Thus, damage to the OHCs results in the reduction of sensitivity of the basilar membrane to weak sounds. Amplification to these sounds is therefore required, in order for the basilar membrane to respond efficiently. IHCs are less susceptible to damage in comparison to the OHCs. However, if they become damaged, this will result in an overall loss of sensitivity. Neural tuning curves
Frequency selectivity
The traveling wave along the basilar membrane peaks at different places along it, depending on whether the sound is low or high frequency. Due to the mass and stiffness of the basilar membrane, low frequency waves peak in the apex, while high frequency sounds peak in the basal end of the cochlea. Therefore, each position along the basilar membrane is finely tuned to a particular frequency. These specifically tuned frequencies are referred to as characteristic frequencies (CF).If a sound entering the ear is displaced from the characteristic frequency, then the strength of response from the basilar membrane will progressively lessen. The fine tuning of the basilar membrane is created by the input of two separate mechanisms. The first mechanism being a linear passive mechanism, which is dependent on the mechanical structure of the basilar membrane and its surrounding structures. The second mechanism is a non-linear active mechanism, which is primarily dependent on the functioning of the OHCs, and also the general physiological condition of the cochlea itself. The base and apex of the basilar membrane differ in stiffness and width, which cause the basilar membrane to respond to varying frequencies differently along its length.
Therefore, assuming normal hearing exists around the dead region, it would produce an audiogram that has a dramatically steep slope between the frequency where a threshold is obtained, and the frequency where a threshold cannot be obtained due to the dead region. However, it appears that this is not the case. Dead regions cannot be clearly found via PTA audiograms. This may be because although the neurons innervating the dead region, cannot react to vibration at their characteristic frequency. If the basilar membrane vibration is large enough, neurons tuned to different characteristic frequencies such as those adjacent to the dead region, will be stimulated due to the spread of excitation. Therefore, a response from the patient at the test frequency will be obtained. This is referred to as “off-place listening”, and is also known as ‘off-frequency listening’. This will lead to a false threshold being found. Thus, it appears a person has better hearing than they actually do, resulting in a dead region being missed. Therefore, using PTA alone, it is impossible to identify the extent of a dead region (See Figure 7 and 8).Consequently, how much is an audiometric threshold affected by a tone with its frequency within a dead region? This depends on the location of the dead region. Thresholds at low frequency dead regions, are more inaccurate than those at higher frequency dead regions.