It is often difficult for a driver to see a person walking on the side of the road at night鈥攅specially 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. Small eye movements may be crucial to observing contrast.
Researchers previously believed contrast sensitivity function鈥攖he minimum level of black and white that a person needs to detect a pattern鈥攚as mainly dictated by the optics of the eye and processing in the brain. Now, researchers, including , explain that there is another factor at play: contrast sensitivity also depends on small eye movements that a person is not even aware of making.
鈥淗istorically these movements have been pretty much ignored,鈥� says . 鈥淏ut what seems to be happening is that they are contributing to vision in a number of different ways, including our contrast sensitivity function.鈥�
When we 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 to be inconsequential, are critical to the visual system in helping us reconstruct a scene, Rucci says. 鈥淪ome 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.鈥�
Think of a scene or object like a computer image made up of different pixels, or points. Each point is a different color, intensity, luminance, and so on. Our eyes take in signals from each of the points and project the signals onto photoreceptors on the retina: the arrangement of these points makes a spatial pattern that we perceive as a scene or object. But, if a spatial pattern is projected as a stationary image, it will fade from view once the retina鈥檚 photoreceptors become desensitized to the signal鈥攍ike a student who becomes bored in class if the teacher repeats the same information over and over again.
Researchers have long known that the tiny eye movements鈥攁lways jittering and taking in different points鈥攃ontinually change the signal to the retina and refresh the image so it does not fade. However, the new research, which was funded by the National Eye Institute, the National Science Foundation, and the Harvard/MIT Joint Research Program, suggests that these movements do more than prevent fading; they are one of the very mechanisms by which the visual system functions, Rucci says. 鈥淭he way the visual system encodes information is based on these temporal changes. Eye movements transform a spatial pattern into temporal changes on the retina.鈥�
The system is similar to that involved in the sense of touch: to glean information about the surface of a solid object, we do not simply place our fingertips on the surface, but also move them along the object. We are able to perceive the object based on the interaction between a sensory process (the tactile receptors in our fingers) and a motor process (the way we move our fingertips). 鈥淪ince our eyes are never at rest even聽when we fixate a point in the visual scene, a similar mechanism holds聽for vision,鈥� says (Italian Institute of Technology) and a co-author of the paper. 鈥淐ontrast sensitivity results from the interaction of two processes: a聽sensory process鈥攖he response properties of neurons in the early visual system鈥攁nd a motor process.鈥�
Measuring eye movements and contrast sensitivity
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 鈥渢hinner and thinner, until the participants eventually said they no longer saw separate bars,鈥� Rucci says. The width of the bars is known as the spatial frequency. For each spatial frequency, researchers measured the minimum level of black and white that participants needed to be able to see a contrast, while, at the same time, carefully measuring their eye movements.
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 contrast sensitivity function. You can see your own contrast sensitivity function in image C, where frequency varies on the horizontal axis and contrast varies on the vertical axis.
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鈥� contrast sensitivity. 鈥淲e found that they are only compatible when we include the motion of the eye movements,鈥� Rucci says. 鈥淲hen we don鈥檛 include this movement factor in the computer model, the simulated neurons don鈥檛 give the same responses that the subjects do.鈥�
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鈥攁nd 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. 鈥淰ision isn鈥檛 just taking an image and processing it via neurons. The visual system uses an active scheme to extract and encode information. We see because our eyes are always moving, even if we don鈥檛 know it.鈥�
