Examples of simple monaural frequency shifts

Acoustic simulations can effectively illustrate the effects of basalward shifts of stimulation - .speech is processed here through a vocoder simulation of a 6 channel cochlear implant. The use of just 6 channels represents well the effective number of channels that many CI users receive, even when the electrode has 16 or 22 electrodes. The sounds each comprise 6 sine waves whose levels represent the spectral shape of the input speech as this changes over time. Where there is no mismatch, the sine wave frequencies are at the centres of the 6 input filters through which the input speech spectrum is sampled. As the basalward shift increases, the sine wave frequencies are increasingly higher than the filter centre frequencies.  The place shifts illustrated here are simulated assuming that the electrodes stimulate auditory nerves whose charateristic frequencies are aligned with the tuning of the basilar membrane according to the well-known Greenwood mapping. This mapping may well overestimate the frequency shift for a given insertion depth but is the best worked-out model that we presently have.

Here are examples of simple sentences processed in this way
Female talker: no mismatch - 2mm basalward shift - 4 mm basalward shift - 6 mm basalward shift
Male talker:    no mismatch - 2mm basalward shift - 4 mm basalward shift - 6 mm basalward shift
 
A shift of 6 mm makes speech virtually unintelligible at first, but studies using acoustic simulations of CI processing show that after only a few hours of exposure intelligibility is markedly improved (e.g., Rosen et al., 1999). This finding points to a perceptual adaptation process that must, at least in the early stages, be based on an adjustment to the listener’s cortical mapping of spectral shape for speech sounds. There is also evidence of such adaptation in CI users (e.g. Fu et al., 2002), and a CI user whose implant provides a shifted frequency to place map will of course be exposed to this shift over a very long time period, with potential for a more complete adaptation to occur. Exposure to a shifted map over periods of months could perhaps lead to remappings of frequency to place in the sub-cortical auditory pathways, although there is at present no evidence in support of this form of adaptation.
 
Combining unshifted acoustic hearing with an implant with upward shifting