.2. Measurements of the acoustic re£ex of the middle ear muscles Contralateral sound stimulation can cause the acoustic re£ex of the middle ear muscles (MEMs). In the present study, the acoustic re£ex threshold (ART) of the MEMs was measured by means of an impedance audiometer (Teledyne Avionics, model TA-2C) with a probe tone of 226 Hz in all the participants. In this device, impedance changes are represented by a compliance value which is shown by an equivalent volume of air in cubic centimeters (cc). Impedance changes induced by contraction of the MEMs were recorded on an X^Y plotter included in this device. Broadband noise, which was used for the sound presented to the contralateral ear to evoke OC activity, was also used as an activator to elicit the AR. 2.3. Measurement of DPOAEs To assess the magnitude of sound-evoked OC e¡ects in each subject, the e¡ects of contralateral sound on the level of DPOAEs (distortion product otoacoustic emissions) at 2f1 3f2 were measured using a measurement system from Etymotic Research (earphone : ER-2; microphone: ER-10B; IBM PC based DSP board: Ariel DSP 16+; software: CUBDIS, v 2.4). Equilevel primaries (L1 = L2 = 55 dB SPL) at a frequency ratio of f2 / f1 = 1.2 were used. As the probe tone at a frequency of 2000 Hz was used for the PTC measurements in the present study, the DPOAEs for the f2 frequency of 2000 Hz were focused on. Broadband noise at 50 dB SPL was used for the sound presented to contralateral ear. The DPOAEs with and 10 DPOAEs without contralateral noise were measured and the average level di¡erences between the two conditions were calculated. 2.4. E¡ects of contralateral noise on PTC To examine the e¡ects of the addition of contralateral noise on the PTC contour, masking thresholds of the masker (1/6 octave band noise) to mask the probe tone (short tone burst) were obtained with the forward masking method as well as by the simultaneous masking method (Vogten, 1974 ; Zwicker, 1974; Zwicker and Schorn, 1978). A schematic paradigm for the presentation of the test signals is shown in Fig. 1. A 2-kHz tone burst (duration 20 ms, rise^fall time 5 ms), 1/6 octave band noise (duration 495 ms, rise-fall time 5 ms) centered at several frequencies (500, 750, 1000, 1500, 1800, 2000 and 3000) and broad band noise (duration 495 ms, rise-fall time 5 ms) were used as a probe (masked) tone, a masker and a noise presented at the contralateral ear (contralateral noise), respectively. A test probe tone at the level of 35 dB SPL was used. As shown in Fig. 1,
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Fig. 1. Schematic paradigm of the tone presentation used in the present study. Contralateral noise was simultaneously presented with the masker. On the other hand, the probe tone to be masked was presented 50 (a), 250 (b) or 450 (c) ms after the onset of the presentation of the masker in the simultaneous masking paradigm, while in the forward masking paradigm, the masked probe tone was presented 5 ms after the masker presentation (d) (see text for further details).
