Magdalena Wojtczak, Anna C. Schroder, Ying-Yee Kong, and David A. Nelson

Transcription

1 EFFECT OF BASILAR MEMBRANE COMPRESSION ON MASKING PERIOD PATTERNS (MPPS) A aer resented to the 138 th Acoustical Society of Aerica Colubus, Ohio Noveber 2, 1999 Magdalena Wojtczak, Anna C. Schroder, Ying-Yee Kong, and David A. Nelson Clinical Psychoacoustics Laboratory Deartent of Otolaryngology University of Minnesota Send corresondence to: Magdalena Wojtczak, Ph.D. 396 UMHC R Delaware St. S.E. Minneaolis, MN Phone: (612) Fax: (612) Eail: Magdalena.Wojtczak-1@tc.un.edu Page 1

2 ABSTRACT MPPs were easured in listeners with noral and iaired hearing using tonal 4-Hz SAM askers and short tonal robes with frequencies that were either identical to or higher than the carrier frequency of the asker. The robe frequencies were 5, 12, and 3 Hz for on-frequency asking, and 12, 24, and 6 Hz for off-frequency asking. In noral-hearing listeners MPPs easured with offfrequency robes had valleys that were uch longer and deeer than valleys observed with on-frequency robes. A siilar result was observed in hearing-iaired listeners in the frequency region of ild hearing losses, where significant residual coression was resuably oerating. However, in the frequency region with substantial hearing loss where coression is substantially reduced or absent, MPPs easured with the on- and off-frequency robes were very siilar. A odel consisting of eriheral filtering, coressive nonlinearity, and a sliding teoral window was used in an attet to redict the data and to estiate the coression index. The results suggest that the siilarity of on- and off-frequency teoral resolution in hearing-iaired listeners ay be due in art to the lack of the coressive nonlinearity that is evident at the level of the basilar ebrane in noral-hearing listeners. [Work suorted by NIH- NIDCD grant DC149 and the Lion s 5M International Hearing Foundation.] INTRODUCTION Masking eriod atterns easured with askers odulated at different rates rovide inforation about teoral rocessing in the auditory syste. A asking eriod attern is obtained when detection of a short robe is easured as a function of the teoral osition of the robe within a odulation cycle of a asker. As the odulation rate of a asker increases, detection of the robe in a valley of the asker enveloe becoes ore difficult and a higher robe level is necessary to reach the threshold. Detection of the robe resented during the eak reains unchanged. Thus, the difference between the detection threshold of the robe easured at a eak and the detection threshold easured at a valley decreases with increasing odulation rate of the asker, reflecting deteriorated teoral resolution. Fig. 1 shows a robe resented during a valley of a asker odulated at two different rates (uer anels). Lower anels of the figure show hyothetical asking eriod atterns easured over two odulation cycles of the asker shown above the. Higher odulation rate roduces a asking attern with shallower valleys. Zwicker (1976), Nelson and Swain (1996) and recently Gregan et al. (1998) easured asking eriod atterns using asker frequencies that were equal to the frequency of a robe (on-frequency askers) and asker frequencies that were lower than the frequency of a robe (off-frequency askers). Their data indicated that for a given odulation rate, and a given odulation deth of the asker, deeer valleys and larger eak-valley differences are observed in asking eriod atterns easured with off-frequency askers than in asking eriod atterns easured with on-frequency askers. Based on this observation they concluded that teoral resolution is better in the uer accessory excitation than it is in the ain excitation. The ai of the resent study was to deonstrate that the better teoral resolution observed with a lower-frequency (off-frequency) asker reflects coressive nonlinearity of auditory rocessing. A odel, in which a linear resonse to the low-frequency asker at the robe-frequency lace and a coressed resonse to the on-frequency asker and the robe were assued, was used to redict asking eriod atterns easured with an on- and off-frequency asker. The sae teoral window was used in both onand off-frequency asking situations. EXPERIMENT Masking eriod atterns were easured in noral hearing listeners. In a 3AFC task, listeners were Page 2

