Why Your Eyes Jitter
The coach’s advice “Keep your eye on the ball” is impossible, because your eyes are constantly in motion with tiny jerks called fixational eye movements or saccades. Why do the eyes move all the time? Some scientists at Boston University decided to find out. Reporting in Nature,1 they found that saccades help you discriminate fine details in the visual field. Rucci et al said,
Our eyes are constantly in motion. Even during visual fixation, small eye movements continually jitter the location of gaze. It is known that visual percepts tend to fade when retinal image motion is eliminated in the laboratory. However, it has long been debated whether, during natural viewing, fixational eye movements have functions in addition to preventing the visual scene from fading. In this study, we analysed the influence in humans of fixational eye movements on the discrimination of gratings masked by noise that has a power spectrum similar to that of natural images. Using a new method of retinal image stabilization, we selectively eliminated the motion of the retinal image that normally occurs during the intersaccadic intervals of visual fixation. Here we show that fixational eye movements improve discrimination of high spatial frequency stimuli, but not of low spatial frequency stimuli. This improvement originates from the temporal modulations introduced by fixational eye movements in the visual input to the retina, which emphasize the high spatial frequency harmonics of the stimulus. In a natural visual world dominated by low spatial frequencies, fixational eye movements appear to constitute an effective sampling strategy by which the visual system enhances the processing of spatial detail.
The brain compensates for these movements so that we are not aware of them (03/29/2002, 11/24/2005, 11/10/2006). This was known, but the reason for the saccades was only suggestive till now. Using new methods, the Boston University team found that subjects with the stabilized vision lost more than 16% of their ability to discriminate fine details in the high-frequency gratings, but showed no change with low-frequency gratings. This result was unexpected:
Thus, fixational eye movements improved discrimination of the orientation of a high-frequency grating masked by low-frequency noise but did not help with a low-frequency grating masked by high-frequency noise. This result is surprising because it contradicts traditional views of the influence of fixational eye movements on vision. Indeed, the pronounced reduction in contrast sensitivity at low spatial frequencies measured by previous experiments with prolonged retinal stabilization predicts a more significant drop in performance with low-frequency than with high-frequency gratings.
Nevertheless, their experiments were robust: the saccades helped most in distinguishing fine detail. The researchers found, furthermore, that the eye movement also helped distinguish detail in very low contrast scenes.
For a controlled experiment, they kept one axis stable and the other in natural motion. As expected, image discrimination was improved on the moving axis.
These results are consistent with the informational content of the modulations of luminance introduced by fixational eye movements. These modulations only convey information about the pattern of noise during motion parallel to the grating, but provide maximal information about the grating when motion occurs on the axis orthogonal to the grating.
The authors provided some differential equations that described how the motions of the eye provide more information from the visual field. In conclusion, they said:
Our results show that vision is impaired at high spatial frequencies in the absence of fixational eye movements. This finding is consistent with the spatial frequency dependence of the temporal modulations resulting from fixational eye movements. Neurons in the early visual system are sensitive to these input modulations. As with the stimuli of experiment one, natural visual environments possess substantial power at low spatial frequencies. Our results indicate that sampling visual information by means of a jittering fixation is an effective strategy for analysing natural scenes, facilitating the processing of spatial detail in the face of otherwise overwhelming low-frequency power.
As indicated, they figured that there must be a function for the phenomenon. This approach motivated them to experiment and find the answer. It was not possible to determine this function with earlier technologies, they said.
Science Now weighed in on this story, commenting that these findings “mark an important step toward settling a 50-year-old controversy.” The article said we still have known surprisingly little about saccades. This new work shows that “the eye’s jitters help the brain pick out fine details, the kind involved in locating a single tree in a forest or a berry in a bush.” This ability is shared with other mammals: “Most animals with sharp central vision, such as humans, monkeys, and cats, make microscopic eye adjustments when they fix their gaze.” Saccades have also been observed in the eyes of birds.2
1Rucci, Iovin, Poletti and Santini, “Miniature eye movements enhance fine spatial detail,” Nature 447, 852-855 (14 June 2007) | doi:10.1038/nature05866.
2See Journal of Neuroscience where researchers described saccades in the eyes of two species of predatory birds possessing binocular vision. The authors did not comment on whether this represents a case of “convergent evolution.”
There was no mention of evolution in this paper. We do not know their feelings about evolution, but these authors have demonstrated in deed that assuming design leads to productive science. They saw a phenomenon; they assumed there was a reason for it. Now we know more about the eye than we did: and it’s a wonderful thing. There is more information and functional design behind these strange eye movements than we imagined.
Their results make sense in hindsight, too (if you’ll pardon the expression). Continuous eye motion allows the neurons and the brain to take numerous snapshots from slightly different angles, so as to glean the maximum amount of information from the visual field. For widely spaced details, this does not add much information, but it adds a lot in low contrast and high-detail situations. Think about that the next time you are reading fine print in low light, like the small black lettering on black plastic that manufacturers are fond of embossing on the backs of TV sets to frustrate consumers when they need to plug in the cable in dim light. Your eyes are subconsciously helping you out. This could have been vital for our primitive ancestors. How could they have plugged in the cable before the flashlight was invented?
Compare this finding with the one about birds that bobble their heads when they walk (04/12/2004). When you see something in nature you don’t understand, try the approach that there must be a reason for it. Science is supposed to be an organized method for finding out the reasons for things. Now, ask yourself the meta-question: what is the reason for reason?