Vision results from light transmission. Light is converted to electrical impulses that are then relayed to the brain. After traveling through the cornea and lens, light reaches the retina and the optic nerve and is converted to electrical energy. This energy is conducted intracranially through the lateral geniculate nucleus to the primary visual cortex in the occipital lobe. The information is subsequently transmitted to higher cortical areas of the brain, thus making the optic nerve and retina vital components of the pathway of light transmission.

According to a recent report by the World Health Organization, 285 million people are visually impaired worldwide, 39 million are blind, and 246 million have low vision (October 2011 Fact sheet WHO). Blindness, the inevitable result of irreparable damage to the visual pathway, is often a result of injury to either the neural retina or the optic nerve. Retinitis pigmentosa (RP) is a leading cause of inherited visual blindness, while age-related macular degeneration (AMD) is a leading cause of degenerative visual loss in the elderly.

Research is currently under way to offer patients suffering from visually debilitating diseases viable prosthetic options for visual rehabilitation. Approaches to visual rehabilitation involve electrical microstimulation of the visual cortex, the lateral geniculate nucleus, the optic nerve, and the retina to generate the impression of light or phosphenes. Beginning in the early 1900s, experiments in cortical stimulation were performed in attempt to assist the blind. For example, in 1929, the German neurosurgeon Foerster observed that electrical stimulation of the cortex enabled his subject to detect a spot of light. This phenomenon of electrically induced light perception was defined as a phosphene (1).

In 1974, an electrode was used in a blind patient to stimulate the visual cortex to generate light perception (2). Experiments conducted by Dobelle et al. and Normann et al. (3,4,5,6) utilized either implanted surface electrodes or intracortical microelectrodes to excite the visual cortex. Over 50 electrodes were implanted over the occipital pole, and although some form of visual perception was obtained in the experiment, it was difficult to control the number of phosphenes induced by each electrode and the interaction between phosphenes. Furthermore, electrical stimulation was seen to cause an increase in the incidence of epileptic activity and to cause meningeal pain induced by high currents and large electrodes.

A contemporary form of the intracortical prosthesis, the Utah electrode array, consists of multiple silicon spikes. In testing of the array, the silicon tissue reaction ranges from none to gliosis and buildup of fibrotic tissue between the array and meninges (7). Recently, research was conducted to determine the feasibility of implanting a neural prosthesis in the lateral geniculate nucleus of the thalamus, which relays visual information from the retina to the visual cortex (8). Initial research demonstrated that electrical stimulation of the lateral geniculate body can generate neural responses in the visual pathway: electrical stimuli delivered
to the thalamus engendered responses similar to those of when images are presented to the eye. Further research is currently under way.

Cortical prosthesis may bypass diseased visual pathway neurons rostral to the primary cortex and allow for visual restoration regardless of the insult to the eye. However, the organization of the visual field is more complex at the level of the primary cortex than at the level of the optic nerve or retina and may result in limited spatial resolution. Intracranial surgery also runs the risk of central nervous system (CNS) infection, epilepsy, and disturbance of blood flow to the optic nerve. Surgical complications carry significant morbidity and mortality risks for the patient.

Given the potential dangers of approaching artificial vision from the cortical approach, other groups have chosen to pursue an ocular prosthesis from within an eye possessing remaining viable tissue. Previous reports show that in spite of the level of vision loss in patients with RP, viable cells in the eye are able to transmit information with 30% of ganglion cells and 80% of inner nuclear layer cells that remain histologically intact after the death of photoreceptors (9). Investigations are ongoing in regard to a viable ocular prosthesis at the levels of suprachoroidal, subretinal, and epiretinal space by utilizing the preexisting signal processing network. This requires that sufficient bipolar cells or ganglion cells remain to elicit a response to electrical stimulation. In cortical prosthesis experiments, an array of electrodes is used to deliver an electrical current to the retina, thus stimulating functional retinal neurons to send signals to the visual cortex to be perceived as phosphenes (Fig. 26.1). In contrast to cortical electrodes, the use of preexisting signal processing network is expected to have higher visual resolution.

Sakaguchi and Tano et al. (10) investigated suprachoroidal transretinal stimulation in which an electrode was inserted into the suprachoroidal space to elicit electrical evoked potential (EEP). In the subretinal approach, explored by Chow et al. (11,12,13) and Zrenner et al. (14), lost photoreceptor function was replaced by a subretinal microphotodiode array (MPDA) that activated the remaining functioning retinal network. In the epiretinal approach, investigated by Eckmiller et al. (15), Humayun et al. (16,17), Rizzo et al. (18,19), and Walter et al. (20,21), devices stimulated the ganglion cells from the vitreous side of the retina.