the contralateral noise was simultaneously presented with the masker. The timing of the presentation of the probe tone to be masked was variable, i.e. it was presented 50, 250 or 450 ms after the onset of the presentation of the masker in the simultaneous masking paradigm. On the other hand, in the forward masking paradigm, the masked probe tone was presented 5 ms after the end of the masker presentation. The masking thresholds were determined by the randomized maximum likelihood sequential procedure (RMLSP) (Suzuki et al., 1996), an adaptive method for the estimation of auditory sensitivity. In this method, the sound level of the next stimulus is determined randomly within a certain range centered at the sound level that yields the maximum likelihood calculated from the subject's responses, in contrast with the `original' maximum likelihood sequential procedure (MLSP), in which the next stimulus is simply determined as the highest level of likelihood (Hall, 1968; He et al., 1998). The generation of the stimulus and the sampling of the listener's responses, as well as the execution of the RMLSP method, were conducted by means of an IBMcompatible computer. Tones were produced by a programmable generator (TDT WG2, DD1) and an attenuator (TDT PA4 in series) and were presented after mixing (TDT SM3). A task was given to judge whether the stimulus of the probe tone was audible or not. After the response by the subject (actually this procedure was conducted using a response button), the next set of stimuli were presented after a delay of 200 ms. To determine the next level of masker, the logistic
function was assumed to be a psychometric function for calculating the likelihood in the run. Our psychometric function (PF) was in the form of PF X ; M; S; 1= 1 expf M3X =Sg
1
with X: the level of the stimulus in decibels, M: the mean of the logistic (the point where the logistic function has a value of 0.5) and S: the standard deviation corresponding to the slope of psychometric functions. The stimuli covered a range of 40 dB ( þ 20 dB) in 1-dB steps. The `threshold' was de¢ned as the track point on the psychometric function corresponding to a correctness level of 50% (Levitt, 1971) using the method of maximum likelihood (Ogura et al., 1989) after 50 or 100 trials. The sound pressure level of the masker was not elevated more than the level at which subjects felt it to be uncomfortably loud (usually 95^100 dB SPL). In the case where the masking thresholds could not be determined due to the above-mentioned limitation of the sound pressure level, determination of the masking threshold for that condition was abandoned. These masking threshold measurements to obtain PTCs were examined with and without the presentation of contralateral noise and the e¡ects of the addition of noise in the contralateral ear were evaluated. Measurement of masking thresholds with and without contralateral sound was alternatively done at least 5 times for each condition and the average values were used for data analysis. Broadband noise at a level of 50 dB SPL was used for the contralateral noise. The threshold
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changes caused by the addition of the contralateral noise were statistically analyzed by the t-test for each condition. All parts of the present study were performed in accordance with the guidelines of the Declaration of Helsinki. 3. Results Representative examples of the e¡ects of contralateral noise on the PTC (frequency of the probe tone = 2.0 kHz) are shown in Fig. 2. Typically, in the tail part of the PTC contour, masking thresholds of the test masker decreased signi¢cantly, while on the other hand, the addition of contralateral noise had only a slight e¡ect around the tip frequency. Threshold reductions caused by the addition of contralateral noise are plotted as a function of the frequency of the masker in Fig. 3. A signi¢cant reduction of masking thresholds was constantly observed for the frequency at the tail part. On the other hand, although the e¡ects of contralateral sound on the frequencies around the tip (1800 and 2000 Hz) were not signi¢cant
in most cases, in some cases, the average masking threshold tended to be reduced, while in others, it tended to be elevated. Although these ¢ndings were observed in the PTCs obtained with both forward and simultaneous masking paradigms, the threshold depressions in the tail part obtained with the forward masking paradigm were often larger than those obtained with the simultaneous one. The e¡ects of changing the timing relation between the presentation of the masker and that of the masked probe tone on the masking threshold at the tail part of the PTC with and without the presentation of contralateral noise (probe tone, 2 kHz ; masker, 1 kHz narrow band noise ; background continuous masker, 2 kHz narrow band noise) were examined in seven ears of four subjects, in which relatively large threshold reductions were observed in the tail part of the PTC (Fig. 4). The reduction of the masking threshold by the addition of contralateral noise tended to be larger when the presentation of the signal tone was delayed after the onset of the masker. Usually, when the signal tone was presented at 500 ms after the onset of the presentation of the masker, i.e. a forward masking situation, the reduction of the threshold was most remarkable.
Fig. 2. Typical example of the e¡ects of contralateral noise on the masked PTC (frequency of the probe tone = 2.0 kHz). PTCs measured by means of simultaneous and forward masking paradigm are represented in the top left (A) and top right (B), respectively. The probe tone was presented 450 ms after the onset of the presentation of the masker in the simultaneous masking paradigm. In the bottom panel, the masking threshold changes caused by the addition of contralateral noise are plotted as a function of frequency of the masker. Asterisks indicate the points at which signi¢cant changes were observed in the masking threshold when the contralateral noise was presented. (P 6 0.05, t-test).
