A variety of invasive procedures have also been developed. So far, however,
only with limited success. The most frequently used method is electrical
stimulation of neural tissue. The idea here is to convert visual signals
collected by a camera or other sensing device into electrical impulses
that are applied to brain tissue. It has been shown that electrical stimulation
can elicit visual percepts in several brain areas. The visual impression
created by stimulation of a single site has often been referred to as
a "phosphene". The areas that have been electrically stimulated
include the retina, the lateral geniculate nucleus of the thalamus, primary
visual cortex and higher visual areas. Among these we believe that primary
visual cortex holds the greatest promise for developing a visual prosthesis.
This area has a well-defined topographic layout. An array of electrodes
can be implanted to selectively stimulate different representations of
the visual field. However, there are numerous problems that must be solved
before such a device can become effective.