One would think that having normal 20/20 vision is due mainly to the anatomical characteristics of the eye. According to a new study, however, tiny eye movements play a large role – and the findings could lead to new treatments for poor sight.
For some time now, scientists have been aware of continuous, involuntary, minuscule shifts in a person's gaze, known as fixational eye movements. Because these movements are so small, we're not even aware that they're taking place.
Previously, it was thought that they played no significant role in how people see the world. More recently, though, it was found that by shifting an image across the retina, these movements expose more of the retina's light-sensing cells to that image. It has been theorized that as a result, the brain is better able to "see" what the eyes are looking at.
Led by Prof. Michele Rucci and grad student Janis Intoy, researchers at the University of Rochester decided to put this theory to the test. They started by having a group of volunteers with 20/20 vision view a standard Snellen eye chart, which was displayed on a video screen. Snellen charts are the type with 11 lines of letters, with the letters in each row being smaller than those in the row above.
When the test subjects initially viewed the chart from a distance of 20 feet (6 m), they could accurately make out the letters down to at least the eighth row. This is standard for people with 20/20 vision.
In a second round of tests, however, the chart moved around slightly on the screen, automatically compensating for the participants' fixational eye movements in real time. This meant that the image of the chart was exposed to fewer of their retinas' receptor cells. The test subjects were thus (on average) only to able to read down to the sixth line, indicating sub-par 20/30 vision.
More research still needs to be conducted, although it is now conceivable that by undergoing therapy to help improve their motor control, near- or farsighted people could experience an improvement in their vision.
"We found that achieving 20/20 vision is not only the outcome of good optics and a healthy retina but also fine motor control, to a level that eludes awareness," says Rucci. "Impairment in visual acuity may originate from eye movements, a factor that is presently not monitored at all."
A paper on the study was published this week in the journal Nature Communications.
Source: University of Rochester via EurekAlert
