Significance of auditory evoked responses (EABR and P300) in cochlear implant subjects

An electrically derived auditory brainstem response (EABR) was recorded int ra-operatively, and the event-related potential (P300) postoperatively in p ost-lingually deaf adult cochlear implantees. EABR and P300 were recorded f rom two groups of 25 subjects each. All subjects had a cochlear mini-22 sys tem implanted at our center between 1994 and 1998. Two parameters of EABR, threshold (T) and the gradient of the amplitude-growth (AG) curve, were use d for correlation with postoperative speech reception scores. The consonant recognition score (CRS) measured 1 month postoperatively showed a signific ant correlation with the AG curve of EABR (Spearman rank order test, p = 0. 004), but not at later points in time. No correlation was found between the T and CRS at any time postoperatively. As the AG curve reflects the number of remaining spiral ganglion cells in the inner ear, a large number of sur viving neurons is thought to be advantageous only for initial speech learni ng. The latency of P300 was measured for correlation with the speech recept ion score. The division of the cochlear implant subjects into a fair and a good hearing group according to their CRS showed that the average latency o f P300 was significantly longer for the fair hearing group than for the goo d hearing group. A follow-up study showed that P300 latency significantly c orrelated with CRS measured at 6 months, 1 year and at a later time. As the latency of P300 is thought to reflect the time for sound processing in the central auditory system, our results indicate that plasticity of the centr al auditory system is more important than that of the peripheral auditory s ystem for speech learning in cochlear implant subjects.