Cochlear Implant Research

This research investigates the auditory-processing capabilities of deaf cochlear-implant listeners with usable electric hearing. The research is divided into four areas of investigation. In the first, experiments investigate the intensity limits for effective sensory channel separation between electrodes. In the second, the limitations and perceptual attributes associated with place and periodicity coding are investigated. In the third area of investigation, the dependence of perceived loudness on stimulus intensity is examined and the limits of intensity coding are determined. In the fourth area, measures of auditory-processing capability, obtained in the previous three areas, are related to the ability to process speech sounds and to current speech-processor schemes.

Relevant Publications:

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Donaldson, G.S. and Smith, S.L. (1999). Speech Performance-Intensity Functions for Nucleus SPEAK and Clarion CIS Cochlear Implant Listeners. Poster presented at the 1999 Annual Meeting of the American Academy of Audiology, Miami, FL

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Donaldson, G.S. and Nelson, D.A. (1997). Effects of pulse duration on discrimination of current amplitude by Nucleus cochlear-implant users. Abstracts 20th ARO Midwinter Mtgs.

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DONALDSON, G.S. and NELSON, D.A. (1999). Place-pitch sensitivity and its relation to consonant recognition by cochlear implant listeners using the MPEAK and SPEAK speech processing strategies. J. Acoust. Soc. Am., (submitted 11/98) [ARO Abstracts 19th Midwinter Mtgs].

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DONALDSON, G.S., VIEMEISTER, N.F. and NELSON, D.A. (1997). Psychometric functions and temporal integration in electric hearing. J. Acoust. Soc. Am., 101, 3706-3721.

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NELSON, D.A., SCHMITZ, J.L., DONALDSON, G.S., VIEMEISTER, N.F. and JAVEL, E. (1996). Intensity discrimination as a function of stimulation level with electrical stimulation. J. Acoust. Soc. Am.,100, 2393-2414.

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NELSON, D.A., VAN TASSEL, D.J., SCHRODER, A.C., SOLI, S. and LEVINE, S. (1995). Electrode ranking of "place pitch" and speech recognition in electrical hearing. J. Acoust. Soc. Am., 98, 1987-1999.

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VAN TASELL, D.J., GREENFIELD, D.G., LOGEMANN, J.L. and NELSON, D.A. (1992). "Temporal cues for consonant recognition: training, talker generalization, and use in evaluation of cochlear implants," J. Acoust. Soc. Am., 92, 1247-1257.

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KIMBERLEY, B.P., LEE, A., SCHELLER, L., LEVINE, S., ADAMS, G. and NELSON, D.A.(1989). "Cochlear- implant hearing performance at the University of Minnesota," J. of Otolaryngology, 18, 24-27.

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Speech Performance-Intensity Functions for Nucleus SPEAK and Clarion CIS Cochlear Implant Listeners.

Performance-intensity (P-I) functions for vowels, consonants and sentences in quiet were obtained from adult users of the Nucleus SPEAK and Clarion CIS cochlear implants. Subjects used their clinical speech processor maps, and adjusted the loudness (volume/sensitivity) controls on their processors so that speech presented at 60 dB SPL (A) was comfortably loud. Results show that speech recognition in quiet is strongly dependent on stimulus level in both SPEAK and CIS users, but that SPEAK users are especially susceptible to the effects of reduced audibility at stimulus levels below 50-60 dB SPL. Performance at these lower levels is limited by relatively high sound field thresholds in SPEAK listeners.

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Intensity discrimination as a function of stimulus level with electric stimulation

Difference limens (DLs) for changes in electric current were measured from multiple electrodes in each of eight cochlear-implanted subjects. Stimuli were 200-ms/phase biphasic pulse trains delivered at 125 Hz in 300-ms bursts. DLs were measured with an adaptive three-alternative forced-choice procedure. Fixed-level psychometric functions were also obtained in four subjects to validate the adaptive DLs. Relative intensity DLs, specified as Weber fractions in decibels [10log(delta-I/I)] for standards above absolute threshold, decreased as a power function of stimulus intensity relative to absolute threshold [delta-I/I = b(I/Io)^a] in the same manner as Weber fractions for normal acoustic stimulation reported in from previous studies. Exponents (a) of the power function for electric stimulation ranged from -0.4 to -3.2, on average, an order of magnitude larger than exponents for acoustic stimulation, which range from -0.07 to -0.11. Normalization of stimulus intensity to the dynamic range of hearing resulted in Weber functions with similar negative slopes for electric and acoustic stimulation, corresponding to an 8-dB average improvement in Weber fractions across the dynamic range. Sensitivity to intensity change [10logb] varied from -0.42 to -13.5 dB compared to +0.60 to -3.34 dB for acoustic stimulation, but on average was better with electric stimulation than with acoustic stimulation. Psychometric functions for intensity discrimination yielded Weber fractions consistent with adaptive procedures and d’ was a linear function of dI. Variability among repeated Weber-fraction estimates was constant across dynamic range. Relatively constant Weber fractions across all or part of the dynamic range, observed in some subjects, were traced to the intensity resolution limits of individual implanted receiver/stimulators. DLs could not be accurately described by constant amplitude changes, expressed as a percentage of dynamic range [delta-A(%DR)]. Weber fractions from prelingually deafened subjects were no better or worse than those from postlingually deafened subjects. The cumulative number of discriminable intensity steps across the dynamic range of electric hearing ranged from as few as 6.6 to as many as 45.2. Physiologic factors that may determine important features of electric intensity discrimination are discussed in the context of a simple, qualitative, rate-based model. These factors include the lack of compressive cochlear preprocessing, the relative steepness of neural rate-intensity functions, and individual differences in patterns of neural survival.

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Electrode Ranking of "Place Pitch" and Speech Recognition in Electrical Hearing

The ability to distinguish electrical stimulation of different electrodes on the basis of "pitch or sharpness" was evaluated with an electrode ranking procedure in fourteen individual users of the Nucleus cochlear implant. Prior to the electrode ranking test, absolute thresholds and maximum comfortable loudness levels were measured, and loudness balancing was accomplished across all usable electrodes. Performance on the electrode ranking task was defined in terms of d’ per mm of distance between comparison electrodes. Large individual differences were found among cochlear-implant users. In subjects with good to excellent place-pitch sensitivity, the electrode ranking task was limited by a ceiling effect; however, in those with poor to moderate sensitivity d’/mm was relatively constant with spatial separation between electrodes. Place pitch was typically ordered from apical to basal electrodes, i.e., basal electrodes were judged to be higher in pitch than more apical electrodes. However, instances of reversals in place-pitch ordering were seen on some electrodes in some subjects. Instances were also seen of better electrode ranking in the apical half of the electrode array than in the basal half, and vice-versa. Analyses of the electrode ranking functions in terms of d’ per stimulus indicated that, in some subjects, perfect performance was reached with as little as 0.75 mm between comparison electrodes, the minimum possible. In other subjects, perfect performance was not reached until the spatial separation between comparison electrodes was over 13 mm, more than three quarters of the entire length of the electrode array. Ten of the subjects also participated in a closed-set recognition task of intervocalic consonants. Although the maximum transmitted information for place of consonant articulation (which is based primarily on spectral speech cues) was only 34%, correlations between place-pitch sensitivity and transmitted speech information were as high as 0.71. This was surprising considering the excellent place-pitch sensitivity exhibited by some of the subjects, and may reflect limitations of the Nucleus speech coding strategy for representing spectrally-coded speech information. The two prelingual subjects performed notably poorer on the speech task than the postlingual subjects, even though one of the prelingual subjects demonstrated very good place-pitch sensitivity.

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