Optic nerve stimulation offers hope for visually impaired

Scientists are developing a technology for the visually-impaired people that bypasses the eyeball entirely and sends messages to the brain, paving the way for a new visual aid for daily living.

Researchers from Swiss Federal Institute of Technology Lausanne (EPFL) and Scuola Superiore Sant'Anna in Italy use a new type of intraneural electrode called OpticSELINE to stimulate the optic nerve. The technology, described in the journal Nature Biomedical Engineering, has been successfully tested in rabbits.

"We believe that intraneural stimulation can be a valuable solution for several neuroprosthetic devices for sensory and motor function restoration. The translational potentials of this approach are indeed extremely promising," said Silvestro Micera, a professor at Scuola Superiore Sant'Anna. Researchers noted that blindness affects an estimated 39 million people in the world. Many factors can induce blindness, like genetics, retinal detachment, trauma, stroke in the visual cortex, glaucoma, cataract, inflammation or infection. Some blindness is temporary and can be treated medically, researchers said.

The idea is to produce phosphenes, the sensation of seeing light in the form of white patterns, without seeing light directly. Retinal implants, a prosthetic device for helping the blind, suffer from exclusion criteria. For example, half a million people worldwide are blind due to Retinitis pigmentosa, a genetic disorder, but only a few hundred patients qualify for retinal implants for clinical reasons. A brain implant that stimulates the visual cortex directly is another strategy albeit risky.

A priori, the new intraneural solution minimises exclusion criteria since the optic nerve and the pathway to the brain are often intact. Previous attempts to stimulate the optic nerve in the 1990s provided inconclusive results. Intraneural electrodes may be the answer for providing rich visual information to the subjects. They are also stable and less likely to move around once implanted in a subject, according to the scientists. "Cuff electrodes are surgically placed around the nerve, whereas intraneural electrodes pierce through the nerve," said Diego Ghezzi from EPFL.

Together, Ghezzi, Micera and their teams engineered the OpticSELINE, an electrode array of 12 electrodes. In order to understand how effective these electrodes are at stimulating the various nerve fibres within the optic nerve, the scientists delivered electric current to the optic nerve via OpticSELINE and measured the brain's activity in the visual cortex.

They developed an elaborate algorithm to decode the cortical signals. The researchers showed that each stimulating electrode induces a specific and unique pattern of cortical activation, suggesting that intraneural stimulation of the optic nerve is selective and informative. As a preliminary study, the visual perception behind these cortical patterns remains unknown.

"For now, we know that intraneural stimulation has the potential to provide informative visual patterns," Ghezzi said. "It will take feedback from patients in future clinical trials in order to fine-tune those patterns. From a purely technological perspective, we could do clinical trials tomorrow," he said.