Place mismatches in the low frequency region


It is likely that in most recipients of cochlear implants, the remaining hearing in the opposite ear is limited to only the lower frequencies. Matthew Smith at UCL has run a study that considers the situation in which a mismatch of frequency-to-place map is present on in the low frequencies. His study simulates the combination of low frequency hearing up to 800 Hz with a cochlear implant on the opposite ear that imposes an upward frequency shift.

Acoustic hearing and the reduced frequency selectivity that is expected with a profound hearing loss is simulated by a two-channel vocoder spanning 200 to 800 Hz.

A contralateral CI is simulated by a five-channel vocoder, with the stimulation places shifted upwards in frequency compared to the speech processor filters.

The lowest two filter bands of the CI simulation cover the same frequencies as those presented to the opposite ear, but the information from these bands is presented to different places in the two ears - so there is a place mismatch for these lower frequencies.

Even after several hours of experience, at least for understanding speech in quiet. normal hearing listeners perform just as well if the lower two channels of the simulated CI are turned off, indicating that they do not make use of this upward shifted information when it is also present in the opposite ear without the shift.

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