Biomedical Engineering Reference
In-Depth Information
A series of beeps (beacons) leads users on the path to their destination. Other beacons
convey useful situational awareness information (Walker and Lindsay, 2006):
•
Navigation beacon
sounds guide the user along a predetermined path, from a start
point, through several waypoints, to arrive at the user's destination.
•
Object sounds
indicate the location and type of objects around the user, such as
furniture, fountains, and doorways.
•
Surface transition
sounds signify a change in the walking surface, such as sidewalk
to grass, carpet to tile, level corridor to descending stairway, and curb cuts.
•
Locations
, such as offices, classrooms, shops, buildings, and bus stops, are also
indicated with sounds.
•
Annotations
are brief speech messages recorded by users that provide additional
details about the environment. For example, “Deep puddle here when it rains.”
Because the system relies on a priori detailed information about the area, any short-term
changes, such as road works, are not registered, and the user needs to use one of the other
prostheses discussed in this chapter. Another limitation is that GPS does not work indoors,
so the researchers are developing a camera-based system to help in these situations.
7.9
VISUAL NEUROPROSTHESES
7.9.1 Historical Perspective
The first recorded description of an electrically induced spot of light (later called a
phosphene) comes from LeRoy in 1755. He was investigating the potential of electricity
to cure various diseases when he discovered that an electrical pulse delivered to the eye
produced the sensation of light. Research in the field continued in 1929 when Foerster
applied electrical stimulation directly to the occipital cortex and reported the perception
of spots of light that he referred to as phosphenes (Lovell, Hallum et al., 2007).
In the 1960s and 1970s, research efforts led by Brindley in England used an array
of 80 platinum surface electrodes embedded under the dura membrane, connected to
80 individual receivers, to generate phosphenes using electrical stimulation. Information
was conveyed to the receivers using electrical induction. It was found that, though it
was possible to stimulate up to 35 individual phosphenes, the currents required were
extremely high, typically between 1 and 10 mA. This was completely impractical from a
visual prosthetic perspective as the total current from an array of electrodes would reach
dangerous levels that could produce seizures.
In the last decade or so, research focus has shifted from the invasive intracranial
neurosurgery required for cortical implants to the less invasive retinal options, which are
discussed in some detail later.
7.9.2 Potential Sites for Visual Neuroprostheses
Of all of the neural structures discussed earlier in this chapter, the photoreceptor layer of
the retina, the ganglion cell layer of the retina, the optic nerve, and the cerebral cortical
region V1 have been proposed as the most likely sites for visual neuroprostheses. These
sites have been proposed because of the relative ease with which they can be accessed
