Neuroprosthetics

Abstract- Throughout the history of mankind, tools have served the role as passive extensions of the body. Recently, the development of neuroprosthesis has changed the scope of how humans interact with tools. Neuroprosthetics enable direct interfacing with the brain and have the great potential for restoring communication and control in disabled individuals. The transformative aspect of direct neural interfaces is that they can be designed as ‘intelligent tools’ that not only carry out intent but also have the capability to assist, evolve, and grow with the user. Unlike other tools, neuroprosthetics exist in a shared space that seamlessly spans the user’s internal representation of the world and the physical environment enabling a much deeper human tool symbiosis. Recent advancements in the engineering of neuroprosthetics are providing a blueprint for how new co adaptive designs change the nature of a user’s ability to accomplish tasks that were not possible using conventional methodologies. This paper analyzes how key advances in science and technology support the development of intelligent neuroprosthesis.

I. INTRODUCTION
Neuroprosthetics is a discipline related to neuroscience and biomedical engineering concerned with developing neural prosthesis. Neural prosthesis is a series of devices that can substitute a motor, sensory or cognitive modality that might have been damaged as a result of an injury or a disease. An example of such devices is Cochlear implants. The development of such devices has a profound impact on the quality of human life, and research in this field intends to resolve disabilities.
There is another side to the application of neural prostheses. These implantable devices can also be used in animal experiments as a tool for neuroscientists in order to develop a better understanding of how the brain works. Accurately probing and recording the electrical signals in the brain would help better understand the relationship among a local population of neurons that are responsible for a specific function. In order to substitute sensory, motor or cognitive modalities, we need to first understand which part of the brain is responsible for those modalities and how those functions are performed. Neuro prosthetics and neuro science have a much intertwined relationship. Neuro prosthesis contribute to better understanding of the neural system and this better understanding helps develop better, more application-specific neural prostheses.

II. HISTORY
The first cochlear implant dates back to 1957. Other landmarks include the first motor prosthesis for foot drop in hemiplegia in 1961, the first auditory brainstem implant in 1977 and a peripheral nerve bridge implanted into spinal cord of adult rat in 1981. Paraplegics were helped in standing with a lumbar anterior root implant (1988) and in walking with Functional Electrical Stimulation (FES).
Regarding the development of electrodes implanted in the brain, an early difficulty was reliably locating the electrodes, originally done by inserting the electrodes with needles and breaking off the needles at the desired depth. Recent systems utilize more advanced probes, such as those used in deep brain stimulation to alleviate the symptoms of Parkinson's disease. The problem with either approach is that the brain floats free in the skull while the probe does not, and relatively minor impacts, such as a low speed car accident, are potentially damaging.
Some researchers, have proposed tethering electrodes to be mounted on the exterior surface of the brain' to the inner surface of the skull. However, even if successful, tethering would not resolve the problem in devices meant to be inserted deep into the brain, such as in the case of deep brain stimulation (DB).

Fig.1:Basic building block of a neuroprosthetic system
III. SENSORY PROSTHETICS
A. Visual prosthetics
A visual prosthesis can create a sense of image by electrically stimulating neuro cells in the visual system. A camera would wirelessly transmit to an implant; the implant would map the image across an array of electrodes. The array of electrodes has to effectively stimulate 600-1000 locations, stimulating these optic neurons in the retina thus will create an image. The stimulation can also be done anywhere along the optic signal's path way. The optical nerve can be stimulated in order to create an image, or the visual cortex can be stimulated, although clinical tests have proven most successful for retinal implants.
A visual prosthesis system consists of an external imaging system which acquires and processes the video. Power and data will be transmitted to the implant wirelessly by the external unit. The implant uses the received power/data to convert the digital data to an analog output which will be delivered to the nerve via micro electrodes.

Fig. 2: Bionic sight and sound. a. The most successful neuroprosthetics b. In common forms of blindness, information-gathering cells called photoreceptors die
B. Auditory Prosthetics
Cochlear implants (CIs), auditory brainstem implants (ABIs), and auditory midbrain implants (AMIs) are the three main categories for auditory prostheses. CI electrode arrays are implanted in the cochlea, ABI electrode arrays stimulate the cochlear nucleus complex in the lower brain stem, and AMIs stimulates auditory neurons in the inferior colliculus. Cochlear implants have been very successful among these three categories. Today Advanced Bionics and Medtronic are the major commercial providers of cochlea implants.

C. Prosthetics for pain relief
The SCS (Spinal Cord Stimulator) device has two main components: an electrode and a generator. The technical goal of SCS for neuropathic pain is to mask the area of a patient's pain with a stimulation induced tingling, known as "paresthesia", because this overlap is necessary (but not sufficient) to achieve pain relief. Paresthesia coverage depends upon which afferent nerves are stimulated. The most easily recruited by a dorsal midline electrode, close to the pial surface of spinal cord, are the large dorsal column afferents, which produce broad paresthesia covering segments caudally.

D.Cognitive prosthesis
Cognitive prostheses seek to restore cognitive function to individuals with brain tissue loss due to injury, disease, or stroke by performing the function of the damaged tissue with integrated circuits. The theory of localization states that brain functions are localized to a specific portion of the brain. However, recent studies on brain plasticity suggest that the brain is capable of rewiring itself so that an area of the brain traditionally associated with a particular function i.e. auditory cortex can perform functions associated with another portion of the brain. i.e. auditory cortex processing visual information. Implants could take advantage of brain plasticity to restore cognitive function even if the native tissue has been destroyed.

fig.3:A neurprosthetic device translates brain signal into actions on a computerscreen
IV. MOTOR PROSTHESIS
Devices which support the function of autonomous nervous system include the implant for bladder control. In the somatic nervous system attempts to aid conscious controls of movement include Functional electrical stimulation and the lumbar anterior root stimulator.
A. Bladder control implants
Where a spinal cord lesion leads to paraplegia, patients have difficulty emptying their bladders and this can cause infection. From 1969 onwards Brindley developed the sacral anterior root stimulator, with successful human trials from the early 1980s onwards. This device is implanted over the sacral anterior root ganglia of the spinal cord; controlled by an external transmitter, it delivers intermittent stimulation which improves bladder emptying. It also assists in defecation and enables male patients to have a sustained full erection.