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5. Neurorobotic Feedback Sensation
The duplex wiring facilitated not only communication to the limb, but also feedback signals from the limb to the Scientists are also simulating tactile perception by electrical excitation feedback to neural cells. University of Utah bioengineers connected electrical leads from a computerized limb to arm nerves of an amputee, who was able to control the limb as the neural impulses were being transmitted from his nerve endings to circuits in the computerized limb. The duplex wiring facilitated not only communication to the limb, but also feedback signals from the limb to the nerves. Thus the team has developed a limb prostheses control mechanism that enables sensations of touch and joint movement in an amputee's nervous system. The system consists of electrodes implanted in muscle nerve fibers, which when stimulated, produce graded discrete sensations of touch or movement (commonly known as the amputee's phantom hand). The electrodes can read and interpret neural firings in the nerve fascicles related with the phantom limb movements to control the robotic arm, allowing the amputee to both discern and apply the appropriate pressure to grasp an object. This neurorobotic feedback mechanism was the inception of "direct neural control of an artificial arm" in amputees .Electrical stimulation of auditory and visual neurosensory perceptions has also been developed. In 1997, electrical engineers at the University of Michigan developed a silicon micromachined electrode array , an electronic apparatus implanted in the cochlea that can produce perception of sound for the hearing-impaired, whose deafness cannot be remedied with sound wave amplification via hearing aids. This cochlear implant, or bionic ear, directly transmits acoustic electrical impulses down auditory nerves inside the spiral hearing organ. The components of the cochlear implant include an audio to electrical signal converter, acoustic microprocessor chip, and microelectrode array transmitter circuitry.
With some training, the profoundly deaf can begin to hear with the implant (see Figure 5), which has been surgically inserted into thousands of patients with damaged cochlear sensory hair cells, which convert acoustic signals into neural firings. In some cases, where the auditory nerves are all defunct, surgeons have to implant auditory receptors directly onto the auditory brainstem. Circumventing the cochlear auditory nerves, this prosthesis comprises of implanting a microelectrode array on the acoustic brainstem. Whereas previously, auditory brainstem implants electrically connected only superficially to the ventral cochlear nucleus, recent neural prosthetic advances allow the microelectrode arrays to be connected to the ventral cochlear neurons .
Figure 5: The cochlear implant.
For those whose eyes are surgically enucleated due to their being blinded and pained with cancers or tumors, orbital implants allow ocular prosthesis (see Figure 6) or artificial eyes, but not sight restoration.In the field of visual neurosensory research, sight is refurbished in the visually-impaired by implanting electrodes on or behind their degenerating retinas. Subretinal implants, consisting of an incredibly fine plate of photo-receptive microphotodiodes, are inserted in between the pigmented epithelium and an outer layer consisting of photoreceptor cells. The neurons in the retina are stimulated with electric currents produced in the microphotodiodes when light is incident on the retina. On the other hand, epiretinal implants, microelectronic circuits that read and process image data received from a remote camera, are coupled with ganglion cells in the retina. Ganglion cell axons transmit image data through the optic nerve to the visual cortex when the epiretinal implant feeds them with signals received from the peripheral camera .
Figure 6: A cross-sectional diagram of visual prosthesis.
The above remedies amalgamate with the retina to aid in its deficiencies. However, for glaucomatous patients, whose ganglion axons may be degenerate, visual prosthesis (connecting a camera to the optic nerve or visual cortex) is required. Currently, however, cortical surface and intracortical stimulation by connecting cameras to the visual cortex has only led the visually impaired to perceive spots of light. Further research is being conducted to fully restore image perception .