Eye movements

It is often difficult for a driver to see a person walking on the side of the road at night especially if the person is wearing dark colors. One of the factors causing this difficulty is a decrease, in contrast, making it hard to segment an object; such as a person, from its background. Researchers previously believed contrast sensitivity function the minimum level of black and white that a person needs to detect a pattern was mainly dictated by the optics of the eye and processing in the brain.
Now, in a study published in the journal eLife, researchers, including Michele Rucci at the University of Rochester, explained that there is another factor at play: contrast sensitivity also depends on small eye movements that a person is not even aware of making. “Historically these movements have been pretty much ignored. “But what seems to be happening is that they are contributing to vision in a number of different ways, including our contrast sensitivity function.”
When they fix our eyes on a single point, the world may appear stable, but at the microscopic level, our eyes are constantly jittering. These small eye movements, once thought too inconsequential, are critical to the visual system in helping us reconstruct a scene. “Some scientists believed that because they are so small, the eye movements might not have much impact; but compared to the size of the photoreceptors on the retina; they are huge, and they are changing the input on the retina.”

Measure Contrast Sensitivity

The Pelli-Robson test is one type of test to measure contrast sensitivity. The test consists of a chart with six letters per line; contrast varying from high at the top left to low at the bottom right. To test your contrast sensitivity function; read the letters starting with the highest contrast in the top left until you are unable to read two or three letters in a single group. Each group has three letters of the same contrast level.
To measure contrast sensitivity; the researchers showed human participants gratings with black and white stripes of different widths (known as spatial frequency, image A). For each frequency; they determined the minimum amount of contrast (separation from black and white, image B) that would enable the subjects to still see the grating pattern. The resulting function of spatial frequency is known as the contrast sensitivity function.
See your own contrast sensitivity function in image C; where frequency varies on the horizontal axis and contrast varies on the vertical axis. In order to measure contrast sensitivity and whether or not eye movements play a role; the researchers showed human participants gratings with black and white stripes. The researchers gradually varied the width of the stripes; making them “thinner and thinner, until the participants eventually said they no longer saw separate bars. The width of the bars is known as the spatial frequency.

Eye Movements

For each spatial frequency, researchers measured the minimum level of black and white that participants needed to able to see a contrast, while, at the same time, carefully measuring their eye movements. The researchers then simulated this task in a computer model of the retina to see if the responses of neurons in the retina matched the human subjects‘ contrast sensitivity. “They found that they are only compatible when we include the motion of the eye movements. “When they do not  include this movement factor in the computer model, the simulated neurons don’t give the same responses that the subjects do.”

Knowing that eye movements do affect contrast sensitivity, researchers are able to input this factor into models of human vision, providing more accuracy in understanding exactly how the visual system processes information and what can go wrong when the visual system fails.

The research also highlights that movement and motor behavior may be more fundamental to vision than previously thought, Rucci says. “Vision is not just taking an image and processing it via neurons. The visual system uses an active scheme to extract and encode information. See because our eyes are always moving, even if we do not know it.”