Biomedical Engineering Reference
In-Depth Information
(fMRI), neuroscientists still don't know how diverse perceptions occurring in
distant parts of the brain are bound together in a single conscious experience.
The state-of-the-art research plan in neurobiology is still to find the neurons
or subcircuits of interconnected neurons, their feedback and inhibitory
mechanisms that would explain brain activities such as thinking, emotions,
intuition, creativity and insight. Neuroscientists are looking for neural
correlates, presuming that the rate and manner of firing correlate with
conscious experiences. The search is focused on finding the neural code and
to discover what exactly is associated with experiences, the mean firing rates or
the chemical behavior of neurotransmitters. Nevertheless, the complexity of
the brain might exist in a much higher dimension, at an unthinkable and
inconceivable frontier.
d
n
4
t
3
n
g
|
2
6.5 Retinal Prosthetic Interfaces
The retina in the human eye is an impressive and very fragile tissue on which the
whole marvelous process of vision is based. It receives and converts the incident
light, be it a single photon in total darkness to the brightest sunlight, on an
impressive dynamic range, remarkable color discrimination and with exquisite
sensitivity. The photostimuli are converted in the retina to a signal which is
later processed by the visual centers of the brain. Millions of photoreceptors
(100-150million rods and cones creating a condensed network in the central
area of the retina, the macula, where the resolution is maximal and vision
sharpest) convert the image in the language of neurons in the bipolar layer,
connecting to the third ganglia layer which transfers the signals to the nerve
fiber layer that transfers the information—now reduced by a factor of 100, and
processed to account
n
3
.
for motion detection and edge enhancement—to
the brain.
Loss of vision due to the degeneration of the retina occurs in age-related
macular degeneration (people over 65), in retinitis pigmentosa (occurring at a
younger age, 1 in 4000 live births), and numerous other medical conditions or
accidents. Artificial means of restoring lost vision cannot replace millions of
optical receptors in the retina, situated at about 5 mm distance from each other.
Nor can it attain the fine resolution of the macula. Moreover, the task of
restoring vision is further complicated by the fact that the brain requires
multiple sensory inputs to correctly reconstruct a three-dimensional image.
This implies that the sensory neurons be stimulated not only in parallel, but
also in a spatially correct order to allow an accurate image encoding to take
place in the visual cortex. The impressive functional intricacies of the eye are
further compounded by the complexity of the brain's interpretation of signals
originating in the retina.
Attempts are being made today to partially restore lost vision using current
microelectronic technologies as visual prosthetics. These prostheses can be
connected directly to the brain or implanted in the eye if the optic nerve is still
intact. Presently, there are three strategies in the technical development for
visual prosthetics—cortical
implants, optic nerve stimulation and retinal
