I have found very
exciting article of Dr. Dobelle “Artificial Vision for the Blind by Connecting a Television Camera to the Visual Cortex”. The article was published in 2000, so I would like to notice that many of the characteristics of the visual implant described below could be improved since that time. Personally I was impressed by the fact that the first cerebral implants aimed to stimulate the visual cortex were set up as early as in the 70s.
Dr. Dobelle along with more than 300 other scientists, physicians,
engineers and surgeons have been developed cortical visual implant, which
unlike some other artificial vision proposals (e.g. retinal stimulators) is applicable
to virtually all causes of blindness. Particularly, the presence of the eyeballs is not necessary in this case.
The research group have also provided a battery powered, electronic
interface which can replace the camera, permitting the sightless volunteer to
directly watch television and use a computer, including access to the Internet.
The complete artificial vision system showing the computer and
electronics package on the belt with output cable to the electrodes on the
brain.
According to the author, the push for his research was made by a seminal
paper published by Giles Brindley’s group in 1968. Their first human
experiments in 1970-1972 involved cortical stimulation of 37 sighted volunteers
who were undergoing surgery on their occipital lobe under local anesthesia to remove
tumors and other lesions. In 1972-1973 they then stimulated the visual cortex
of three blind volunteers who were temporarily implanted for a few days with
electrode arrays.
Their subsequent
experiments have involved four blind volunteers implanted with permanent
electrode arrays using percutaneous connecting pedestals. Two volunteers were implanted
in 1974. One array was removed 3 months after surgery as planned, and the
second one after 14 years. The second volunteer
agreed to continue participation but his implant was removed due to a blood
borne infection that did not originate with the implant. The first five volunteers were operated on at
the University of Western Ontario in London
Canada.
Two additional blind volunteers, including the subject of this article, were
implanted in 1978 at the Columbia-Presbyterian
Medical Center
in New York City.
They have both retained their implants for more than 20 years without infection
or other problems.
The Volunteer and
Implant
Dr.Dobelle describes in details a 62 year old patient,
who traumatically lost vision in one eye at age 22, and was totally blinded at
age 36 by a second trauma. He was continually employed, before and after losing
his sight, as an administrator by the State of New York. He retired in 1997 after 32 years
of service. The electrode was implanted in 1978 when he was 41 years old. The implanted pedestal and intracranial electrode array
were used to experimentally stimulate the visual cortex, on the mesial surface
of the right occipital lobe, for more than 20 years. However, the fifth generation external electronics
package and software are entirely new, taking advantage of cutting edge
technology that has only recently become available. An X-ray of the implanted
visual cortex electrode array is shown in Figure below:
X-ray of electrode array
on the mesial surface of the right occipital lobe.
The original surgery in 1978 was performed under local anesthesia, and
implants in future patients can probably be performed on an outpatient basis by
most neurosurgeons.
Phosphenes and Their
Map in The Visual Field
The Dr.’s Dobelle visual prosthesis produces black and white display of
visual cortex “phosphenes” analogous to the images projected on the light bulb
arrays of some sports stadium scoreboards.
When stimulated, each electrode produces 1-4 closely spaced phosphenes.
Each phosphene in a cluster ranges up to the diameter of a pencil at
arms length. The Dr.’s Dobelle work team determined
that the phosphene map occupies an area roughly 8 inches in height and 3 inches
wide, at arms length.
The Electronics
Package
The 292 X 512 pixel charge coupled devices (CCD) black and white
television camera is powered by a 9 V battery, and connects via a
battery-powered National Television Standards Committee (NTSC) link to a
sub-notebook computer in a belt pack. This f 14.5 camera, with a 69° field of
view, uses a pinhole aperture, instead of a lens, to minimize size and weight.
