The relation between speech recognition and within-channel or across-channel (i.e., spectral tilt) intensity discrimination was measured in nine CI users (11 ears). Within-channel intensity difference limens (IDLs) were measured at four electrode locations across the electrode array. Spectral tilt difference limens were measured with (XIDL-J) and without (XIDL) level jitter. Only three subjects could perform the XIDL-J task with the amount of jitter required to limit use of within-channel cues. XIDLs (normalized to %DR) were correlated with speech recognition (r = 0.67, P = 0.019) and were highly correlated with IDLs. XIDLs were on average nearly 3 times larger than IDLs and did not vary consistently with the spatial separation of the two component electrodes. The overall pattern of results was consistent with a common underlying subject-dependent limitation in the two difference limen tasks, hypothesized to be perceptual variance (how the perception of a sound differs on different presentations), which may also underlie the correlation of XIDLs with speech recognition. Evidence that spectral tilt discrimination is more important for speech recognition than within-channel intensity discrimination was not unequivocally shown in this study. However, the results tended to support this proposition, with XIDLs more correlated with speech performance than IDLs, and the ratio XIDL/IDL also being correlated with speech recognition. If supported by further research, the importance of perceptual variance as a limiting factor in speech understanding for CI users has important implications for efforts to improve outcomes for those with poor speech recognition.
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4IFC DLs would be approximately 1.2 times 2IFC DLs if the same stimuli and discrimination were tested in both cases. This ratio was estimated by fitting Weibull functions to two alternative psychometric functions.
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The ImpResS software was jointly developed by the University of Melbourne and Medical Research Council (UK), and the SPEAR research processor was developed by the Hearing Cooperative Research Centre in Melbourne.
The research was supported by Veski and Lions fellowships to the first author. The Bionics Institute acknowledges the support of the Victorian Government through its Operational Infrastructure Support program.
Conflict of Interest
The authors declare that they have no conflict of interest.
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McKay, C.M., Rickard, N. & Henshall, K. Intensity Discrimination and Speech Recognition of Cochlear Implant Users. JARO 19, 589–600 (2018). https://doi.org/10.1007/s10162-018-0675-7