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Effect of interphase-gap and pulse-duration on evoked-potential amplitudes and loudness in cochlear implantees
International Congress Series 1273 (2004) 60 – 63
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Effect of interphase-gap and pulse-duration on evoked-potential amplitudes and loudness in cochlear implantees C. McKay*, L. Fewster Department of Otolaryngology, University of Melbourne, 384–388 Albert Street, 3002 Melbourne, Victoria, Australia
Abstract. There is evidence that individual differences in the effects of interphase-gap and pulseduration are related to the degree of neural survival. This experiment explored the effect of interphase gap and pulse duration on loudness in cochlear implantees. Also, the current change required to equalise loudness after an interphase gap or pulse duration change was compared to the current required to maintain constant EABR and NRT amplitude for the same interphase gap or pulse duration change. It was found that these measures varied among implantees and some of the objective measures were correlated with the psychophysical measures. D 2004 Elsevier B.V. All rights reserved. Keywords: Cochlear implant; Compound action potential; Electrically evoked auditory brainstem response; Loudness
1. Introduction There is evidence that individual differences among implantees in the effect of changing interphase gap or pulse duration are related to the degree of neural survival. In two psychophysical studies [1,2], the relationship to neural survival was theoretically proposed. Miller et al. [3] showed that the strength-duration function of evoked potentials (i.e. the effect of pulse duration changes) in guinea pigs became shallower over time after deafening, presumably due to ongoing neural degeneration. Finally, a recent study using guinea pigs [4] showed that both interphase gap and pulse duration effects on evoked potentials were correlated with histological measurements of neural survival. * Corresponding author. Tel.: +613 9667 7506; fax: +613 9667 7518. E-mail address: [email protected] (C. McKay). 0531-5131/ D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ics.2004.08.031
C. McKay, L. Fewster / International Congress Series 1273 (2004) 60–63
61
Fig. 1. Box plots showing the current reduction required to maintain equal loudness when pulse duration is doubled. The left panel shows the MP data and the right panel shows the BP data (pulse durations in ms).
Thus, measuring the effects of interphase gap or pulse duration in implantees may provide useful information about the degree of individual neural survival. In this paper, we explored the variability among implantees of the effects of interphase-gap and pulse duration on loudness, and we compared the effect of the same changes on evoked potential amplitudes in the same subjects, using the same stimuli. 2. The loudness measurements Ten cochlear implantees participated in this part of the experiment (5 CI24M users and 5 CI24R users). Stimuli were 33 Hz pulse trains of 500 ms duration on three electrode positions. The mode of stimulation was monopolar (MP) and bipolar (BP). The current reduction required to equalise loudness when pulse duration was doubled was measured by a loudness-balancing task at several supra-threshold levels and by measuring threshold of detection. Pulse durations used were 26–416 As. The adjustment in current required to maintain equal loudness when interphase gap was changed from 8 to 50 As was measured at threshold and 80% of the dynamic range in MP mode (pulse duration 26 As), and at threshold in BP mode (pulse duration 52 As). Three electrode positions were investigated in each subject. The current adjustments required to maintain equal loudness when pulse duration was doubled are shown in Fig. 1. The current reduction varied between approximately 2 and 8 dB. The mode did not affect the measurement significantly. The median adjustment became larger and the variance smaller as the reference pulse duration decreased. For pulse durations over 208 As, there were a significant number of current reductions greater than 6
Fig. 2. Box plot showing the range of current adjustments required to maintain equal loudness when interphase gap is widened from 8 to 50 As (data collated across subject/electrode).
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C. McKay, L. Fewster / International Congress Series 1273 (2004) 60–63
Fig. 3. The relationship between the effects on loudness of pulse duration (52–104 ms) and interphase gap (8–50 As) changes. Each symbol is one subject/electrode.
dB at threshold level. (For these points, the amount of charge per pulse at threshold decreased with a doubling of pulse duration, in opposition to the usual finding). In some conditions, the amount of adjustment decreased with an increase in overall level (as noted before Ref. [1]). This relationship was stronger for longer reference pulse durations than for shorter, and stronger for users of the CI24M implant (straight array) than for users of the Contour array (CI24R). The current adjustments required to maintain equal loudness when interphase gap was widened from 8 to 50 As are shown in Fig. 2. The adjustment for interphase gap change was smaller ( p=0.004) in BP than in MP mode. This effect is at least partially due to the longer pulse duration used in the BP mode [2], but is also consistent with higher average spike probabilities for neurons activated in BP mode. In contrast to previous findings [2], there was no significant effect of level in the dynamic range for MP mode. If both of these measures (effects of interphase gap and pulse duration) are related to the degree of neural survival, then the two measures should be correlated with each other. Fig. 3 shows that the two measures were indeed correlated. 3. The NRT and EABR measurements Growth functions for EABR and NRT were obtained using stimuli which differed only in pulse duration or interphase gap (the same stimuli as part 2). For each pulse-duration/
Fig. 4. The current adjustment required to equalise EABR or NRT amplitude when pulse duration is doubled plotted against the current adjustment required to equalise loudness for the same stimulus pair. The correlation is significant in the EABR case ( p=0.03), and nearly significant in the NRT case ( p=0.08).
