Implants are being researched
as a viable way to restore vision in patients suffering from retinal
pigmentosa as well as macular degeneration. The epiretinal implants
typically work in a two component system that consists of an extraocular
and intraocular part. The extraocular part contains an image sensor
that is responsible for catching visual images, an artificial neural
net which can imitate the functions of different layers of the retina,
and a transmitter. The neural net transform the visual images control
signals that the electrodes pick up and become stimulated. The signal
is passed on to the internal eye and then is picked up by the intraocular
part of the implant. The intraocular component consists of a receiver
for the relayed signals and also power for the implant. Radiofrequency
links are being used currently and optical links may be used in the
future. There is an integrated circuitry component that decodes the
signals for the electrodes and controls the stimulation array to produce
action potentials in the upper ganglion cell layers to cause visual
sensations (Trieu, H. K. et al. 1998).
Boston Retinal Implant Project:
In the diseases retinitis pigmentosa
and macular degeneration the light sensing photoreceptors start to degenerate
however the surrounding machinery such as bipolar and ganglion cells
stay healthy. The retinal prosthesis is a microelectronic retinal implant
that is aimed to take over the function and replace the light sensing
photoreceptors which can no longer perform their purpose. The way that
the retinal prosthesis implant does this is by electrically stimulating
the surrounding healthy cells that make up the retina (Rizzo, Joseph F 2005).
This retinal based neuroprosthesis is based on the foundation that the electrical stimulation of the still workable cells will induce the sensation in distinct points of light, the phosphenes. “Geometrical visual percepts can be generated by delivering appropriate multi-site patterns of electrical stimulation which in turn can lead to the perception of shapes and images.” This is known as the scoreboard approach (Rizzo, Joseph F 2005).
The device works by receiving
two types of input; information of the visual scene known as the visual
signal as well as power and data to work the electronics and stimulate
the retina to generate the matching visual image. This is carried out
by wireless communication and radiofrequency transmission which is attached
to a transmitting coil on the arm of special glasses which transmits
the data and power to the coils located on the prosthesis (Rizzo, Joseph F 2005).
A visual scene is captured
by the camera and observed and analyzed in order to be converted into
a pattern of electrical stimulations. The data information are transmitted
wirelessly to a coil that is found in the prosthesis. The electrical
current passes from individual electrodes implanted in the retina and
stimulates the cells in the correct areas of the retina that correspond
to the features of the visual scene (Rizzo, Joseph F 2005).
Retinal Prosthesis Project:
The Retinal Prosthesis Project is based on designing an epiretinal implant that consists of two different components, the retina encoder and the retina stimulator. The retina encoder transforms a visual scene into nerve signals like the retina does. The implant is calibrated to meet each individual’s needs in order to produce optimal results. The encoder works with a photosensor array for pattern reception and transmission to transmit signals and energy which are found in the glasses. The retina stimulator is a flexible microcontact foil that consists of a receiver for the signal from the retina encoder and stimulation microcontacts, the matching controlling device stimulates the ganglion cells. The retina stimulator sends the information by more than 100 microcontacts to the ganglion cells that are lined to the optic nerve (The EPI-RET Project, 1999).