Considerations in matching acoustic and electrical hearing


Work in HearCom on combining acoustic and electrical hearing addresses the first of these differences. In a standard cochlear implant, sound frequencies covering a range from around 125 to 8000 Hz are mapped to the different places of stimulation determined by the insertion depth of the implanted electrode array. The electrode array is typically 16mm in length and located in the outer two turns of the 35 mm long cochlea, so that the lowest frequency nerve fibres in the innermost apical turn are not stimulated. As a result, the 125 to 8000 Hz range of acoustic frequencies are encoded at cochlear places that are shifted towards the outer basal end of the cochlea as compared to the places at which they would be encoded in acoustic hearing. Depending on the insertion depth achieved, this basalward shift may be equivalent to a mismatch of acoustic frequency to cochlear place of one octave or more. The electrode used in “hybrid” devices is only 10 mm in length, and in consequence the deepest inserted electrode is even further from the low frequency nerve fibres. It is not clear exactly which auditory nerve fibers are stimulated by electrodes at different positions, but this will depend in the degree of degeneration of the nerves and perhaps on whether the electrodes are located close to the organ of corti or to the modiolus.
Such mismatches of acoustic frequency to cochlear place are believed to cause considerable difficulties at first for implant recipients with acquired hearing losses.
see examples of simple monaural frequency shifts
When a CI is combined with a hearing aid, and is fitted in a standard way with a basalward shift of frequency to place map, this mapping will inevitably be in conflict with the frequency to place mapping of the acoustic modality. The presence of two conflicting frequency to place maps could pose far more of a problem than when the listener only hears speech through a cochlear implant.  
See examples of conflicting frequency maps across the full frequency range
It is of course likely that in most recipients of cochlear implants, the remaining hearing in the opposite ear is limited to only the lower frequencies. The studies outlined above do not reflect this factor. Additional studies at UCL have considered the situation in which a mismatch of frequency-to-place map is present on in the low frequencies.
See examples of maps that conflict only in the low frequency region of residual hearing