A new study reveals that our eyes absorb important information from the surroundings during periods of rapid movement, when the eyes shift abruptly from one point of focus to another, according to a new study in the July 23 issue of Science Advances. People corrected their gaze to locate a moving target by using information from motion streaks gathered during these eye movements, which scientists call saccades.
The findings contradict the long-held assumption that our eyes are temporarily blind during saccades, helping to explain how we perceive the world as stable even though our eyes dart incessantly from object to object.
"In previous experiments we found that human observers performed rather poorly when their task was to explicitly detect motion streaks, which were induced by rapidly moving objects strictly during saccades," said Richard Schweitzer, a former doctoral student at the department of psychology in Humboldt-Universität zu Berlin and a postdoctoral researcher at the Cluster Science of Intelligence (SCIoI) in Berlin. "It was therefore surprising to us that the current experiments, in which observers performed an everyday oculomotor task, showed that intra-saccadic information was consistently used even though motion manipulations during the tests were never even noticed."
Our Secret Saccades
For more than a century, scientists have assumed that the eyes are incapable of absorbing visual information during saccades, which last less than 50 milliseconds each and serve to project images onto the fovea of the eye's retina, where our vision is the sharpest. This assumption is, in part, rooted in the fact that we are normally not aware that we are constantly making saccades — it seems intuitive that the brain would suppress these signals.
"It's a nice party trick to try to see your own saccades in a mirror — you'll realize that you will not be able to," said Schweitzer. "In normal visual environments, we remain fully unaware of the sensory consequences of our own saccades."
Previous studies have supported the idea that vision is impaired during saccades, arguing that there are active mechanisms in place to discard such visual information that could otherwise harm our sense of visual stability.
"It is probably precisely this strong intuition that visual input during saccades is a kind of nuisance to the system that fosters the idea of elimination from processing," said Schweitzer.
While previous research has tended to focus on the brain's impressive ability to prevent us from seeing a " grey out " each and every time we experience saccadic eye movements, Schweitzer and Martin Rolfs, a professor in the department of psychology at Humboldt-Universität zu Berlin and lead principal investigator of the study, took their work in a different direction.
"We were inspired by a question that has so far received only very little attention, that is, whether self-induced, intra-saccadic visual information may be useful to the visual system in any way, even if we are not consciously aware of it," said Schweitzer. He noted that in order to explore this question, the researchers needed to design experiments in which an object's identity and motion could be manipulated independently of one another, even during saccades.
"Implementing such a paradigm was indeed no easy task, as both eye tracking and visual presentation systems need to operate synchronously at extremely high temporal and spatial resolution," said Schweitzer.
Tracking Targets With Motion Streaks
To investigate whether the eyes actually gather visual data from motion streaks during saccades, Schweitzer and Rolfs used cutting-edge hardware, including a 1440-hertz high-speed projector that generates about 70 images per saccade and a 2000-hertz binocular eye tracker, to manipulate a visual scene during participants' rapid eye movements. This technology far surpassed the computer monitors often used by visual scientists, which usually operate at between 60 and 160 hertz.
"These technological advances allowed us to mimic visual consequences of saccades, making the investigation of the potential functional role of intra-saccadic vision even possible," said Schweitzer.
Leveraging these technologies, the researchers swiftly rotated a designated eye movement target among an array of six patches with unique patterns as participants' eyes sought out the target. They noticed that the participants performed corrective eye movements that enabled them to successfully locate the target, although briefly inverting an object during its motion negatively impacted gaze correction. This suggested that the continuity of motion streaks perceived during saccades helped observers find the target.
"The new results of Schweitzer and Rolfs now show that the brain adapts […] the retinal motion streak generated by an eye movement," wrote Jasper Fabius and Stefan Van der Stigchel in a related Perspective . "Together [with other recent insights], these data demonstrate that eye movements are a fundamental component in the human visual system — not a nuisance, but useful."
While we may live our everyday lives unaware of our saccades and the visual input our eyes covertly obtain during them, Schweitzer noted that they may impact us in unexpected ways. For example, when our eyes make saccades across single, small LED light sources that flicker at relatively high frequencies in dimly lit spaces, such as car lights at night (but not rooms brightly lit by LEDs), a visual phenomenon occurs in which we briefly see dotted or dashed trajectories — an issue some manufacturers are working to resolve. Research has suggested that people more sensitive to these phantom arrays also tend to be more susceptible to visual discomfort.
"This phantom array is a genuine intra-saccadic visual phenomenon and elucidates that vision during saccades might not be as irrelevant as previously thought," said Schweitzer.
Next, the researchers plan to test biologically inspired models of visual function in simple robotic systems to understand how moving agents use vision, motor, and proprioception information to achieve visual stability.
[Credit for associated image: Endo :]/ Flickr]