Biomedical Engineering Reference
In-Depth Information
FIGURE 7-40
Schematic showing the components of a visual neuroprosthesis [Adapted
from (Finn and LoPresti 2003).]
from a surgical perspective and because their structures show some correspondence in a
visuotopic map. Other possibilities include the supra-choroidal space in the eye and the
LGN in the brain.
At this stage, very strong stimulation is required to evoke a useful response (percept)
with the result that only a crude sense of vision may be possible. However, considering
the rapid progress that has been made with cochlear prostheses over the past decade, there
is hope that a second generation of visual prostheses will be more subtle and effective in
stimulating a relatively accurate visual response.
7.9.3 Components
To make any visual prosthesis acceptable, its components must be integrated into normal
systems usually worn by individuals. These include eyeglasses to house the camera and
a small processor about the size of a PDA or phone to perform the image processing
function. A telemetry device would provide both power and the video signal to an embed-
ded prosthesis containing the stimulation electronics. This would generate the appropriate
currents to feed each of the elements of a neural interface array. These components are
shown schematically in Figure 7-40.
7.9.3.1 Video Encoder
A video encoder mimics the lost function of the photoreceptors in the retina by transform-
ing the visual image into its electronic form. This could be a photodiode array, a dedicated
CCD array, or a miniaturized video camera mounted within the frames of a pair of glasses
worn by the patient.
For retinal-based visual neuroprostheses, the encoder would reside within the plane
of the retina. This uses existing optics of the eye to form a projection with the result that
acquisition of image data would function more naturally without the requirement of head
movement to track as the eyes would move normally.
Unfortunately, the spatial resolution of any of these systems will be limited, not by the
video encoder but by the limited numbers of electrodes in the neural interface. Temporal
resolution is not seen to be a problem as the human visual system is relatively slow (about
30 Hz). Therefore, conventional miniaturized camera technology, already available, would
be ideal for use as a video encoder.
7.9.3.2 Signal Processor
The foundation on which all visual prostheses are built is the assumption that the visual
system is built into a hierarchical sequence of maps. It is assumed that excitation of
neurons close to each other spatially will excite other neurons at various levels in the
visual pathways that are also close together. If this mapping were perfectly conformal and
linear, then a high-quality neural image of the visual space would be found at each level of
the visual pathways. This is not true, unfortunately, and the visuotopic map is conformal
