The Eye's Have It
This recent article in Scientific American on how eye movements are stablized in the brain is potentially Nobel Prize material in my humble opinion. Understanding a process so basic and fundamental that it occurs without our awareness and yet without it, we could not normally function (nor would we have been able to evolve) is truly an extraordinary achievement.The problem: How does the brain reconcile and stabalize the images received from our eyes. Our eyes constantly dart around about 3-4 times a second in little hops called saccades. Yet we do not perceive this motion. We see a stable, stationary picture presented to us. This stablalizing effect has been in debate for a over a century. Brain researchers have long assumed that the brain must keep track of the impulses that cause these tiny motions, so as to subtract their effect from our visual awareness. Now researchers have identified a circuit in the monkey brain that seems to play this "stabalizing" role.
Ignoring the motion of our eyes allows us to focus on changes in our environment. The alternative would be chaos, says brain researcher Robert Wurtz of the National Institutes of Health in Bethesda, Md. "It's almost as if you have a movie camera on top of a bronco and it's jumping around," Wurtz says. "If you watched the movie it would make you sick." 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...
Wurtz and his colleague Marc Sommer, now at the University of Pittsburgh, stumbled onto the presumed corollary discharge pathway while stimulating the brain region that controls eye movements in live monkeys. Sommer noted that a current applied to this area, called the superior colliculus, elicited a delayed response in the frontal cortex, which is associated with attention and decision making, Wurtz recalls. The delay suggested a relay of neurons ending at the frontal cortex.
Researchers had already observed that brain cells in this region seem to anticipate where the eye's center of focus will move to after an impending saccade, making it a reasonable place for corollary discharges to end up.
Other experts find the result convincing. "For a long time we've known that mechanism had to be there, and they've shown how it works," says neurophysiologist Douglas Munoz of Queens University in Ontario. Besides solving this puzzle, adds 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.
While this first step toward understanding is great, I do cringe at the following statement made in the article - "brain cells in this region seem to anticipate where the eye's center of focus will move to ...".
The term anticipate used by the researcher seems a bit out of place. He is attempting to personify brain cells with anticipatory response - which is (to put it very nicely) unlikely. Brain cells do not anticipate anything - they process information.
When we interact with our environment the brain processes those interactions. From birth we are subjected to the stimuli inherent to our reality (environment). We receive these stimuli through our five senses. Over time and through repitition, our brains become hard-wired to recognize and subconsciously respond to various aspects of our environment based on past experience. This hard-wiring, or imprinting process in the brain is known as neuroplasticity (think classical or Pavlovian conditioning) .
When I see the color "red" I think of many things. "Red" can mean "apple", or "stop", or "blood". Based on past experience, my brain retains these numerous concepts of "red". When I next encounter a situation involving "red", the brain processes the enviromental context in which "red" is currently occurring, and subconsciously determines the appropriate association (I cut my finger so red = blood. I am driving and see a traffic light so red = stop). All these associations are based on prior interactions and experience of the concept "red" imprinted in the brain.
A better way to describe what is happening when the brain stabalizes our jerky eye motions would be to say the brain creates and retains a "background image" in another location. Through neuroplasticity, we know things far away from us (say mountains or the sky) are stationary and will be in the same place when the next saccade occurs - it is therefore "stable". The brain can now focus on detailing objects with the potential to display motion during the next saccade (a tree on a windy day or clouds passing overhead). The brain would then need only reconcile the portions of a new incoming image that have the potential for perceptable change with that of the stable "background image". Thus negating the need for the brain to anticipate future events.
I think it would be interesting if the researchers looked at another seemingly simple, yet overlooked occurance in nature that may be in direct correlation with their research- why lower level animals are able to walk almost immediately after birth. And why humans take between 9-16 months before they even begin to take those first tentative steps.
I would suggest this occurs because animal brains do not need to process as much information as the human brain does. Their brains are geared solely for survival. Without the distraction of high-level consciousness, the animal brain is able to "focus" more rapidly on survival processes. The faster it can stabalize visual reality, in conjunction with being born with the appropriate musculature, the faster they become ambulatory and un-eaten by predators.
There is no immediacy for humans to begin walking, nor for our bodies to develop quickly. Because of our nuturing process, we are not in any immediate danger at birth nor for a long time afterwards. Our bodies develop slowly. This gives our brain time. Time to "concentrate" on other things. Time to "concentrate" on all the other things that make us uniquely human (speech, face and symbol recognition, high-level consciousness, etc.).
Labels: brain, neuroscience, research, science, vision
