[Rice Catalyst Issue 14]

each stimulated electrode contributed to a
phosphene at one predictable visual field
location, resulting in a pattern of artificial
vision that could be used to help blind
individuals navigate their environment or
perform other visual tasks [10].
FIGURE 2: Visual fields and visual maps. A, The right sides of both retinas project to the left
lateral geniculate nucleus (LGN), which in turn projects to the left primary visual cortex
(area V1). B, The upper parts of the visual fields project to lower parts of the contralateral
visual cortex [3].
phosphenes, or sensations, can be caused
by physical pressure on the eye, electrical
stimulation of the visual system, or other
types of sensory stimulation [2]. In the
case of electrical stimulation, optogenetics
uses light to stimulate relevant neurons
and create the sensation of light, even in
the absence of true visual stimulation.
Specifically, this sensation is created by
taking advantage of the retinotopic organization
of the primary visual area V1 of
the cerebral cortex, where the first stage
of cortical information processing occurs
and a complete map of the visual field is
generated [Figure 2]. Precise perturbations
in the targeted neurons of this area lead to
the creation of these light percepts.
One of the main advantages of optogenetics
is that it allows for high-resolution
and selective stimulation of neurons due
to the ability to target specific genetic sites
with the LoxP system. Because optogenetics
has a high specificity, it activates
only the targeted neurons, resulting in
a high degree of specificity and spatial
control, and a more accurate and detailed
visual perception. Research has shown
that optogenetics can provide reliable and
targetable light sensation in the absence of
visual stimulation [4]. While optogenetics
is still in the early stages of development
and testing, early clinical trials have shown
effective restoration of vision in retinal
degeneration [5].
However, there are some challenges with
optogenetics, specifically the limitations of
current light delivery methods and neuron
structure itself. In order to activate photoreceptors,
a substantial amount of energy
must be conferred to surpass the action
potential threshold [6]. Furthermore, the
limitation of current light delivery methods
prevent stimulation of deeper nervous
tissue. Activation of underlying brain tissue
requires higher energy pulses, which can
lead to thermal injury, causing damage or
destruction of cells and tissues [7].
ELECTRODE-BASED SOLUTIONS
Electrodes can be inserted through shuttle
microwires [Figure 3], which then form a
bidirectional connection between neural
implants and the controlling computer.
This connection allows control of the
neural implant through both receiving and
transmitting information by the electrode.
Electrode solutions involve five components:
an amplifier, filter, analog-to-digital
converter (ADC), stimulator, and communication
interface [8]. One strategy
to induce sight using electrodes is called
visual cortical prosthesis (VCP), which uses
electrical current to stimulate the visual
cortex [9]. Previous studies have taken
advantage of the retinotopic organization
of the visual cortex to produce form vision
in both sighted and blind humans. In these
studies, an array of multiple electrodes are
implanted in different locations to create
multiple phosphenes. In an electrode array,
In contrast to optogenetic-based solutions,
solutions involving electrodes have a minimal
energy loss. Llectrode-based solutions
directly stimulate the retina or the visual
cortex using electrical signals. One of the
main advantages of electrode-based solutions
is that they can be highly efficient, as
electrical signals can be delivered directly to
cells. However, one of the main challenges
associated with electrode-based solutions
is that it can be less selective than optogenetics
due to the size limit of individual
electrodes. This size limit results in less
accurate and detailed visual perception.
Another challenge is that electrode-based
solutions can trigger the body’s immune
response. When a foreign object, such as
an electrode, is implanted in the brain, the
brain’s immune system may recognize it as
a potential threat and attempt to remove it.
When it occurs in response to an implanted
electrode, it can lead to a process known as
the foreign body reaction, during which the
immune system releases a variety of inflammatory
molecules that can cause damage
to the tissues surrounding the electrode.
This can lead to the formation of scar tissue
and the buildup of immune cells, which can
interfere with the function of the electrode
and reduce its effectiveness over time.
Current research has been conducted to
evaluate the safety of electrodes in creating
artificial vision. One study used a direct
optic nerve electrode (AV-DONE) in a blind
patient with retinitis pigmentosa (RP) [13],
which is a genetic eye disease that affects
the retina and results in the inability to
perceive light [14]. The AV-DONE consists
of three wire electrodes, which were implanted
into the optic disc of a patient. The
researchers then induced visual sensations
by electrical stimulation through each elec-
FIGURE 3: A, Schematic showing the needle-thread
temporary engaging mechanism
[11]. B, Photograph of the insertion
process. Arrows indicate a needle penetrating
tissue proxy, advancing the thread
to the desired depth [12].
2 6 | C A T A L Y S T 2022-2023