**6. Current research**

With the CI field involving a wide range of professionals including, surgeons, nurses, audiologists, engineers, physicists, speech and language therapists, teachers of the deaf, hearing therapists, rehabilitation specialists, psychologists and health economists, research related to the field can cover a very wide range. Here some of the key topics that are most closely connected to extending current practice will be reviewed.

It is clear that when less trauma is inflicted during surgery that outcomes are better [60], this being the case whether or not there is residual hearing at risk [61, 62]. Hearing preservation is thus a key topic for surgeons. The design of less traumatic electrode arrays [48–50, 63] and the development of less traumatic

**101**

array insertion.

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

surgical techniques [64], including the use of robot assisted insertion [65], are factors that can lead to reduced trauma. Providing real time feedback to the surgeon during electrode array insertion is a hotly researched area. Electrocochleography (ECochG), where acoustic stimulation of the ear produces a cochlear microphonic signal [66–68] that the surgeon can use to gauge proximity to structures, such as the basilar membrane, appears promising with clinical systems due to launch in 2019. The use of drugs to reduce the body's reaction to implantation is also an area with some connection to minimizing trauma. Steroids such as Dexamethasone or antimitotic drugs [69] have been applied to suppress a fibrotic reaction during and immediately after surgery. Some longer term benefits have been shown but mainly in lower electrode contact impedances rather than significant outcome advantage [70]. Longer term deployment of steroids and other drugs has been proposed for some time [71] but has not yet seen clinical practice. Drugs such as neurotrophins [72] have been proposed to enhance the spiral ganglion but carry considerable risk of uncontrolled sprouting of new fibers that may not lead to any improvement in electrical stimulation [73]. Likewise the regrowth of hair cells or other cochlear structures [74] is an extremely challenging problem, although simply reconnecting peripheral processes that have been damaged while leaving intact hair cells [75] may be more manageable in the foreseeable future. While not a drug, near infrared light has been shown to promote tissue healing [76], helping reduce the extent of hearing loss following cochlear stress [77] and has been proposed as an approach that might also enhance the cochlea's ability to survive the traumas of electrode

Deployment, development and assessment of sound coding strategies continues with a variety of goals. Optimizing compression parameters to maximize speech understanding [78], reviewing the effect of various parameters as well as individualized fitting approaches [79, 80] all promise improvements for strategies that are already available but could be fitted better. Likewise, tools to guide clinicians in fitting for performance, rather than simply for comfort [45] should also lead to substantial improvement in outcomes. Seeking improvement via limiting current spread, through tripolar stimulation [81, 82], phased array [83] or manipulation of field interactions [84] have so far not shown a general improvement, although benefits for some recipients have been demonstrated. New sound coding strategies that attempt to improve temporal information such as FSP [85], or reduce masking effects such as MP3000 [31] have been developed and introduced into clinical practice. There may be more benefit in reducing battery power from the likes of MP3000 or Optima than in any improvement in outcome. Connected to improving outcome, research into the

listening effort required to understand speech has been increasing [86, 87].

the test methodology used involves a questionable comparison of conditions.

The very nature of the speech tests used to evaluate CI systems is an active area of research. Standardization across languages has involved the use of matrix tests [88]. These tests involve a fixed syntax and a closed set of keywords so can be selfadministered and used essentially indefinitely. However, the sentences are not fully representative of natural sentences. Avoiding fixed presentation levels, something necessary to evaluate the function of AGC has led to development of roving presentation level tests [89–91]. These tests provide insights into where a particular subject may have problems, so could be useful in supporting programming. As with the STARR tests, multiple speakers have been included in tests such as the AzBio [92] and taken much further with the coordinate response matrix test that can run on a multiple loudspeaker array and hence mimic a more realistic test environment [93]. Improved outcomes have been shown thorough use of the recently developed wireless technology supporting integrated bimodal [94, 95] and CROS [96]. EAS has also been shown to produce substantial improvements in outcome [97] although

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

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

provide less chance of the ABI's electrode pad moving out of position, risking substantial non-auditory stimulation. Outcomes with ABI devices are generally substantially poorer than with CIs. In some series there is essentially no open-set speech understanding possible [55], while in others the speech understanding is limited, with only occasional high levels of speech understanding [56]. The reasons for poor performance with ABIs are not fully understood. Beyond potential movements of the electrode pad, there are specialized auditory functions being carried out in the cochlear nucleus, meaning that simply assuming raw tonotopic stimulation patterns may not be sufficient. Additionally, those receiving ABI devices may have many other issues beyond deafness and these could also explain

Stimulation of even higher structures in the auditory system has been attempted

While placement of an electrode array in the scala tympani, or where necessary in the scala vestibuli, leaves the electrode contacts quite close to their target neurones they are still some 1–3 mm away. This separation prevents discrete stimulation of local neural populations as discussed above. It has been proposed that an electrode array could be inserted directly into the auditory nerve, or failing this inside the modiolus. Promising results have been shown from acute experiments in cats [59]. Recording form electrodes placed in the inferior colliculus indicates that intra-neural simulation is more localized than stimulation using an electrode array placed in the scala tympani. There are considerable challenges to overcome before intra-neural stimulation could be considered for humans. The surgical access is not straightforward, risking losing all residual hearing. The human auditory nerve being in the order of 1 mm diameter would require very small stimulating contacts. While the stimulation currents required would be smaller than those needed for a traditional CI, charge density considerations require careful consideration. Also, how well an electrode array can be placed and be tolerated in the auditory nerve, without the destruction of auditory fibers or the formation of granulation tissue will need to be carefully studied. Finally, the structure of the auditory nerve is complex, with axons from different parts of the cochlea rolling into the tubular

through an auditory mid-brain implant (AMBI), where the electrode array is inserted into the inferior colliculus. Currently this is restricted to a pure research device [57]. Within the inferior colliculus it is possible to access a tonotopic organization, using a shortened version of a traditional CI electrode array, 10 mm long as opposed to 20–30 mm for most CI arrays. However, when accessing the auditory system at an even higher level than with an ABI device, the amount of pre-processing that should have already been done leaves a crude CI type coding strategy only able to support very limited outcomes. Already at this level higher stimulation rates are

inappropriate, leaving limited sound coding strategy options [58].

nerve, making tonotopic targeting an additional challenge.

