gms | German Medical Science

Electrical stimulation by a cochlea implant (CI) may partly restore hearing function after profound bilateral hearing loss. CI users often show reduced speech perception abilities in noisy environments when one has to separate the signal from the noise. Thus auditory selective attention seems to be impaired in CI patients. Until recently, ERP research in auditory selective attention has not focused on CI users. Moreover, recording the mismatch negativity (MMN), a negative deflection of the ERP provides an excellent opportunity to evaluate whether attentive auditory selection is guided by intact preattentive auditory discrimination in CI users.

We recorded 32-channel event-related potentials (ERPs) for active and passive listening conditions in a series of auditory discrimination experiments. Besides hitrates, the Nd-attention effect and the P300 were analysed to quantify attentive auditory stimulus selection. The mismatch negativity (MMN) and the P3a-component were measured to trace preattentive auditory stimulus discrimination in the passive listening condition. Exp. 1 was conducted in 9 postlingually deafened unilateral CI users. ERPs were recorded for i) frequent standards (800 Hz,p=0.8) ii) rare frequency deviants (2100 Hz,p=0.1) iii) rare auditory novel stimuli (p=0.1). Exp. 2 was conducted in the same group. Rare deviants (p=0.1 each) were presented at -90° and -30° azimuth as well as 30° elevation with regard to the frequent standard (p=0.7). Exp. 3 replicated Exp. 1 in 5 patients 30 days after implantation, whereas Exp.4 included a selective pitch discrimination task in the same 5 subjects.

CI users showed excellent performance for rare auditory novels even 30 days after implantation (Exp.1,3). Overt responses were strongly diminuished for the less salient frequency deviants. A stable Nd-attention effect initiated 100 ms after stimulus onset. The MMN was evident for both, deviant and novel stimuli, and indicated the integrity of the preattentive change detection system for novel stimuli and for the less salient deviant stimuli. In Exp. 2 the MMN was obtained for spatially defined deviants, although active discrimination was only slightly above chance. Finally, subjects were able to focus their attention on the selected pitch, as was indicated by the Nd-effect and the P300 (Exp. 4). Pitch deviants in the unattended channel evoked a typical MMN and  indicated intact monitoring of an unselected auditory stream.

The results indicate that even weak auditory changes are preattentively monitored in CI users but partly fail to activate attentive stimulus detection. ERP recordings provide a promising tool to objectively characterize auditory perception in CI users.