The world we see is usually stable. Still, our eyes are continuously moving, making two to three 'saccades' per second. For over a century, scientists have tried to explain why we saw a stable world. Now, two U.S. researchers have found why our shifty eyes don't drive us crazy. In fact, they confirmed what was suggested more than 50 years ago: when our brain asks our eyes to move, it also sends a copy -- an e-mail? -- of this command to our visual system. But read more...
Before going further, below is an illustration showing how we maintain our visual stability across saccades. "When we scan a visual scene with quick eye movements known as saccades (top), the retinas send a series of 'snapshot' images to the brain that must be integrated properly." (Credit: Marc Sommer/Robert Wurtz)
The paper from which this image comes (see below) has a more extended caption: "(a) A subject may look at an apple (dotted line) and make a saccade (orange arrow) to a pepper. (b) Several such saccades (arrows 1-3 at top left) will cause sequential images on the retina (top right). The visual system is thought to use corollary discharge to interpret these retinotopic visual changes. A percept of visual stability can be constructed by integrating corollary discharge and retinal images (bottom). (c) We hypothesize that a corollary discharge used for spatial visual analysis rises from superior colliculus (SC) through mediodorsal thalamus (MD) to the frontal eye field (FEF)." (Credit: Marc Sommer/Robert Wurtz/Nature)
Now, let's look at the details of what these researchers have found.
Sommer and Wurtz showed in a 2002 Science paper that a pathway from brainstem to frontal cortex conveys a corollary discharge signal in the brains of monkeys. They suggested that this pathway might cause visual neurons of the cortex to suddenly shift their receptive field--their window on the world--just before a saccade.
[Now] they showed that the receptive fields in cortex are shifted because of the corollary discharge from the brainstem. To do this, they exploited the fact that the signals are relayed via the thalamus, a crucial intermediary. By knocking out the relay neurons, they interrupted the pathway. They found that receptive field shifts were curtailed by more than half.
In "Brain Pathway Brings Order to Visual Chaos," Scientific American gives additional explanations.
Researchers believe the brain solves this problem through a process called corollary discharge. Every time the brain sends the eyes a signal to twitch, it sends a copy, or corollary signal, to another location in the brain, sort of like the way your e-mail client sends copies of your e-mails to their own folder, Wurtz explains.
Other researchers seem convinced by Sommer and Wurtz experiments.
Besides solving this puzzle, [says] James Lynch of the University of Mississippi, the group's "imaginative and exceedingly difficult" experiments also mark a new step in the ability to pinpoint the flow of information in the brain. Sommer says future experiments may inactivate more of the thalamus to see if monkeys have a harder time distinguishing their own saccades from changes in their environment.
So what will be these future experiments? Let's return to the University of Pittsburgh news release.
In future studies, Sommer and his graduate students at Pitt will perform the first direct test of the visual stability hypothesis. To determine whether shifting receptive fields are responsible for visual stability, the shifts will be disrupted in monkeys trained to detect visual motion. The monkeys could then report whether their world appears to be moving around abnormally as eye movements are made.
This research work is available as an advance online publication from Nature under the name "Influence of the thalamus on spatial visual processing in frontal cortex." Here are two links to the abstract and to the full paper (PDF format, 4 pages, 912 KB). But for more information, don't miss this supplementary info (PDF format, 18 pages, 2.30 MB). The above illustration has been extracted from this document.
Finally, don't you think it's funny that 'Today's Trivia' question from Scientific American is also eye-related? Here is the question of the day: How often do people blink? And the answer is: "Infants blink about twice a minute--a rate that increases until puberty, when it levels off. From puberty onward, people blink an average of 15 times a minute."
Sources: University of Pittsburgh news release, November 8, 2006; JR Minkel, Scientific American, November 8, 2006; and various websites
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