With the CI field involving a wide range of professionals including, surgeons, nurses, audiologists, engineers, physicists, speech and language therapists, teachers of the deaf, hearing therapists, rehabilitation specialists, psychologists and health economists, research related to the field can cover a very wide range. Here some of the key topics that are most closely connected to extending current practice will be reviewed. It is clear that when less trauma is inflicted during surgery that outcomes are better [60], this being the case whether or not there is residual hearing at risk [61, 62]. Hearing preservation is thus a key topic for surgeons. The design of less traumatic electrode arrays [48–50, 63] and the development of less traumatic

some of the difference in outcome.

**100**

**6. Current research**

surgical techniques [64], including the use of robot assisted insertion [65], are factors that can lead to reduced trauma. Providing real time feedback to the surgeon during electrode array insertion is a hotly researched area. Electrocochleography (ECochG), where acoustic stimulation of the ear produces a cochlear microphonic signal [66–68] that the surgeon can use to gauge proximity to structures, such as the basilar membrane, appears promising with clinical systems due to launch in 2019.

The use of drugs to reduce the body's reaction to implantation is also an area with some connection to minimizing trauma. Steroids such as Dexamethasone or antimitotic drugs [69] have been applied to suppress a fibrotic reaction during and immediately after surgery. Some longer term benefits have been shown but mainly in lower electrode contact impedances rather than significant outcome advantage [70]. Longer term deployment of steroids and other drugs has been proposed for some time [71] but has not yet seen clinical practice. Drugs such as neurotrophins [72] have been proposed to enhance the spiral ganglion but carry considerable risk of uncontrolled sprouting of new fibers that may not lead to any improvement in electrical stimulation [73]. Likewise the regrowth of hair cells or other cochlear structures [74] is an extremely challenging problem, although simply reconnecting peripheral processes that have been damaged while leaving intact hair cells [75] may be more manageable in the foreseeable future. While not a drug, near infrared light has been shown to promote tissue healing [76], helping reduce the extent of hearing loss following cochlear stress [77] and has been proposed as an approach that might also enhance the cochlea's ability to survive the traumas of electrode array insertion.

Deployment, development and assessment of sound coding strategies continues with a variety of goals. Optimizing compression parameters to maximize speech understanding [78], reviewing the effect of various parameters as well as individualized fitting approaches [79, 80] all promise improvements for strategies that are already available but could be fitted better. Likewise, tools to guide clinicians in fitting for performance, rather than simply for comfort [45] should also lead to substantial improvement in outcomes. Seeking improvement via limiting current spread, through tripolar stimulation [81, 82], phased array [83] or manipulation of field interactions [84] have so far not shown a general improvement, although benefits for some recipients have been demonstrated. New sound coding strategies that attempt to improve temporal information such as FSP [85], or reduce masking effects such as MP3000 [31] have been developed and introduced into clinical practice. There may be more benefit in reducing battery power from the likes of MP3000 or Optima than in any improvement in outcome. Connected to improving outcome, research into the listening effort required to understand speech has been increasing [86, 87].

The very nature of the speech tests used to evaluate CI systems is an active area of research. Standardization across languages has involved the use of matrix tests [88]. These tests involve a fixed syntax and a closed set of keywords so can be selfadministered and used essentially indefinitely. However, the sentences are not fully representative of natural sentences. Avoiding fixed presentation levels, something necessary to evaluate the function of AGC has led to development of roving presentation level tests [89–91]. These tests provide insights into where a particular subject may have problems, so could be useful in supporting programming. As with the STARR tests, multiple speakers have been included in tests such as the AzBio [92] and taken much further with the coordinate response matrix test that can run on a multiple loudspeaker array and hence mimic a more realistic test environment [93].

Improved outcomes have been shown thorough use of the recently developed wireless technology supporting integrated bimodal [94, 95] and CROS [96]. EAS has also been shown to produce substantial improvements in outcome [97] although the test methodology used involves a questionable comparison of conditions.

With increasing numbers of CI recipients needing management by financially constrained health care systems, methods of improving patient management are being developed. These include the use of consumer devices, smart phones and tablets, to run Apps that can evaluate a CI user's speech understanding and qualitative condition as well as remotely analyzing the status of their implant and sound processor hardware [98] and delivering rehabilitation material usable by adult CI recipients and the families of young children. This whole area will necessarily see much development in the coming years.

The pressures on CI manufacturers to reduce size and improve comfort and ease of use continue, with cosmetic considerations playing a large role in the choice of which CI systems are selected. Reliability of both the implanted and external parts of the CI system needs to be continuously improved, underpinning consistent device use and reducing costs inherent in managing failures. Further pressures on cost are also critical to address so that the enormous unmet need for cochlear implants in developing countries can also be met.
