Cognitive event-related brain potentials (ERPs) recorded during a 2-tone discrimination (oddball) task were examined in 8 patients with cochlear implants. The aim of this study is to investigate the effects of discrimination difficulty formed by 4 conditions of stimulus contrast: the target stimuli were set at 2000, 1500, 1200, or 1100 Hz, and nontarget stimuli were set at 1000 Hz throughout. The averaged ERPs recorded after the target stimuli revealed N100, P200, N200, and P300 peaks, whereas those after the nontarget stimuli showed only N100 and P200 peaks. The peak latencies of the ERP components of early information processings (N100 and P200) were not significantly increased among the 4 conditions. In contrast, the latencies of late cognitive components (N2b and P300, whose peak latencies were longer than 250 msec) and reaction times were significantly increased as target discriminability decreased. In conclusion, early information processings were not delayed by the discrimination difficulty, but cognitive processings were delayed in patients with cochlear implants. Because the N2b amplitude, which is considered to be associated with controlled processings, was much larger than that previously reported from subjects with normal hearing, it is suggested that patients with cochlear implants make a considerable effort to process their poor auditory information.
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