It also incorporates an electronic “iris” for automatic exposure control. The
sub-notebook computer incorporates 233 MHz processor, 32 MB of RAM and a 4 GB
hard disk. It also has an LCD screen and keyboard. The belt pack also contains
a second microcontroller, and associated electronics to stimulate the brain. This
stimulus generator is connected through a percutaneous pedestal to the
electrodes implanted on the visual cortex. The computer and electronics package
together are about the size of a dictionary and weigh approximately 10 pounds,
including camera, cables, and rechargeable batteries. The battery pack for the
computer will operate for approximately 3 hours and the battery pack for the
other electronics will operate for approximately 6 hours. This general
architecture, in which one computer interfaces with the camera and a second
computer controls the stimulating electronics, has been used by Dr.’s Dobelle team
in this, and four other substantially equivalent systems, since 1969. The
software involves approximately 25,000 lines of code in addition to the sub-notebooks’
operating system. Most of the code is written in C++, while some is written in
C. The second microcontroller is programmed in assembly language.
To control costs and ensure easy maintenance, the commercial
off-the-shelf (COTS) components are used. The computer, stimulating electronics,
and software are all external, facilitating upgrades and repairs.
Performance of the
System
Тhe Dr’s Dobelle system provides low parafoveal tunnel vision. The
picture captured by the patient is black and white with plus field defects (due to gaps between
phosphenes; there is no depth perception.
The patient learnes to use the system within 1 one-day sessions, and he
continues to
practice 3-4 hours per day 2 or 3 days per week. With scanning the patient can routinely recognize
a 6 inch square “tumbling E” (see pic. [а]) at five feet, as well as Snellen letters [b], HOTV test [c], Landolt rings [d], and Lea figures [e] of similar size. These psychophysical tests are summarized
in Figure below:
The patient can also count fingers. With the exception of finger
counting, these acuity tests have been conducted using pure black characters on
a pure white background at an illumination greater than 1,000 lux. The volunteer
can recognize a 2-inch high letter at 5 feet. This represents acuity of roughly
20/400.
Paradoxically, larger characters are slightly more difficult for this volunteer
because they extend well beyond the limits of his visual “tunnel”. The rapid
fall-off with characters smaller than 20/1200 is also quite reproducible.
Similar acuity results have been achieved with the
television/computer/Internet interface replacing the camera, although scanning
is slower.
Although stimulation of visual cortex in sighted patients frequently produces
colored phosphenes, the phosphenes reported by this volunteer (and all previous
blind volunteers to the best of their knowledge) are colorless. Probably, this is the result of post-deprivation deterioration
of the cells and/or senaphtic connections required for color vision.
Consequently, color vision may never be possible in this volunteer or in future
patients. However, optical filters could help differentiate colors, and it is
also conceivable that chromatic sensations could be produced if future patients
are implanted shortly after being blinded, before atrophy of the neural network
responsible for color vision.
Contrast is entirely a function of the
software, with adjustment by the experimental team depending on the
experimental situation. The system also allows “reversal” in which the world
looks much like a black and white photographic negative. Reversal is
particularly useful when presenting black characters on a white background. These
characters are then reversed by the computer so they appear as a matrix of
white phosphenes on the patient’s (otherwise dark) visual field. The phosphene
map is not congruent with the center of the volunteer’s visual field.
Phosphenes also move with eye movement. However, the volunteer’s ability to
fixate with this artificial vision system is a function of aiming the camera
using neck muscles, rather than eye muscles.
Edge Detection
Picture of the 38 inch high child mannequin, with a second ski cap
placed at a random location on the wall. B, Same scene as above, after
edge-detection using Sobel filters and black/white reversal. The blind
volunteer is able to easily find the cap and detect the wall outlets.
Similarly, doorways appear as an outline of white phosphenes on a black
background. All processing can be performed and transmitted to the patient at 8
frames/second.
Ultrasonic Rangefinder
While using edge detection, it is particularly helpful for the blind
patient to know how far the wall is located behind the mannequin.
By placing an electrostatic transducer on the left lens of the patient’s
eyeglasses (lateral to the camera and below the laser pointer) Dobelle’s team has begun exploring the
supplementary information that can be provided by modulating brightness, blink rate
and identity of selected phosphenes.
One final comment is that none of the
seven blind volunteers of Dr’s Dobelle study have ever exhibited epileptic symptoms or other
systemic problems related to the implant. Based on clinical experience during
the last 30 years, implanting thousands of patients in more than 40 countries
with other types of neurostimulators (to control breathing, pain, and the urogenital
system), Dobelle’s team
believes that the
principal risk of their artificial vision device is infection, which might
require removal of the implant in addition to antibiotic therapy.
