Visual Prostheses

Abstract

A visual prosthesis is an active implantable device intended to restore vision in a blind person. Present day systems are only partially implanted. A number of different approaches are under development. These are mainly classified on the basis of their interface to the visual nervous system. One particular approach resorts to chemical activation of the retina. All others use electrical stimulation applied subretinally, epiretinally, transretinally, to the optic nerve, on the visual cortex, or in the visual cortex. These systems can typically be described as a chain comprising a camera or image grabbing unit, a processor, a telemetry unit, an implanted stimulator and an implanted electrode. Localized phosphenes have been obtained and exploited to convey a primitive form of visual information to the brain. Feasibility has been demonstrated, but a clinical evaluation has yet to come. Many aspects of these devices are still open to rapid and significant development.

Low-Power Wireless Data Transfer System for Stimulation in an Intracortical Visual Prosthesis

“…The inductive link allows for short transmission ranges in the order of centimeters. This fits well with the application of the visual prosthesis, where transmission through a small layer of tissue (skin), at an implantation depth of 3 to 7 mm [ 23 ] is sufficient (see also Figure 1 c) and security may be a concern. Phase shift keying (PSK): has a better theoretical bit error rate performance than other modulation schemes, such as amplitude shift keying (ASK) and frequency shift keying (FSK).…”

“…The inductive link allows for short transmission ranges in the order of centimeters. This fits well with the application of the visual prosthesis, where transmission through a small layer of tissue (skin), at an implantation depth of 3 to 7 mm [ 23 ] is sufficient (see also Figure 1 c) and security may be a concern.…”

Low-Power BPSK Inductive Data Link for an Implanted Intracortical Visual Prosthesis

“…Table I shows the key parameters of the simulation. The skin is usually about 5 mm thick [9]. Therefore the target coil separation distance is 5 mm, and it is extended to a coil separation distance of 10 mm for extreme cases.…”

“…The desired data rate is in the range of 0.5-4 Mbps for the visual prosthesis. Since the communication range from beneath the skin to above the skin is below 10 mm [9], an inductive link is the most suited type of link. To keep power consumption low, the carrier frequency is chosen to be below 20 MHz.…”

System Design of a Low-power Wireless Link for Neural Recording in a Visual Prosthesis

“…The typical normalized signal to noise ratio (Eb/No) required by on-off keying modulation to reach a bit error rate of 10 −7 with non-coherent demodulation is 15 dB [15]. Assuming the data rate equals the bandwidth which is easy to reach with IR-UWB, the signal to noise ratio (SNR) required While this maximum allowable path loss of 37 dB can be quite small for free space path of over a few meters, in the context of the visual prosthesis, it is primarily sought to communicate from beneath the skin to just above the surface of the head which is about 5 mm [16]. Losses through 5 mm of skin are well below the estimated 37 dB for P L MAX .…”