A brain–machine interface (BMI) is a system that provides direct communication between the brain and an external device. It is designed to assist or repair human cognitive or sensory-motor functions. BMI research has studied sensory systems with a focus on the use of neuro-prosthetic devices to restore impaired hearing or vision.
Because of cortical plasticity, signals from implanted prostheses can be processed by the brain after rehabilitation.
The most widely used neuro-prosthetic devices are cochlear implants. A cochlear implant is activated by sound waves, and signals generated from it are filtered and decomposed into an envelope and a temporal structure, which are then converted into electrical energy. The electrical energy then stimulates the auditory nerve. Speech perception scores typically continue to improve during the first 3–12 months of implant use, suggesting that plastic changes occur in the brain to use sparse inputs better.
In retinal prosthesis, a camera takes an image, and a computer processes and transmits it wirelessly to the implant. The implant maps the image across an array of electrodes, and it stimulates neurons in the retina and sends neural signals to the visual cortex. The recipients then perceive a monochromatic pattern of dots.
Several approaches exist in rental prosthesis; epiretinal prosthesis in which electrode array is inserted under the retina, subretinal prosthesis in which electrode array is fixed on the retina, suprachoroidal prosthesis in which electrode array is inserted in the suprachoroidal space or in the sclera pocket.
Currently, the best decimal visual acuity achieved by retinal prosthesis is 0.037 by subretinal prosthesis. Even though the visual acuity is still poor, patients with implants can read large letters. To achieve an improvement of the quality of life for blind patients implanted with a retinal prosthesis, not only an improvement of surgical procedures and engineering advancement but also a development of effective rehabilitation are necessary.
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