3 Page 3 Wojtczak, Schroder, Kong and Nelson, JASA (1999) asked to detect a short robe in the resence of a longer odulated asker. The robe was a 6-kHz tone burst resented for a duration of 8 s, including 2-s raised-cosine ras. The teoral osition of the robe within a cycle of the asker enveloe was varied across blocks of trials. Carrier frequencies of 3 khz for the off-frequency asker and 6 khz for the on-frequency asker were used in the exerient. The asker was resented for 5 s and was 1%-odulated at a rate of 4 Hz. Masking eriod atterns were easured using four levels of the asker: 65, 75, 85 and 9 db. Not every listener was available for all conditions. RESULTS Masking eriod atterns easured with a 3-kHz asker and a 6-kHz robe are shown in Fig. 2. Different anels contain data collected at different asker levels, as indicated in each anel. As shown in the figure, asking atterns observed with the off-frequency asker have wide and dee clearly defined valleys. For low asker levels the thresholds are liited by the absolute threshold of the robe. For higher asker levels the difference between the threshold easured at the eak and at the valley of the odulated asker is larger than it is with an on-frequency asker, as shown by the data resented in Fig. 3. Masking eriod atterns easured with the 6-kHz (on-frequency) asker have very narrow valleys and the eak-to-valley differences are generally sall coared with the differences shown for the offfrequency asker. Narrow and shallow valleys of the asking eriod atterns suggest that teoral rocessing is deteriorated when the asker and the robe have the sae frequencies coared with the case where the asker has a uch lower frequency than the robe. The sae general result was shown by Zwicker (1976), Nelson ands Swain (1996) and Gregan et al. (1998), although the shaes of the MPPs are defined in soewhat greater detail here. PREDICTIONS A odel roosed by Oxenha and Moore (1994) was used to redict the easured asking eriod atterns. The odel consisted of eriheral filtering followed by half-wave rectification and coression, which were in turn followed by a teoral window, and a decision device. A teoral window roosed by Moore et al. (1988), each side of which was defined by two ROEX functions, was used to siulate teoral rocessing. To aly the odel to the situation where the asker and the signal have different frequencies and overla in tie, soe assutions needed to be ade. Basilar-ebrane echanical data (Ruggero, 1992; Ruggero et al., 1997) and sychohysical data of Oxenha and Plack (1997) indicate that when the asker frequency f and the robe frequency f r are equal, the resonse to the two stiuli resented siultaneously can be written as R = a ( X + X ) + r r, where X +X r is the half-wave rectified su of the asker (x ) and the robe (x r ), and is the coression exonent for the robe and the on-frequency asker. The resonse to the asker alone is R = a. X The odel assues that the robe is detected in the resence of the asker when the resonse to the asker lus robe, at the outut of the teoral window, exceeds the resonse to the asker alone at the outut of the teoral window by soe constant aount in db (with criterion K). This can be written as TW{( X + X ) log TW{ X } } = K r 2, (1)

4 where TW{X} denotes a stiulus X at the outut of the teoral window. Page 4 Wojtczak, Schroder, Kong and Nelson, JASA (1999) When the frequency of the asker is lower than the frequency of the robe (f <f r ), the resonse to the asker at the robe frequency lace can be exressed as R q b X =, (2) where q is the exonent characterizing the growth of the resonse to the asker at the robe frequency lace. Because the frequency of the asker was uch lower than the frequency of the robe, q was assued to be equal 1 in the calculations of the redicted asking eriod atterns. It was further assued that the sae resonse (R ) could be roduced by a signal (x s ), whose frequency is the sae as the frequency of the robe (f s =f r ), and therefore the signal is subject to the sae coression as the robe. For such a signal R a X s =. (3) Fro Eqs.(2) and (3), it is ossible to find the alitude of such a signal and exress it in ters of the asker X s b ( ) a 1/ q / X. =. (4) The robe will be detected in the resence of signal x s, when the resonse to both stiuli exceeds the criterion K. Using Eq.(1) for the criterion, substituting Eq.(4) for X s and setting C=b/a roduces the following forula for the criterion: 1/ q / TW{( C X + X r ) 2 log 1/ q / TW{( C X ) } } = K. (5) The robe was detected when its alitude x r reached a value for which the criterion K was fulfilled. This aroach is very siilar to the aroach reresented by the odel roosed by Goldstein (199). For each listener, only one condition (the off-frequency asker resented at the highest level at which a given listener was tested) was siulated using five free araeters, four araeters of the teoral window and the coression exonent. The araeters of the teoral window were then ket the sae for all the reaining conditions. All the reaining off-frequency siulations were run with two free araeters (the constant C and the coression exonent ), and the on-frequency siulations were run with one free araeter (the coression exonent ). One of the standard Matlab otiization rocedures (by Nelder and Mead) was used to iniize the su of the squared deviations between the data and the redictions. Fig. 4 shows the data (sybols) and the redictions (solid lines) for the 3-kHz asker. Each anel shows the data and the redictions for a different subject. The agreeent between the data and the redictions is very good. However the redictions were not as accurate for the on-frequency asker, as is shown for one listener in Fig. 5. The greatest discreancy between the redictions and the data was observed near the botto of the valley. The coression exonents resulting fro the on-frequency siulations were also higher than could be exected based on the results for the off-frequency siulations and higher than the exonents reorted reviously for cochlear coression. Because no highass noise was used in the exerient to revent off-frequency listening, it is likely that the listeners detected the robe through an auditory channel tuned to a higher frequency region. This roble was not taken into consideration by the odel. Such off-frequency listening would ost likely affect thresholds near the eak

