NEW YORK — There are plenty of things to see and do outdoors, but reading a book may be the best thing for your eyes. Researchers from the State University of New York’s College of Optometry are shedding new light on what keeps our eyes healthy. Their study finds daylight improves visual brain cells, helping people to distinguish faint details better.
The discovery could lead to improvements in smartphone screens, further protecting humans from poor vision habits. Too much screen time has already been linked to a phenomenon dubbed “computer vision syndrome.”
“The amount of light is continuously changing in our visual world,” says lead author Dr. Hamed Rahimi-Nasrabadi in a statement per SWNS. “Our work demonstrates image brightness changes our sensitivity to light and dark contrast to efficiently extract information from natural scenes.”
“Findings from the investigation conclude you can now feel good when you decide to read your favorite book outdoors. You can say it is scientifically proven visual contrast increases outdoors and, therefore, reading under bright light stimulates your visual brain more effectively, allows you to see the letters better, and helps your eyesight,” the SUNY researcher adds.
Shining the light on the visual spectrum
Experiments on cats and humans showed bright light stimulates neurons in the visual cortex. The amount of reflected light, or luminance, can change by more than 10,000 times throughout the day.
These fluctuations mean the white and black parts of a road sign can reflect 1,000 times more light at noon than at night. In a series of tests, the team measured the response of visual neurons with white and black stimuli at between 300 to 1,000 luminance per square meter.
Researchers fitted adult felines with contact lenses to protect their corneas and focused visual stimuli on the retina. The team measured the responses in the visual cortex as the animals were exposed to large dark or light squares. The team then confirmed the findings in humans wearing EEG (electroencephalography) skullcaps that monitor brainwaves.
“To our surprise we found the shape of the luminance-response function changed with both the range and black and white polarity,” Dr. Rahimi-Nasrabadi says.
What is contrast?
Contrast is a sensory property that makes stimuli stand out. Writers, designers, and musicians all use contrast to emphasize striking differences in text, visual displays, and melodies. In vision, luminance contrast describes the difference in light intensity between a bright object and its darker surroundings.
It is what makes a dark letter easier to read than a grey letter in a white page. Vision research has operated for decades under the assumption that luminance contrast does not change with light intensity. That is, a dark letter in a white page is assumed to have the same contrast outdoors under the brightest light than indoors under the dimmest light.
If contrast was not preserved across different light intensities, a black letter indoors would become white outdoors because it reflects much more light. The new study shows this decades-old assumption is incorrect and can cause important measurement errors.
It has implications in basic research, the eye clinic and the multiple disciplines that depend on accurate estimates of visual contrast. For example, it can lead to inaccurate measurements of visual sensitivity in eye disease, the rendering of contrast in medical images or the architectural design of spaces for the visually impaired and elderly.
Changing the theory surrounding vision and light
The findings demonstrate that, for visual sharpness, contrast sensitivity is strongly dependent on the amount of light. Increasing the amount of light shifts the contrast sensitivity for stimuli darker or lighter than their surroundings in opposite directions.
It improves the discrimination of the darkest contrasts – making it easier to see subtle differences in eye shadow – while severely impairing the discrimination of the brightest contrasts.
“The opposite shifts of dark and light contrast with light intensity can be demonstrated in neurons of the visual cortex, natural scenes, and appear to be well preserved across different species of mammals,” Rahimi-Nasrabadi explains. “The new findings can be also used to improve current algorithms of image processing and metrics of visual contrast.”
They turn the theory that cortical neurons signal luminance contrast independently of light intensity on its head.
“If this was true, a bright sky would look very dark.”
“Our measurements of ON and OFF cortical functions could help to simulate more accurately human luminance vision and prevent bright digital images from appearing too dark,” the SUNY team writes in the journal Cell Reports.
Computer vision syndrome is an umbrella term for conditions that result from looking at screens at arm’s length or closer for long periods of time. They include dry eyes, caused by a lack of blinking and eyestrain due to the brightness or glare.
SWNS writer Mark Waghorn contributed to this report.