We see contrast because our eyes are always moving
They’re not always big movements, but they’re important. Natalie Parletta reports.
The ability to see contrast is an important visual function, particularly when we are walking down stairs, driving at night or looking for things in bad light.
Contrast sensitivity function – which is different to visual acuity – is the minimum amount of light and dark that we need to see to detect an object or pattern.
Until now, researchers have thought that seeing contrast relies on eye optics and brain processing. However, a new study, published in the journal eLife, reveals that tiny eye movements play a critical role.
“Historically these eye movements have been pretty much ignored,” says Michele Rucci, professor of brain and cognitive sciences at the University of Rochester in the US. “But what seems to be happening is that they are contributing to vision in a number of different ways, including contrast sensitivity function.”
If we fix our eyes on a single point, the world may appear still. But microscopically, our eyes are always moving – known as “fixational eye movements”. Without these movements continually refreshing visual input to the retina, an image can fade from view.
To test the role of eye movements in detecting contrast, the researchers showed five females aged 21-31 with normal vision gratings with black and white stripes, making them progressively thinner – known as spatial frequency – until volunteers could no longer see separate bars.
For each spatial frequency, they measured the minimum amount of black and white needed to see a contrast while tracking participants’ eye movements.
When they simulated the task in a computer model of the retina and associated neurons, the researchers found that contrast sensitivity was only achieved when they included the eye movements.
“When we don’t include this movement factor in the computer model, the simulated neurons don’t give the same responses that the subjects do,” says Rucci.
It’s a bit like the system involved in our sense of touch, explains lead author Antonino Casile from the Italian Institute of Technology.
To feel the texture of a surface, it is not enough to just touch it – we also need to move our fingers along the object. We process information from the interaction between our fingertips’ tactile sensors and movement.
Similarly, contrast sensitivity results from the interaction between the sensory process in the brain’s visual system and the motor process of eye movement, Casile says.
The findings, write the authors, “are highly robust, bear multiple consequences, and lead to important predictions”.
“Vision isn’t just taking an image and processing it via neurons,” says Rucci. “The visual system uses an active scheme to extract and code information. We see because our eyes are always moving, even if we don’t know it.”
Importantly, researchers will be able to include eye movement in models of human vision to better understand how it processes information and how it can go wrong.