**3.1 Technical considerations**

In the interests of simplicity some of the more technical aspects of electrical stimulation have been avoided in the text above, allowing focus on the broader application. In this section some of these more technical issues will be discussed. Where a reader is not interested in technical detail this section can be ignored.

Ohm's Law states that the electrical voltage difference required to drive a given current is directly proportional to the resistance through which the current has to flow. With changes in the resistance, or more generally the impedance if we consider frequency effects, a voltage source would lead to uncontrolled changes in the current being output. As described below this could lead to uncontrolled changes in loudness over time. Most of today's CI systems deliver electrical stimulation through one or more current sources. As the name implies, this circuit attempts to deliver the current requested of it regardless of how much electrical resistance is offered by the body. A current source is said to be in compliance when it is delivering the current requested of it. Given a finite amount of voltage being available within an implant, for example 8 volts, there will be a maximum impedance into which the implant's maximum output current can be delivered. With a typical stated maximum output current of 2000 μA, the maximum impedance for which this could be delivered will be 4000 Ohms (8 volts divided by 2000 μA, or 8/2 × 10<sup>−</sup><sup>3</sup> = 4000). For higher impedances the maximum stated output current will not be available. For lower impedances the implant will be limited to its maximum output current value, ensuring safe operation.

Since CI systems are designed to provide stimulation for essentially all waking hours, day after day over decades, their output must not damage the neural tissue that they are intended to stimulate. One obvious source of damage is the delivery of direct current (DC). If a DC current is applied to the body it will result a process called electrolysis. Here there will be a continuous transport of ions, charged atomic particles, leading to dissolution of the platinum electrode contact and destruction of the cochlear tissue: obviously a catastrophic situation. Research in animals indicates that even very small amounts of DC, 400 nA, is enough to cause tissue damage [14]. Several mechanisms are used in a CI to prevent the delivery of DC. Firstly, a balanced stimulus waveform is used, almost always a symmetrical pulse having two complementary phases (**Figure 7a**), although so long as the two phases contain an equal and opposite area they need not be symmetrical (**Figure 7b**). At the simplest interpretation, the first phase, referred to as cathodic, will depolarize neurones hence producing the electrical stimulation that we seek to achieve. The second, anodic, phase balances the stimulation resulting in no nett current being delivered to the body, thereby avoiding DC. Even with very careful design, there is likely to be some small imbalance between the two phases. To account for any in-balance,

**91**

cally 30 μC/cm<sup>2</sup>

**Figure 7.**

for example 400 μC/cm<sup>2</sup>

and largely ignored.

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

following delivery of a stimulation pulse the electrode contact is connected to ground, ideally removing any residual DC. Finally, an electronic component called a capacitor is placed in series with the stimulating electrode contact. A capacitor does not allow DC to pass, so offers yet more protection in case some fault in the electronics interferes with either of the previous two protection mechanisms. Together these mechanisms appear successful in avoiding the delivery of DC. Devices do fail, particularly in the pediatric population, with reimplantation rates over tens of years being reported at 8% from the well-established Sydney clinic [15]. Typically half of these failures are medical issues and half device failures. However, in virtually all cases it is possible to re-implant the patient, with outcomes almost always being as

*or asymmetrical phases (b) where the area of each phases is identical.*

*Stimulation waveforms with balanced cathodic and anodic phases may have either symmetrical phases (a),* 

good as those obtained when the original device was working well [16].

While we speak about stimulation current it is really the electrical charge that is at issue. Charge is simply the product of current times time and has units of Coulombs, C. Electro-chemical considerations mean that an electrode has a maximum charge injection capacity, such that a given size and material will only be able to handle a given charge limit in a reversible way, so that all of the charge injected in one phase can be removed in the second phase [17]. This is necessary to avoid the DC as discussed above. A conservative value for the maximum charge density, typi-

into the implant's controlling software, ensuring that this limit is never exceeded. Animal experiments confirm that chronic stimulation with higher charge densities,

to the loss of auditory neurones [19]. The loudness sensation produced by electrical stimulation is related to the amount of charge delivered in one phase of the stimulation waveform. There is also an effect of the rate at which stimulation pulses are delivered. However, for the stimulation rates used in clinical practice, typically 500–2000 pulses per second per channel (ppps/ch) the effect of rate is quite small

Stimulation current flows through cochlear tissue as a result of voltage differences developed along the current's path. How these voltage differences are arranged along the length of a neurone's peripheral or medial process, or indeed across the cell body, determines which neurones depolarize, leading to action potentials being generated. The action potential may propagate to the medial synapse with the cochlear nucleus and hence initiate activity on the auditory pathway, leading to a sense of hearing being detected in the brain. Rather than stimulating a single neuron, typically hundreds, or even thousands, of neurones in a region of the cochlea will be addressed by a single electrode contact. These patterns

