February 1, 2006
La Jolla, CA – For the first time, scientists have caught a glimpse of the brain as it predicted the future location of a fast moving object in real time.
Vision seems so effortless: We open our eyes and let the world stream in. But we don’t ‘see’ with our eyes – we actually ‘see’ with our brains, and it takes time for the world to arrive there. From the time light hits the retina till the signal is well along the brain pathway that processes visual information, at least 70 milliseconds have passed. During this time, a baseball that clocks in at a rather lame 85 mph has already traveled 10 feet! For the player to hit the ball, experience notwithstanding, his brain has to compensate for the delay.
“Our brain is a computing device that takes time to compute,” says John Reynolds, Ph.D., a neurobiologist at the Salk Institute for Biological Studies who headed the study reported in the current issue of the journal Neuron. “If you are looking at a painting it doesn’t matter that you’re experiencing events that occurred a fraction of a second ago. In the real world you often have to interact with moving objects and then you need to compensate for this delay,” he explains.
And that’s just what the brain does, concluded the Salk scientists when they recorded the activity of individual brain cells in the visual cortex while they were activated in response to a moving object that briefly changed color on a computer screen.
The visual system is composed of a set of interconnected maps that represent the visible space around us, just like conventional maps mirror geographical areas. Any brain cell activity within these maps reflects the location of a visual stimulus, such as a fastball about to leave a pitcher’s hand in the real world. But by the time the signal shows up on the visual maps in the batter’s brain, the baseball has already traveled a sixth of the distance between the pitcher’s mound and home plate.
While it had been known from earlier studies that the visual system had a mechanism in place that shifts a moving object’s perceived location from where it was when we ‘saw’ it to where it most likely would be by the time the brain was done processing the incoming information, it was unclear just how the brain achieved this feat.
“We have found evidence that at least one of these maps becomes distorted in just the way you would need to account for the perceptual shift,” explains Salk Institute graduate student Kristy A. Sundberg, the study’s first author. “This map distortion may be part of the mechanism the brain uses to shift the perceived location of the moving stimulus along its motion trajectory, in order to compensate for the brain’s computational delay.”
To find out how the brain compensates for the computing delay, the researchers took advantage of an optical illusion known as the ‘flash-jump’ effect click here for a short demonstration.
In the slightly modified version the scientists used for their experiments, a flashing stimulus moves across the screen and briefly changes color while it continues on its journey. When asked where the color change occurred in relation to a fixed symbol on the computer screen, most people can’t pinpoint the correct location but instead report ