**3. Electrical stimulation principles**

In the earlier chapters of this book the auditory system has been described in some detail, including pathology that can result in the most debilitating degrees of hearing loss: severe to profound deafness. Fortunately, electrical stimulation can be delivered without external, middle or indeed even an inner ear. However, in the large majority of clinical cases the auditory nerve is intact and can be accessed via a very poorly or non-functioning cochlea: it being this damage that the CI bypasses. The operating principle of a CI is that small electrical currents are able to initiate activity on the auditory nerve that crudely mimics the activity produced by a normally functioning cochlea. Taking advantage of the cochlea's tonotopic organization, currents representing high frequency sounds are delivered to the base, while currents representing lower frequency sounds are delivered to more apical cochlear locations. This is achieved through the use of an electrode array containing multiple separate electrode contacts placed along the scala tympani (**Figure 4**). The number of intra-cochlear electrode contacts in clinical service varies by manufacturer between 12 and 24. In addition, either the shorting of adjacent contacts, or simultaneous delivery of synchronized stimulation patterns on multiple electrode contacts, seeks to increase the number of stimulation sites available [11–13].

In **Figure 4** an electrode array is shown placed in the scala tympani. Here the exposed electrode surface, from which stimulation current is delivered, faces towards the modiolar wall, behind which the auditory nerve's spiral ganglion cell bodies are located in Rosenthal's canal. The remainder of the array is composed of a soft silicone that supports the contacts and the insulated wires connecting the electrode contacts to the implant's electronics. A modiolar facing contact surface orients the electrical stimulation towards the primary stimulation targets, the spiral ganglion cell bodies. Since the perilymph fluid in the scala tympani is electrically conductive, it allows current to flow through the cochlea and achieve stimulation of neural elements. A downside is that current also tends to spread along the scala, rather than addressing only the area local to the electrode contact where we would like it to act. If peripheral processes still remain in the cochlea, extending from the cell bodies to the organ of Corti, then these may also be targets for electrical stimulation. Unfortunately it is not currently possible to accurately know the status of any individual's cochlea. It appears reasonable to assume that a great deal of the variability illustrated in **Figure 1** comes from variations in the health of those individuals' cochleae, this variation being part dependent on environment and disease and the individual's particular physiology, as well as how any one individual's immune system reacts to the insertion and presence of the electrode array itself.

The most common type of stimulation paradigm used in CIs today is monopolar stimulation (**Figure 5a**). Here the implant introduces current into the cochlea via a relatively small electrode contact. A typical surface area might be 0.2 mm2 . The density of current close to the electrode contact introduces a higher probability of activating elements close to the contact, the probability decreasing for elements

#### **Figure 4.**

*View of how an electrode array will be positioned within the scala tympani of the cochlea, here with the electrode contacts facing towards the modiolar wall behind with the spiral ganglion cell bodies are located.*

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**Figure 6.**

*body and one on the electrode lead assembly.*

*Electrical Stimulation of the Auditory System DOI: http://dx.doi.org/10.5772/intechopen.85285*

*where current is returned by two adjacent contacts.*

**Figure 5.**

at increasing distance. Monopolar stimulation current is returned to the implant using a distant extra-cochlear electrode that has at least 10 times the intra-cochlear electrode contact's surface area. This keeps the returning current density low, avoiding stimulation at this remote site. Typically there are two return electrodes on a cochlear implant, in case anything goes wrong with one of them. One is placed on the body of the implant while the other is on a separate flying lead, or on the electrode lead assembly but located outside of the cochlea. In **Figure 6** an implant

*Electrical stimulation configurations: (a) monopolar where current is returned to a large distant extra-cochlear return electrode, (b) bipolar where current flows between adjacent intra-cochlear electrodes and (c) tripolar* 

Alternative stimulation paradigms are sometimes used, but mainly for research. **Figure 5b** shows a bipolar stimulation paradigm. Here two intra-cochlear electrode contacts, separated by around 1 mm, operate as a pair. Stimulation is introduced by one contact and returned by the other. This in theory restricts stimulation to a small part of the cochlea, so should help with the spread of current mentioned above. However, in practice current introduced by one contact is returned to the other without activating enough neural tissue to create a loud enough hearing sensation. Hence, it is often necessary to form a bipolar pair using contacts that are not adjacent but for example are spaced 2 mm or more apart. In addition, the arrangement of contacts along the cochlea means that there will be a plane of zero field between the contacts, leading to a need to use higher currents and thus produce a wider spread in stimulation. In **Figure 5c** tripolar stimulation is shown. Now a group of three electrode contacts are used together. Stimulation is introduced via the middle contact and ideally half returned via the adjacent contact on either side. This avoids

*A cochlear implant showing its various components. Note the two ground or return contacts, one on the case* 

can be seen with its various component parts indicated.