[18], taking account of the electrode dimensions, is programmed

, results in the dissolution of platinum but interestingly not

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

**Figure 7.**

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

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

In the interests of simplicity some of the more technical aspects of electrical stimulation have been avoided in the text above, allowing focus on the broader application. In this section some of these more technical issues will be discussed. Where a reader is not interested in technical detail this section can be ignored. Ohm's Law states that the electrical voltage difference required to drive a given current is directly proportional to the resistance through which the current has to flow. With changes in the resistance, or more generally the impedance if we consider frequency effects, a voltage source would lead to uncontrolled changes in the current being output. As described below this could lead to uncontrolled changes in loudness over time. Most of today's CI systems deliver electrical stimulation through one or more current sources. As the name implies, this circuit attempts to deliver the current requested of it regardless of how much electrical resistance is offered by the body. A current source is said to be in compliance when it is delivering the current requested of it. Given a finite amount of voltage being available within an implant, for example 8 volts, there will be a maximum impedance into which the implant's maximum output current can be delivered. With a typical stated maximum output current of 2000 μA, the maximum impedance for which this could be

paradigm, despite the apparent large current spread that this entails.

delivered will be 4000 Ohms (8 volts divided by 2000 μA, or 8/2 × 10<sup>−</sup><sup>3</sup>

For higher impedances the maximum stated output current will not be available. For lower impedances the implant will be limited to its maximum output current

Since CI systems are designed to provide stimulation for essentially all waking hours, day after day over decades, their output must not damage the neural tissue that they are intended to stimulate. One obvious source of damage is the delivery of direct current (DC). If a DC current is applied to the body it will result a process called electrolysis. Here there will be a continuous transport of ions, charged atomic particles, leading to dissolution of the platinum electrode contact and destruction of the cochlear tissue: obviously a catastrophic situation. Research in animals indicates that even very small amounts of DC, 400 nA, is enough to cause tissue damage [14]. Several mechanisms are used in a CI to prevent the delivery of DC. Firstly, a balanced stimulus waveform is used, almost always a symmetrical pulse having two complementary phases (**Figure 7a**), although so long as the two phases contain an equal and opposite area they need not be symmetrical (**Figure 7b**). At the simplest interpretation, the first phase, referred to as cathodic, will depolarize neurones hence producing the electrical stimulation that we seek to achieve. The second, anodic, phase balances the stimulation resulting in no nett current being delivered to the body, thereby avoiding DC. Even with very careful design, there is likely to be some small imbalance between the two phases. To account for any in-balance,

= 4000).

**3.1 Technical considerations**

value, ensuring safe operation.

**90**

*Stimulation waveforms with balanced cathodic and anodic phases may have either symmetrical phases (a), or asymmetrical phases (b) where the area of each phases is identical.*

following delivery of a stimulation pulse the electrode contact is connected to ground, ideally removing any residual DC. Finally, an electronic component called a capacitor is placed in series with the stimulating electrode contact. A capacitor does not allow DC to pass, so offers yet more protection in case some fault in the electronics interferes with either of the previous two protection mechanisms. Together these mechanisms appear successful in avoiding the delivery of DC. Devices do fail, particularly in the pediatric population, with reimplantation rates over tens of years being reported at 8% from the well-established Sydney clinic [15]. Typically half of these failures are medical issues and half device failures. However, in virtually all cases it is possible to re-implant the patient, with outcomes almost always being as good as those obtained when the original device was working well [16].

While we speak about stimulation current it is really the electrical charge that is at issue. Charge is simply the product of current times time and has units of Coulombs, C. Electro-chemical considerations mean that an electrode has a maximum charge injection capacity, such that a given size and material will only be able to handle a given charge limit in a reversible way, so that all of the charge injected in one phase can be removed in the second phase [17]. This is necessary to avoid the DC as discussed above. A conservative value for the maximum charge density, typically 30 μC/cm<sup>2</sup> [18], taking account of the electrode dimensions, is programmed into the implant's controlling software, ensuring that this limit is never exceeded. Animal experiments confirm that chronic stimulation with higher charge densities, for example 400 μC/cm<sup>2</sup> , results in the dissolution of platinum but interestingly not to the loss of auditory neurones [19]. The loudness sensation produced by electrical stimulation is related to the amount of charge delivered in one phase of the stimulation waveform. There is also an effect of the rate at which stimulation pulses are delivered. However, for the stimulation rates used in clinical practice, typically 500–2000 pulses per second per channel (ppps/ch) the effect of rate is quite small and largely ignored.

Stimulation current flows through cochlear tissue as a result of voltage differences developed along the current's path. How these voltage differences are arranged along the length of a neurone's peripheral or medial process, or indeed across the cell body, determines which neurones depolarize, leading to action potentials being generated. The action potential may propagate to the medial synapse with the cochlear nucleus and hence initiate activity on the auditory pathway, leading to a sense of hearing being detected in the brain. Rather than stimulating a single neuron, typically hundreds, or even thousands, of neurones in a region of the cochlea will be addressed by a single electrode contact. These patterns

of stimulation are interpreted as sound input by the higher levels of the auditory system, leading to the sense of electrical hearing. The next section will describe how sounds detected by the CI system's microphone will result in the generation of electrical stimulation patterns.