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Fig. 4. The e¡ects of changing the timing relation between the presentation of masker and that of the masked probe tone on the masking threshold at the tail part of the PTC with and without the presentation of contralateral noise (probe tone, 2 kHz; masker, 1 kHz narrow band noise). The X-axis and the Y-axis indicate the timing of the presentation of the probe tone (relative time to the onset of the presentation of masker) and the reduction masking threshold, respectively. The data points represented by the ¢lled circles in the ¢gure indicate that the threshold change caused by the addition of contralateral sound was signi¢cant (P 6 0.05, t-test); on the other hand, those shown by open circles indicate that the change was not signi¢cant.
Fig. 3. A threshold reduction caused by the addition of contralateral noise are plotted as a function of the frequency of the masker. The probe tones were presented 450 and 500 ms after the onset of the presentation of the masker in the simultaneous masking paradigm and forward masking paradigm, respectively. A reduction of masking thresholds was constantly observed for the frequency at the tail part. The data points represented by the ¢lled circles in the ¢gure indicate that the threshold change caused by the addition of contralateral sound was signi¢cant (P 6 0.05, t-test); on the other hand, those shown by open circles indicate that the change was not signi¢cant.
In Fig. 5, the magnitude of the reduction of the masking thresholds (in the condition of forward masking) by the addition of contralateral noise is compared with that of depression in DP measurement. A signi¢cantly positive relation between the two factors is observable. The measurements of acoustic re£ex revealed that there was no middle ear muscle contraction in response to the contralateral presentation of the BBN at the level of 50 dB SPL in any subjects in the present study.
Fig. 5. The magnitude of the reduction of the masking thresholds (average threshold reductions for the masker at frequencies of 750 and 1000 Hz in the condition of forward masking) caused by the addition of contralateral noise are compared with those obtained from the contralateral sound e¡ects on DPOAEs (f2 = 2 kHz). A signi¢cantly positive relationship between the two factors seems to be observable.
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Fig. 6. Schematic drawing to consider the results obtained from the masking threshold measurements with the forward masking paradigm from the viewpoint of known OC e¡ects. (For further details, see text.)
4. Discussion In the measurement of PTC, in which the masking threshold to mask the probe tone presented at a low level is obtained, it is thought that the auditory nerve ¢bers whose CFs are near those of the probe tone frequency may play an important role. That is, as long as the level of probe tone is low, only the ANFs whose CFs are around the frequency of the probe tone respond to the probe tone. Therefore, in a sense, the masking threshold of the masker used to mask the audibility of the probe tone in PTC measurement would strongly re£ect the threshold of the masker to mask the excitation of these focused ANFs which respond to the low level CF tone. In Fig. 6, hypothetical schematic drawings explaining the results obtained from the present study based on the known OC-e¡ects are shown. When considering the OC-mediated e¡erent e¡ects elicited by contralateral noise on the PTC measurements, the e¡ects on the measurement of masking thresholds were a¡ected not only via the masker but also via the masked probe tone, i.e. the e¡ects on the masking threshold might re£ect the interaction of both e¡ects. Therefore, as mentioned above, if it can be assumed that the e¡ects on the particular ANFs whose CFs are near the frequency of the probe tone play an important role in PTC measurement (the measurement masking thresholds for low level probe tone), the magnitude of the OC-mediated e¡ects on such measurement re£ects the di¡erent OC e¡ects according to the frequency of the sound. Considering the well-known characteristic of the different OC e¡ects on the responses of ANFs which
varies according to the frequency of the tone, namely, the remarkable e¡erent mediated e¡ects observable not for an o¡-CF tone but rather for the CF tone of the recorded ANF (Guinan and Gi¡ord, 1988b; Warren and Liberman, 1989), contralateral e¡ects would be small when using the o¡-CF tone. On the other hand, OC-mediated contralateral e¡ects were larger for the tone of the same frequency as the probe tone. Therefore, in the case where the o¡-CF tone was used as a masker, the e¡ects of OC-mediated suppression mainly occurred for the probe tone, the masker having little e¡ect (top panel in Fig. 6). This imbalance may cause the masking threshold reduction of the o¡-CF masker by the addition of contralateral noise. On the other hand, when the frequency of the masker is the same as that of the probe tone, OC-mediated suppression