Cochlear hearing loss often involves loss of function of the outer and inner hair cells within the cochlea, although there may also be metabolic disturbances and structural abnormalities. Loss of outer hair cell function has the following perceptual effects:

(1) loss of absolute sensitivity (related to reduced amplitude of vibration on the basilar membrane);

(2) loss of frequency selectivity, associated with broader tuning on the basilar membrane, which contributes to difficulties in understanding speech, especially in noise;

(3) an abnormally rapid growth of loudness with increasing intensity (recruitment) associated with steeper input-output functions on the basilar membrane.

The first and third of these effects can be compensated by amplification combined with automatic gain control (AGC), although there is still much controversy about the ‘best’ way of implementing AGC. The second effect cannot be fully compensated, although directional microphones can be helpful, and digital processing to increase spectral contrast may be of some benefit.

Loss of inner hair cell function has the following perceptual effects:

(1) loss of absolute sensitivity (related to less efficient transduction);

(2) more noisy transmission of information to the central auditory system, which contributes to difficulties in understanding speech;

(3) in extreme cases (‘dead regions’), no transduction from certain regions of the cochlea.

Effect (1) can be compensated by amplification, but effects (2) and (3) are much more difficult to deal with. Again, directional microphones can help. There is some evidence that ‘transposition’ (transforming high frequencies to lower ones) can be beneficial for people with dead regions. The combination of a hearing aid and a cochlear implant may also be beneficial.

Research Symposium – Building Hearingted by amplification, but effects (2) and (3) are much more difficult to deal with. Again, directional microphones can help. There is some evidence that ‘transposition’ (transforming high frequencies to lower ones) can be beneficial for people with dead regions. The combination of a hearing aid and a cochlear implant may also be beneficial.