Simulating the effect of cochlear-implant electrode insertion depth on speech understanding.

Normally hearing listeners were presented with vowels, consonants, and sentences for identification through an acoustic simulation of a five-channel cochlear implant with electrodes separated by 4 mm (as in the Ineraid implant). The aim of the experiment was to simulate the effect of depth of electrode insertion on identification accuracy. Insertion depth was simulated by outputting sine waves from each channel of the processor at a frequency determined by the cochlear place of electrodes inserted 22-25 mm into the cochlea. The results indicate that simulated insertion depth had a significant effect on performance. Performance at 22- and 23-mm simulated insertion depths was always poorer than normal, and performance at 25 mm simulated insertion depth was, most generally, the same as normal. It is inferred from these results that, if insertion depth could be unconfounded from other coexisting factors in implant patients, then insertion depth would be found to affect speech identification performance significantly.