e¡ects occur for the masker as well as for the probe tone (bottom panel of Fig. 6). These same directional e¡ects seem to result in little net change in respect of the measurement of masking threshold. It is known that sound evoked OC e¡ects usually grow after the beginning of the presentation of contralateral sound; according to studies in which ANFs were investigated, it takes more than several hundred milliseconds to obtain almost maximum e¡ects (Warren and Liberman, 1989 ; Kawase et al., 1993). The results obtained in the present study, in which the reduction of the masking threshold by the addition of contralateral noise tended to be larger as the presentation of the signal tone was delayed after the onset of the masker, may be explained by the time course of the sound evoked OC e¡ects. In addition, as for the relatively larger e¡ects of con-
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tralateral noise on the masking threshold depression at the tail part of the PTC measured by the forward masking method, the following evidence on the interaction between the OC-mediated suppression and the two-tone suppression may be relevant. According to a report by Winslow and Sachs (1988), the OC-mediated suppressive e¡ects tend to be small when CF tones are suppressed by the o¡-CF suppressor. It is thought that the suppression mechanism is not involved in the forward masking condition but rather in simultaneous masking. Therefore, the di¡erence of the involvement of the suppression mechanism in masking threshold measurement between the two methods may a¡ect the magnitude of the contralateral noise e¡ects on the reduction of the masking threshold. Thus, it appears that the psychoacoustical phenomena obtained in the present study can be explained by the OC e¡ects. In addition, the signi¢cantly positive relation between the e¡ects of contralateral noise on the masking threshold in the tail part of the PTC and those on the level of the DPOAE (Fig. 5) support the hypothesis that the OC-e¡ects are involved in these phenomena. In general, when considering the e¡ects of contralateral noise on the auditory events in the opposite ear, several factors other than OC-mediated e¡ects are possible. Even as a peripheral factor, middle ear muscle contraction elicited by contralateral sound and cross talk of the contralateral sound might be possible. As for the former possibility, if the middle ear muscle re£ex is involved, contraction of muscle would possibly attenuate the sound energy of the low frequency masker. This possibly causes the elevation of the masking thresholds (i.e. e¡ects opposite the phenomenon observed in the present study). Moreover, the acoustic re£ex thresholds of the subjects who participated in the present study were higher than the sound pressure level of the BBN used as contralateral sound. Therefore, involvement of the acoustic re£ex seems to be less possible in the present phenomenon. As for the cross talk e¡ects of the contralateral sound on the opposite ear, it may be possible to decrease the masking thresholds by masking the probe tone directly by the cross talk phenomenon. It appears to be hard to exclude this possibility completely ; however, if these e¡ects are involved in the phenomenon obtained in the present study, the depression e¡ects of contralateral sound application should have been obtained in the tip frequency region as well. The actual signi¢cance of the tail depression of the PTC in the presence of the low level CF tone is not clear. However, considering the important role of the excitation of the ANF for the o¡-CF tone, depression of the tail may play some role in relation to the spread of excitation such as loudness sensation. As is well known, although background broadband masking can
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elevate the thresholds of the signal, the loudness sensation grows more rapidly than normal. As a result, loudness of the high level sound becomes equal to that under conditions of quiet. If the spread of excitation is a key factor in this situation, OC-mediated e¡ects may contribute to this phenomenon. As suggested by the ANF study, in the background masking condition, the tuning function of the masked ANF shows an upward shift not only at the tip but also at the tail. This may reduce the spread of excitation. Therefore, OCmediated tail depression in this situation seems to facilitate the e¡ective spread of excitation. Considerable evidence with regard to the OC-e¡erent system has been revealed. However, the practical role of this system is not yet fully understood. Thus, psychoacoustical approaches to this system, such as the present endeavor, are felt to be important. Acknowledgements The authors wish to thank Professor M.C. Liberman for helpful comments on an earlier version of the manuscript. This study was supported by a grant from the Ministry of Education, Science and Culture (Grant-in Aid for Scienti¢c Research (B) 11557123 and (C) 11671668).
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