C. McKay, L. Fewster / International Congress Series 1273 (2004) 60–63
63
Fig. 5. The current adjustment required to equalise NRT amplitude for interphase gap change, plotted against the current adjustment required for equal loudness for the same stimulus pairs.
interphase gap difference, the current difference required to equalise response amplitude was determined. These current adjustments were compared to the current adjustments required to equalise loudness for the same stimulus pairs. Fig. 4 compares the psychophysical and evoked-potential measures for pulse duration changes. It can be seen that the two types of measure produced analogous results (e.g. the longer reference pulse duration required less current adjustment in each case). The current adjustments to equalise loudness for interphase gap change were not correlated with the current adjustments required to equalise NRT amplitude for the same change (Fig. 5), although the adjustments were generally in the same range (1–2 dB). 4. Conclusions The effect of changing pulse-duration and interphase-gap varied among implantees. For pulse duration changes, the effect was similar using both psychophysical and objective measurements. The effect of interphase-gap on loudness was not correlated to the effect on NRT amplitude, perhaps due to the small size of the effect. These measures may be used in future research to determine the perceptual correlates of neural degeneration for cochlear implant users. Acknowledgments This work was supported by the Australian National Health and Medical Research Council and the Garnett Passe & Ronney Williams Memorial Foundation. References [1] C.M. McKay, H.J. McDermott, The perceptual effects of current pulse duration in electrical stimulation of the auditory nerve, J. Acoust. Soc. Am. 106 (1999) 998 – 1009. [2] C.M. McKay, K.R. Henshall, The perceptual effects of interphase gap duration in cochlear implant stimulation, Hear. Res. 181 (2003) 94 – 99. [3] C.A. Miller, M.J. Faulkner, B.E. Pfingst, Functional responses from guinea pigs with cochlear implants: II. Changes in electrophysiological and psychophysical measures over time, Hear. Res. 92 (1995) 100 – 111. [4] P. Prado-Guitierrez, et al., Effect of interphase-gap and pulse-duration on electrically evoked potentials in deafened guinea pigs, Proceedings of the VIII International Cochlear Implant Conference, Indianapolis, Indiana, 2004 (May).
www.ics-elsevier.com
Effect of interphase-gap and pulse-duration on evoked-potential amplitudes and loudness in cochlear implantees C. McKay*, L. Fewster Department of Otolaryngology, University of Melbourne, 384–388 Albert Street, 3002 Melbourne, Victoria, Australia
Abstract. There is evidence that individual differences in the effects of interphase-gap and pulseduration are related to the degree of neural survival. This experiment explored the effect of interphase gap and pulse duration on loudness in cochlear implantees. Also, the current change required to equalise loudness after an interphase gap or pulse duration change was compared to the current required to maintain constant EABR and NRT amplitude for the same interphase gap or pulse duration change. It was found that these measures varied among implantees and some of the objective measures were correlated with the psychophysical measures. D 2004 Elsevier B.V. All rights reserved. Keywords: Cochlear implant; Compound action potential; Electrically evoked auditory brainstem response; Loudness
1. Introduction There is evidence that individual differences among implantees in the effect of changing interphase gap or pulse duration are related to the degree of neural survival. In two psychophysical studies [1,2], the relationship to neural survival was theoretically proposed. Miller et al. [3] showed that the strength-duration function of evoked potentials (i.e. the effect of pulse duration changes) in guinea pigs became shallower over time after deafening, presumably due to ongoing neural degeneration. Finally, a recent study using guinea pigs [4] showed that both interphase gap and pulse duration effects on evoked potentials were correlated with histological measurements of neural survival. * Corresponding author. Tel.: +613 9667 7506; fax: +613 9667 7518. E-mail address: [email protected] (C. McKay). 0531-5131/ D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ics.2004.08.031
C. McKay, L. Fewster / International Congress Series 1273 (2004) 60–63
61
Fig. 1. Box plots showing the current reduction required to maintain equal loudness when pulse duration is doubled. The left panel shows the MP data and the right panel shows the BP data (pulse durations in ms).