5 significantly ore than it would affect thresholds near the valley. Therefore, siulations for the onfrequency asker were rerun with the su of the squared deviations between the data and the redictions couted for all the data oints excet four oints near each eak of the asking eriod atterns. Those siulations led to the redictions shown in Fig. 6. There is uch better agreeent between the redictions and the data within the valley of each asking eriod attern, but the discreancy between the data and the redictions increases when the robe is oved closer towards each eak of the asking eriod atterns. Also, the coression exonents resulting fro those siulations were ore siilar to the coression exonents observed with the off-frequency asker. The araeters of the teoral window and the coression exonents resulting fro the siulations were siilar to those used by Oxenha and Moore (1994) to fit their cobined forward and backward asking data. Table I shows the coression exonents for all levels of the on- and off-frequency askers used in the exerient and in the siulations. Higher coression exonents were observed for the offfrequency asker resented at 65 db SPL. This could be exected, because the asker resented at such a low level did not roduce uch asking at the robe-frequency lace, and therefore, the level of the robe at threshold easured and couted at the eak of the odulated asker was very low. For low levels the resonse to the robe is believed to be nearly linear. Overall, the odel roduces very good redictions for the off-frequency asker and reasonably good redictions for the on-frequency asker, suggesting that the difference in the shae of asking eriod atterns easured with an on- and off-frequency asker is ainly due to the difference in the rate of resonse growth resulting fro the on- and off-frequency stiulation. It is not clear whether the sychohysically observed coression directly reflects coression of the basilar-ebrane echanical resonse. The coression exonents obtained fro the siulations were higher than those reorted by Ruggero et al. (1997). However, they agree with coression exonents observed in other sychohysical easureents and siulations (Oxenha and Moore, 1994; Gregan et al., 1998). ACKNOWLEDGMENTS This work was suorted largely by grant DC149 fro NIDCD. It was also suorted in art by the Lion s 5M International Hearing Foundation. REFERENCES Oxenha, A.J. and Plack, C.J. (1997). A behavioral easure of basilar-ebrane nonlinearity in listeners with noral and iaired hearing, J. Acoust. Soc. A. 11, Ruggero, M.A., Rich, N.C., Recio, A., Narayan, S.S. and Robles, L. (1997). Basilar-ebrane resonses to tones at the base of the chinchilla cochlea, J. Acoust. Soc. A. 11, Page 5

6 Table I. Coression exonents obtained fro MPP siulations. Masker Level Masker Position YYK KEK MXW PJL 9 db On-Frequency Off-Frequency db On-Frequency Off-Frequency db On-Frequency Off-Frequency db On-Frequency Off-Frequency Alitude Teoral Resolution Measured with MPPs 2 Alitude Tie [s] Tie [s] Probe at Threshold Nuber of Cycles Probe at Threshold Nuber of Cycles Fig. 1. A asker odulated at two different rates with a signal resented at a valley (uer anels) and hyothetical asking eriod atterns (lower anels) easured over two cycles of the resented askers. Page 6

7 OFF-FREQUENCY MASKING PERIOD PATTERNS MASKER 3 Hz; PROBE 6 Hz PROBE LEVEL AT THRESHOLD [db SPL] YYK PJL KEK TAL MXW 9 db SPL 85 db SPL 75 db SPL 65 db SPL Fig. 2. Masking eriod atterns easured with a 3-kHz (off-frequency) asker and a 6-kHz robe. Different sybols show the data for different subjects. Page 7

8 ON-FREQUENCY MASKING PERIOD PATTERNS MASKER 6 Hz; PROBE 6 Hz 1 8 PROBE LEVEL AT THRESHOLD [db SPL] YYK KEK TAL MXW 9 db SPL 85 db SPL db SPL 65 db SPL Fig. 3. Masking eriod atterns easured with a 6-kHz (on-frequency) asker and a 6-kHz robe. Page 8

9 MEASURED AND PREDICTED MPP OFF-FREQUENCY MASKER (3 khz) db 85 db 6 85 db 75 db PROBE LEVEL AT THRESHOLD [db SPL] db 75 db 65 db YYK PJL db 85 db 75 db 75 db 65 db 65 db 2 KEK MXW Fig. 4. The data (sybols) and the redictions (solid lines) for the 3-kHz asker and the 6-kHz robe. Page 9

10 MEASURED AND PREDICTED MPP Masker 6 Hz; Probe 6 Hz PROBE LEVEL AT THRESHOLD [db SPL] db SPL 75 db SPL 75 db SPL db SPL Fig. 5. The data (sybols) and the redictions (solid lines) for the 6-kHz asker and the 6-kHz robe for one listener. Page 1

11 MEASURED AND PREDICTED MPP ON-FREQUENCY MASKER (6 khz) 1 8 PROBE LEVEL AT THRESHOLD [db SPL] YYK KEK MXW Fig. 6. The data (sybols) and the redictions (solid lines) for the 6-kHz asker and the 6-kHz robe. The redictions were obtained fro siulations in which five oints near each eak were excluded fro the coutation of the su of squared deviations between the data and the redictions. Page 11