This page explains the basic principles of audiogram reading with CST (Cochlear Sampling Theory) and how to use it in hearing aid fitting. It will also explain what this means for tuning and the new OtoMerge tuning targets that we will call CST-T (Cochlear Sampling Theory- Target). This new approach to tuning is based on the CST and not tonotopy. The CST offers a different model of inner ear physiology. By taking this physiology into account, a pattern-physiopathology system is associated with CST-T. A pattern involves one or more physiopathologies in the inner ear. This leads to a choice of CST-T for adjusting the amplification of hearing aids. This new approach differs from the target approach used in hearing aids today. On the one hand by using a different model of physiology and therefore of physiopathology in the interpretation of the audiogram. And on the other hand by amplifying the residual coding part only of the inner ear which can route the sound to the auditory chain and therefore the brain. The amplification, frequency and compression parameters are also guided by the CST reading of the audiogram.
1-A different model
The starting point of this paper is the CST. You are all familiar with the tonotopy theory of how the inner ear works. The inner and outer hair cells are arranged along the basilar membrane. Like a piano keyboard, the frequencies coded by the ear are distributed along the membrane. The low sounds at the apex towards the high sounds at the level of the oval window.Unfortunately, this model is wrong, this is demonstrated by Roland Carrat in The Cochlear Sampling Theory (mettre en hyperlien). The inner ear is like the other human senses. Hearing, taste, smell and touch operate on patters. It would be surprising if it were different for the inner ear. It is thanks to this pattern coding method (without frequency selection in the case of the ear) that the brain memorizes the different sensory forms.
The senses are converters from the analog world to a digital one which is the nervous system and the brain.
The inner ear works on this model as it is an analog-digital converter. The Cochlea converts the sound wave into an electrical signal (for the nervous system).We started from this theory to build new auditory targets CST-T.
2-Read the audiogram
The first step of this method is the interpretation of the audiogram with the CST. The audiogram cannot be read by frequency band. Because there is simply no frequency localization along the cochlear membrane as described by the theory of tonotopy. The loss of perception in certain areas of hearing is linked to different physiopathologies of the inner ear. As described in Carrat's CST there are 6 major patterns that can be used for diagnosis. Several causes can be responsible for hearing loss. These 6 large patterns can therefore be associated and create a composite audiogram. For example, loss of outer hair cells AND loss of inner hair cells. This is the form found in a loss described as “presbycusis”. A downward curve from 250Hz to 8kHz. The correct interpretation of the audiogram is equivalent to the correct identification of the different physiopathologies that have caused the loss of auditory perception.
The data of the anamnesis and the audiogram are crossed in order to refine the diagnosis. The anamnesis is essential because patterns can be hidden behind another pathology in the case of composite curves.
3- New targets, CST-T
In the context of existing targets created from the theory of frequency localization (tonotopy) it is the number of dB HL which determines the amplification to be added to a given frequency band. This amplification is weighted by other parameters depending on the strategy of the target in question and acoustic parameters (earmold vent for example).It is not only the volume of amplification per frequency that is determined.
Thresholds and compression ratios are also decided by target calculations.In the case of the CST-T, it is the pathologies that determine the target used for adjustment. We therefore switch from a model based on a loss in dB HL per frequency band to switch to a model based on the pathophysiology of the inner ear (note : outer ear and middle ear pathologies stay the same).
There are therefore different targets in our model according to the pathophysiology of the inner ear. This is the big difference with conventional targets which consider that each hearing loss can be compensated with the same algorithm that calculates the target.
4- Fitting Hearing Aids
After having identified the pathologies in question and the target intended for it, it is necessary to determine how to amplify. Part of the answer lies in the previous step which determined which target to use.The target in question will focus on two main axes that are common to all of the CST-T of our model.On the one hand, use the residual coding channel of the inner ear for the amplification of the hearing aid. And if necessary, use the frequency algorithms offered by hearing aid manufacturers.On the other hand, as we have seen, the ear is an analog-digital converter, the analog part, which is the acoustic wave, must be respected as much as possible. With the CST-T we will take care to respect the temporal envelope as best as possible. And in particular the most fragile parts that are the transients.
These two main axes can come into conflict. This is the case when it is necessary to use frequency algorithms to "push" the signal into the residual coding channel of the patient's inner ear. The acoustic wave is then modified in its frequency component. But it is respect for the residual coding channel that takes precedence over respect for the acoustic wave. The primary objective is to send as much useful information into the residual coding channel that can be routed along the auditory chain and therefore to the brain.Hearing aids are made to work by frequency band. We will use these frequency bands to serve our model even though we know that the inner ear does not work that way.
This page was a quick presentation on how the Cochlear Sampling Theory (Roland Carrat) can be used to do the diagnosis of hearing loss to fit hearing aids. The tonotopy doesn’t make a lot of sense as a physiology model for the inner ear. Because the diagnosis is flawed the hearing aid fitting can’t be good. That’s why patient usually describe the amplification as « metallic sound » or « weird ». The principal cause of that, is the amplification of sounds that can’t be coded by the inner ear and then transmitted to the brain. In contrary the CST is very useful to know what pathology is implicated in the hearing loss and how to fit the hearing aid to bring more sound informations to the brain.
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