The Best of Both Worlds: Electroacoustic Stimulation (EAS) – also called Hybrid Stimulation
The term ‘hybrid’ is much better known in the automobile industry. It means that two different systems are used which complement each other when driving a vehicle. The term is also used in relation to hearing. Here it refers to the use of two different systems for one ear: electrical impulses generated by a cochlear implant processor and acoustic signals provided by a conventional hearing aid. This is known as electroacoustic stimulation (EAS) or hybrid stimulation. The impulses induced by the cochlear implant transfer high-frequency auditory information, while the signals induced by the hearing aid transfer low-frequency auditory information. If hearing deteriorates over time, the processor can be reprogrammed so that all frequencies can still be perceived.
If an individual still has good hearing at low frequencies and benefits from hearing aid amplification, a hybrid electrode is used in order to preserve the residual hearing. This electrode is shorter and much more delicate than a conventional CI electrode, and is inserted into the inner ear using a special surgical method.
A hybrid system not only allows preservation of one’s current state of hearing, but also enables the use of a combination of (a) the well-known acoustic hearing sensations provided by a hearing aid and (b) the new electrically induced hearing sensations for the high frequencies.
Both parts are controlled by only one device, which requires a single power source. The user can adapt more effectively to different situations – at an event or at a restaurant, for example – and can perform better at separating background noise from voices. However, even with this system (just like with all other auditory systems), singling out one individual voice is still a problem. The additional use of a directional microphone may help. This amplifies the voice of the speaker directly in front of it to a normal level, and other speakers to a quieter level.