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Background
Cochlear Implants (CIs) provide near normal speech intelligibility in quiet environments to individuals suffering from sensorineural hearing loss. Perception of speech in situations with competing background noise and especially music appraisal however are still insufficient. Hence, improving speech perception in ambient noise and music intelligibility is a core challenge in CI research. Quantitatively assessing music intelligibility is a demanding task due to its inherently subjective nature. However, previous approaches have related electrophysiological measurements to speech intelligibility, a corresponding relation to music intelligibility, can be assumed. Recent studies have investigated the relation between results obtained from hearing performance tests and Spread of Excitations (SoEs) measurements. SoE functions are acquired by measuring Electrically Evoked Compound Action Potentials (ECAPs) which represent the electrical response generated in the neural structures of the auditory nerve. The parameters designed to describe SoE functions are used to estimate the dispersal of the electric field in the cochlea. The quality of spatial separation of the electrical field generated by adjacent electrodes are assumed to correlate with hearing performance measures.
Aim of study
This study investigated the relation of parameters derived by ECAP measurements and perceptive skills which aim to access the level of speech and music intelligibility in CI users. In addition, the ratings assessed in a questionnaire on self-rated music intelligibility were correlated to a test battery consisting of measures for speech reception threshold (SRT) in noise (Oldenburger Satztest (OLSA)) and music intelligibility (Adaptive Melody-Pattern-Discrimination Test (AMPDT)). We hypothesised that results from this test battery correlated to subjective ratings and measures describing SoE functions.
Methods
The patient collective covered 17 well-experienced bilateral CI listeners (8 females, 9 males) between the age of 14 and 77 years with a minimum CI experience of two years. Music enjoyment and self-rated musicality was evaluated by means of a questionnaire. The AMPDT included two psychoacoustic tests: timbre difference discrimination threshold (TDDT) and background contour discrimination threshold (BCDT). The accentuation of harmonics in a foreground melody created a background melody. Accentuation was realised by sound level increment, frequency detuning and onset asynchrony. Subjects had to detect target intervals comprising both foreground and background melody by discriminating timbre differences in a Three-Interval Three-Alternative Forced-Choice (3I3AFC) procedure. In a One-Interval Two-Alternative Forced-Choice (1I2AFC) procedure, subjects had to classify the background melody’s contour. SoE was measured via a spatial forward-masking paradigm. A basal, medial and apical recording electrode was measured. Probe electrodes were one electrode position apical to the recording. The width of normalised SoE functions was calculated at their 25 % and 50 % level (excitation distance (DIST)). Furthermore, exponential functions were calculated for SoE profiles with more than three data points for each side. The OLSA assessed SRT in noise. The noisy environment was presented through an array of four loudspeakers (MSNF). The Fastl noise-condition allows to make use of gap listening representing the temporal characteristics of speech as a fluctuating noise. The OLnoise-condition is a continuous noise resulting in a maximum portion of masking.
Results
We found that background melody contour classiffication (BCDT) is more challenging to CI users than the detection of small perceptual timbre differences (TDDT). Background melody contour classification was possible with harmonic accentuation by sound level increment whereas accentuation by onset asynchrony was more demanding. CI users failed in background melody contour classification obtained by frequency detuning. SRTs assessed in the OLSA were significantly lower in the OLnoise than in the Fastl noise masking condition. A number of N = 90 SoE functions were acquired from ECAP measurements, in which N = 48 showed a clearly present ECAP response. The DIST at the 25 % and 50 % level was narrower for the basal than for the apical and medial electrode. SoE functions showed asymmetric profiles with larger amplitudes towards the basal end of the cochlea. Correlation analysis between the AMPDT, OLSA and DISTs showed no significant correlation. Correlation analysis between the AMPDT, OLSA and the questionnaire’s results could not prove that musical activities (music listening, singing or playing instruments) improve music intelligibility. However, CI supply has restored the importance of music, self-rated musicality and musical enjoyment in this study’s subjects.
Conclusions
The present study’s results imply that CI listeners are only able to detect distinct timbre alterations throughout the course of a musical piece whereas they cannot discriminate background melodies hidden in a pattern of complex harmonic sounds. Furthermore, SoE measurements do not seem to be an adequate tool to predict neither speech nor music intelligibility in CI listeners, contrary to our initial hypothesis. This finding is consistent with a number of studies who did not find a correlation between music or speech intelligibility and channel interactions assessed by SoE measurements. It can be concluded that albeit CI supply restores musical enjoyment in patients with sensorineural hearing loss, music perception is still poor and does not significantly improve by regular musical activities such as listening to music, singing or playing instruments.