With results that may be very different from what scientists and patients had previously assumed, researchers used simulations to create short videos that mimic what vision would be like after two different types of sight recovery therapies.
“This is the first visual simulation of restored sight in any realistic form,” says Ione Fine, associate professor of psychology at the University of Washington. “Now we can actually say, ‘This is what the world might look like if you had a retinal implant.’”
The goal of the project, Fine says, is to provide information about the quality of vision people can expect if they undergo sight restoration surgery, an invasive and costly procedure, to create what some might think of as a bionic eye.
“This is a really difficult decision to make,” she says. “These devices involve long surgeries, and they don’t restore anything close to normal vision. The more information patients have, the better.”
Rods And Cones
For many of people who have vision problems, the vision loss occurs after light enters the eye and lands on the retina, a thin layer at the back of the eye that contains millions of nerve cells. Among those are cells called rods and cones, which convert light into electrical impulses that are transmitted to vision centers in the brain.
Loss of rods and cones is the primary cause of vision loss in diseases such as macular degeneration or retinitis pigmentosa.
But those diseases leave most remaining neurons within the retina relatively intact, and various technologies under development aim to restore vision by targeting the surviving cells.
This is a pivotal time for the industry, Fine says, with one company that has a device on the market and several others set to enter the market in the next five to 10 years.
Two of the most promising devices, she says, are electric prostheses, which enable vision by stimulating surviving cells with an array of electrodes placed on the retina, and optogenetics, which insert proteins into the surviving retinal cells to make them light-sensitive.
But the devices have a major shortcoming, says Geoffrey Boynton, since stimulating the surviving cells in a retina is unlikely to produce vision that is close to normal.
“The retina contains a vast diversity of cells that carry distinct visual information and respond differently to visual input,” adds Boynton, a psychology professor and coauthor of the study published in the journal Philosophical Transactions B.
“Electrically stimulating the retina excites all of these cells at the same time, which is very different from how these cells respond to real visual input.”
Objects May Be Sharper Than They Appear
There are similar issues with optogenetics, Boynton says. “The optogenetic proteins that are currently available produce sluggish responses over time, and they are limited in the number of different cell types that they can separately target.”
These limitations in both technologies mean that patients may see fuzzy, comet-like shapes or blurred outlines, or they may experience temporary visual disappearances if an object moves too fast.
Previous simulations of restored vision have used a “scoreboard model,” a grid of dots similar to the scoreboard at a football game, in which each electrode produces a visible dot in space. Together, that collection of dots is intended to demonstrate what someone with restored vision will see.
Fine says the new simulations show that the scoreboard model, which is sometimes used to test devices, doesn’t provide a good representation of the quality of vision sight restoration technologies are likely to produce. More realistic models are needed, she says, to give patients, clinicians and researchers a better idea of how those technologies will work in the real world.
And better simulations could provide valuable information about how implants need to be improved to produce more natural vision.
“As these devices start being implanted in people, we can compare different types of devices and the different perceptual outcomes of each,” she adds. “The path to fully restored eyesight is an elusive target. We need to start developing more sophisticated models of what people actually see.
“Until we do that, we’re just shooting in the dark in trying to improve these implants.”
[thrive_text_block color=”light” headline=””]Ione Fine, Geoffrey M. Boynton
Pulse trains to percepts: the challenge of creating a perceptually intelligible world with sight recovery technologies
Phil. Trans. R. Soc. B 2015 370 20140208; DOI: 10.1098/rstb.2014.0208[/thrive_text_block]
Photo: David Goehring/flickr