Thus, measuring the effects of interphase gap or pulse duration in implantees may provide useful information about the degree of individual neural survival. In this paper, we explored the variability among implantees of the effects of interphase-gap and pulse duration on loudness, and we compared the effect of the same changes on evoked potential amplitudes in the same subjects, using the same stimuli. 2. The loudness measurements Ten cochlear implantees participated in this part of the experiment (5 CI24M users and 5 CI24R users). Stimuli were 33 Hz pulse trains of 500 ms duration on three electrode positions. The mode of stimulation was monopolar (MP) and bipolar (BP). The current reduction required to equalise loudness when pulse duration was doubled was measured by a loudness-balancing task at several supra-threshold levels and by measuring threshold of detection. Pulse durations used were 26–416 As. The adjustment in current required to maintain equal loudness when interphase gap was changed from 8 to 50 As was measured at threshold and 80% of the dynamic range in MP mode (pulse duration 26 As), and at threshold in BP mode (pulse duration 52 As). Three electrode positions were investigated in each subject. The current adjustments required to maintain equal loudness when pulse duration was doubled are shown in Fig. 1. The current reduction varied between approximately 2 and 8 dB. The mode did not affect the measurement significantly. The median adjustment became larger and the variance smaller as the reference pulse duration decreased. For pulse durations over 208 As, there were a significant number of current reductions greater than 6
Fig. 2. Box plot showing the range of current adjustments required to maintain equal loudness when interphase gap is widened from 8 to 50 As (data collated across subject/electrode).
62
C. McKay, L. Fewster / International Congress Series 1273 (2004) 60–63
Fig. 3. The relationship between the effects on loudness of pulse duration (52–104 ms) and interphase gap (8–50 As) changes. Each symbol is one subject/electrode.
dB at threshold level. (For these points, the amount of charge per pulse at threshold decreased with a doubling of pulse duration, in opposition to the usual finding). In some conditions, the amount of adjustment decreased with an increase in overall level (as noted before Ref. [1]). This relationship was stronger for longer reference pulse durations than for shorter, and stronger for users of the CI24M implant (straight array) than for users of the Contour array (CI24R). The current adjustments required to maintain equal loudness when interphase gap was widened from 8 to 50 As are shown in Fig. 2. The adjustment for interphase gap change was smaller ( p=0.004) in BP than in MP mode. This effect is at least partially due to the longer pulse duration used in the BP mode [2], but is also consistent with higher average spike probabilities for neurons activated in BP mode. In contrast to previous findings [2], there was no significant effect of level in the dynamic range for MP mode. If both of these measures (effects of interphase gap and pulse duration) are related to the degree of neural survival, then the two measures should be correlated with each other. Fig. 3 shows that the two measures were indeed correlated. 3. The NRT and EABR measurements Growth functions for EABR and NRT were obtained using stimuli which differed only in pulse duration or interphase gap (the same stimuli as part 2). For each pulse-duration/
Fig. 4. The current adjustment required to equalise EABR or NRT amplitude when pulse duration is doubled plotted against the current adjustment required to equalise loudness for the same stimulus pair. The correlation is significant in the EABR case ( p=0.03), and nearly significant in the NRT case ( p=0.08).
C. McKay, L. Fewster / International Congress Series 1273 (2004) 60–63
63
Fig. 5. The current adjustment required to equalise NRT amplitude for interphase gap change, plotted against the current adjustment required for equal loudness for the same stimulus pairs.
interphase gap difference, the current difference required to equalise response amplitude was determined. These current adjustments were compared to the current adjustments required to equalise loudness for the same stimulus pairs. Fig. 4 compares the psychophysical and evoked-potential measures for pulse duration changes. It can be seen that the two types of measure produced analogous results (e.g. the longer reference pulse duration required less current adjustment in each case). The current adjustments to equalise loudness for interphase gap change were not correlated with the current adjustments required to equalise NRT amplitude for the same change (Fig. 5), although the adjustments were generally in the same range (1–2 dB). 4. Conclusions The effect of changing pulse-duration and interphase-gap varied among implantees. For pulse duration changes, the effect was similar using both psychophysical and objective measurements. The effect of interphase-gap on loudness was not correlated to the effect on NRT amplitude, perhaps due to the small size of the effect. These measures may be used in future research to determine the perceptual correlates of neural degeneration for cochlear implant users. Acknowledgments This work was supported by the Australian National Health and Medical Research Council and the Garnett Passe & Ronney Williams Memorial Foundation. References [1] C.M. McKay, H.J. McDermott, The perceptual effects of current pulse duration in electrical stimulation of the auditory nerve, J. Acoust. Soc. Am. 106 (1999) 998 – 1009. [2] C.M. McKay, K.R. Henshall, The perceptual effects of interphase gap duration in cochlear implant stimulation, Hear. Res. 181 (2003) 94 – 99. [3] C.A. Miller, M.J. Faulkner, B.E. Pfingst, Functional responses from guinea pigs with cochlear implants: II. Changes in electrophysiological and psychophysical measures over time, Hear. Res. 92 (1995) 100 – 111. [4] P. Prado-Guitierrez, et al., Effect of interphase-gap and pulse-duration on electrically evoked potentials in deafened guinea pigs, Proceedings of the VIII International Cochlear Implant Conference, Indianapolis, Indiana, 2004 (May).