**1. From the beginning to current practice**

Electrical stimulation of the human auditory system is generally traced back to the pioneering experiments of Alessandro Volta, inventor of the battery. When Volta applied 50 volts to his own head, he reported hearing an unpleasant boiling sound [1]. However, the forerunner of a modern CI system is just over 60 years old: opportunistic stimulation of the auditory nerve [2] of a bilaterally deaf patient receiving a facial nerve graft. During the two decades following this work, various clinical studies [3–10] saw the implantation of single and then multi-channel cochlear implant (CI) systems in people suffering profound deafness. While many of these pioneers suffered ridicule at the hands of the mainstream scientific community, clinical considerations prevailed. The early devices that were produced in academic institutions were transferred to commercial organizations, these often building on prior medical device experience, for example experience gained in the pace maker field.

Today over half a million people, from babies under 6 months of age to adults in their late 90s, have been implanted with a CI. While it can be argued that the CI is the most successful medical device ever created, the outcomes are still highly variable (**Figure 1**). In the best of cases, CI users can make fluent use of a telephone, understand speech in adverse listening conditions where there is considerable competing noise and reverberation, hence enjoying independence spanning social lives and careers that would have been unimaginable without their CI device. Even where speech understanding is limited, a release from the isolation of deafness through access to environmental

*The Human Auditory System - Basic Features and Updates on Audiological Diagnosis and Therapy*

**Figure 1.**

*Percent correct scores on the CNC word test ranked from poorest to best for 113 cochlear implant users showing a large variation in outcome reproduced from Holden et al. [79].*

sounds, a reduction in the level of tinnitus and support of lip-reading with a reduction in the effort required for oral communication are all worthwhile benefits from use of a CI. It should also be noted that in many cases those most satisfied with their implant are not those who receive the highest scores on standardized tests of speech understanding.

The following sections will describe how electrical stimulation of the auditory system is achieved, with the main focus being on CI systems. The factors that influence outcome, so far as they are known, will be described, along with the challenges in delivering clinical service, both today and into the future. With the future in mind the major research topics that are currently being addressed will be outlined.
