[Durand et al., 2014] addresses three key issues that need to be dealt with in order to drive the neural interface field forward.
1. The ability to access remotely and reliably internal neural signals.
2. A translation strategy taking basic research to the clinic.
3. Fundamental tool development procedures for neural interfacing
The first item, we have already touched upon. It will require the development of an implantable amplifier and a wireless transmission system. It being wireless will also help translation to clinical research. It not only alleviates the need for the patient to be physically tethered to a neural recording system, but it also gives advantages for experimental use in the form of greater
mobility and less movement artefacts due to cable movement. For BCI-applications, wireless solutions are being developed [Jackson et al., 2006b], but unfortunately they cannot be directly translated for the use of PNS interfaces. First of all, they are often too bulky to implant under the skin in the upper arm. Secondly, they rely on being embedded in the skull, as it is a solid and stable surface. Using a head mounted solution is something one would like to avoid, as it requires a fairly large subcutaneous cable under the skin with all its implications on the signal quality. Miniaturisation of neural processing and decoding equipment to dedicated micro processing chips is the last step, as it will alleviate the need for heavy processing machines.
Lastly, regarding the development of fundamental tools for neural interfacing, it is obvious that the biocompatibility of the neural interfaces needs to increase. The biochemistry field will play an important role in this, as the development of better biocompatible coatings could reduce the rejection rate of the neural interfaces greatly. On the other hand, improvements in robotic systems, like embedding strain gauges and anti-slip mechanisms in the prosthetic hand itself, will also aid in increasing the dexterity, while simultaneously reducing computational demands [Carrozza et al., 2002]. These, however, both fall out of the scope of this thesis. What does not, and which is often forgotten, is how standardisation of procedures can help the development of tools. For example, regarding the implantation techniques, the wheel is figuratively speaking redeveloped by every surgeon, as there is no standard procedure for it. This can and will cause great disparity in the success rate of the application of neural interfaces. The same can be said regarding the neural processing algorithms and the electrical stimulation pulses. Do note that standardisation comes at the end of the development process. First, further research on among others, the development of efficient neural recording/decoding techniques as well as effective stimulation strategies is required before the scientific community can come to a consensus for a single standard, which will help taking the trial-and-error factor out the equation.
Conclusion
In this thesis I investigated how well the TIME array can function as a peripheral nervous system interface for both motor control of prosthetic devices and somatosensory feedback. While it is shown that TIME arrays can be implanted in the median and ulnar nerve in the upper arm of a rhesus macaque, without adverse effects. The surgical procedures are not standardised yet and there is no way to ensure that the TIME arrays will be targeting the nerve fascicles.
The recording capability of the TIME was tested with a motor decoding task, in which the animal was trained in a delayed grasping task. Neural activity was detected in some of the recordings, but it was too sparse for meaningful grip type decoding, due to the amount of noise introduced by, among others, the length of the subcutaneous cable.
Electrical stimulation of the nerve for somatosensory feedback was investigated with a two-alternative forced choice task. The animal succeeded in the discrimination of tactile vibration cues simultaneously applied to the median and ulnar region of the hand. Due to premature array failure, the longevity of the TIME arrays was too short to complete the training of the somatosensory discrimination task with electrical stimulation to the nerve. However, the animal did respond well to the electrical stimulation and no signs of discomfort were observed.
In order to use thin-film arrays such as the TIME arrays for peripheral nervous system record-ings, it is required to move towards a solution with an implantable amplifier in order to improve the signal-to-noise ratio. A more biocompatible and stable solution is necessary to establish long-term nerve stimulation experiments in non-human primates and ultimately in human pa-tients.
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Supplemental Material
A.1 Vibration motor analysis
To analyse the voltage versus frequency relation of the vibration motors used in the tactile vibration stimulation glove in the somatosensory discrimination task, I was looking for a quick, reliable, and cheap method to test the motors for their durability and the inter-motor rotations per minute (RPM) consistency. The MSc students Laura Jens and Luis ´Angel Pardo S´anchez assisted me during the development and testing of the analysis method as well the measuring the motor specifications.