*Electrical Stimulation of the Auditory System DOI: http://dx.doi.org/10.5772/intechopen.85285*

**Figure 5.**

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

large majority of clinical cases the auditory nerve is intact and can be accessed via a very poorly or non-functioning cochlea: it being this damage that the CI bypasses. The operating principle of a CI is that small electrical currents are able to initiate activity on the auditory nerve that crudely mimics the activity produced by a normally functioning cochlea. Taking advantage of the cochlea's tonotopic organization, currents representing high frequency sounds are delivered to the base, while currents representing lower frequency sounds are delivered to more apical cochlear locations. This is achieved through the use of an electrode array containing multiple separate electrode contacts placed along the scala tympani (**Figure 4**). The number of intra-cochlear electrode contacts in clinical service varies by manufacturer between 12 and 24. In addition, either the shorting of adjacent contacts, or simultaneous delivery of synchronized stimulation patterns on multiple electrode contacts,

In **Figure 4** an electrode array is shown placed in the scala tympani. Here the exposed electrode surface, from which stimulation current is delivered, faces towards the modiolar wall, behind which the auditory nerve's spiral ganglion cell bodies are located in Rosenthal's canal. The remainder of the array is composed of a soft silicone that supports the contacts and the insulated wires connecting the electrode contacts to the implant's electronics. A modiolar facing contact surface orients the electrical stimulation towards the primary stimulation targets, the spiral ganglion cell bodies. Since the perilymph fluid in the scala tympani is electrically conductive, it allows current to flow through the cochlea and achieve stimulation of neural elements. A downside is that current also tends to spread along the scala, rather than addressing only the area local to the electrode contact where we would like it to act. If peripheral processes still remain in the cochlea, extending from the cell bodies to the organ of Corti, then these may also be targets for electrical stimulation. Unfortunately it is not currently possible to accurately know the status of any individual's cochlea. It appears reasonable to assume that a great deal of the variability illustrated in **Figure 1** comes from variations in the health of those individuals' cochleae, this variation being part dependent on environment and disease and the individual's particular physiology, as well as how any one individual's immune

seeks to increase the number of stimulation sites available [11–13].

system reacts to the insertion and presence of the electrode array itself.

The most common type of stimulation paradigm used in CIs today is monopolar stimulation (**Figure 5a**). Here the implant introduces current into the cochlea via a relatively small electrode contact. A typical surface area might be 0.2 mm2

density of current close to the electrode contact introduces a higher probability of activating elements close to the contact, the probability decreasing for elements

*View of how an electrode array will be positioned within the scala tympani of the cochlea, here with the electrode contacts facing towards the modiolar wall behind with the spiral ganglion cell bodies are located.*

. The

**88**

**Figure 4.**

*Electrical stimulation configurations: (a) monopolar where current is returned to a large distant extra-cochlear return electrode, (b) bipolar where current flows between adjacent intra-cochlear electrodes and (c) tripolar where current is returned by two adjacent contacts.*

at increasing distance. Monopolar stimulation current is returned to the implant using a distant extra-cochlear electrode that has at least 10 times the intra-cochlear electrode contact's surface area. This keeps the returning current density low, avoiding stimulation at this remote site. Typically there are two return electrodes on a cochlear implant, in case anything goes wrong with one of them. One is placed on the body of the implant while the other is on a separate flying lead, or on the electrode lead assembly but located outside of the cochlea. In **Figure 6** an implant can be seen with its various component parts indicated.

Alternative stimulation paradigms are sometimes used, but mainly for research. **Figure 5b** shows a bipolar stimulation paradigm. Here two intra-cochlear electrode contacts, separated by around 1 mm, operate as a pair. Stimulation is introduced by one contact and returned by the other. This in theory restricts stimulation to a small part of the cochlea, so should help with the spread of current mentioned above. However, in practice current introduced by one contact is returned to the other without activating enough neural tissue to create a loud enough hearing sensation. Hence, it is often necessary to form a bipolar pair using contacts that are not adjacent but for example are spaced 2 mm or more apart. In addition, the arrangement of contacts along the cochlea means that there will be a plane of zero field between the contacts, leading to a need to use higher currents and thus produce a wider spread in stimulation. In **Figure 5c** tripolar stimulation is shown. Now a group of three electrode contacts are used together. Stimulation is introduced via the middle contact and ideally half returned via the adjacent contact on either side. This avoids

#### **Figure 6.**

*A cochlear implant showing its various components. Note the two ground or return contacts, one on the case body and one on the electrode lead assembly.*

the zero potential plane problem of bipolar stimulation and theoretically provides a tighter containment of stimulation. Again, in practice there is a need to recruit a given number of neurones to signal sufficient loudness and this means increasing the tripolar stimulation current on the middle contact. Eventually the current on the return electrodes will become high enough to cause stimulation, resulting in a wider spread of current than intended. In many cases it is not even possible to increase the current far enough to achieve sufficient loudness for a tripolar configuration. In such cases some of the current has to be returned to the remote extracochlear return electrode, a configuration referred to a partial tripolar. This even further increases the current spread. So, with these practical limitations in mind it is not difficult to see why monopolar is universally used as the default stimulation paradigm, despite the apparent large current spread that this entails.
