Auditory Enhancement in Cochlear-Implant Users Under Simultaneous and Forward Masking

Abstract

Auditory enhancement is the phenomenon whereby the salience or detectability of a target sound within a masker is enhanced by the prior presentation of the masker alone. Enhancement has been demonstrated using both simultaneous and forward masking in normal-hearing listeners and may play an important role in auditory and speech perception within complex and time-varying acoustic environments. The few studies of enhancement in hearing-impaired listeners have reported reduced or absent enhancement effects under forward masking, suggesting a potentially peripheral locus of the effect. Here, auditory enhancement was measured in eight cochlear-implant (CI) users with direct stimulation. Masked thresholds were measured under simultaneous and forward masking as a function of the number of masking electrodes, and the electrode spacing between the maskers and the target. Evidence for auditory enhancement was obtained under simultaneous masking, qualitatively consistent with results from normal-hearing listeners. However, no significant enhancement was observed under forward masking, in contrast to earlier results with normal-hearing listeners. The results suggest that the normal effects of auditory enhancement are partially but not fully experienced by CI users. To the extent that the CI users' results differ from normal, it may be possible to apply signal processing to restore the missing aspects of enhancement.

Auditory enhancement under simultaneous masking in normal-hearing and hearing-impaired listeners

“…6), with thresholds between 50 and 70 dB HL, still showed some residual enhancement, suggesting that differences in degree of hearing loss cannot account for the different outcomes of the two studies. The finding of enhancement in simultaneous but not forward masking is consistent with the recent findings in CI users (Kreft and Oxenham, 2017), again suggesting the possibility of two mechanisms, with only one affected by cochlear hearing loss. A similar twomechanism explanation was recently proposed by Wang et al (2015 to account for differences between CI users and NH listeners in tasks involving loudness context effects.…”

“…The enhancement of vowel formants is similar in nature to the enhancement of a target under simultaneous masking. A study of CI users that examined enhancement under both simultaneous and forward masking found evidence for enhancement under simultaneous masking, but not forward masking (Kreft and Oxenham, 2017). One possible explanation is that the mechanisms yielding relative but not absolute enhancement (reflected in simultaneous masking and vowel perception) are intact and therefore rely on mechanisms more central than the cochlea, whereas the mechanisms yielding absolute enhancement (reflected in forward masking and loudness matching tasks) are not, either because they are peripheral in nature, or because they rely on a peripheral input that is not sufficiently well represented by the CI.…”

Spectral contrast effects and auditory enhancement under normal and impaired hearing

“…One interesting finding is that enhancement is observed under simultaneous masking, but not under forward masking for both cochlear-implant users [18] and hearing-impaired listeners [19,20]. This intriguing difference suggests a potential difference in mechanism underlying enhancement in simultaneous vs. forward masking, which has yet to be fully elucidated.…”

“…Auditory Enhancement Two recent studies in our lab have explored auditory enhancement in both cochlear-implant users [18] and listeners with sensorineural hearing loss of presumed cochlear origin [19]. In both cases, detection thresholds for a pure tone in the presence of spectrally flanking masker tones were reduced by the introduction of a precursor that was a copy of the masker.…”

“…Finally, it has become clear that not all forms of auditory enhancement are the same. Although the detection threshold for a simultaneously masked target tone was enhanced by the presence of a precursor in cochlearimplant users [18], a more recent study has shown that supra-threshold effects of enhancement are not necessarily found in cochlear-implant users [21]. In that study, listeners were asked to judge whether a probe tone had been presented in the preceding mixture of a target tone and spectrally flanking masker tones.…”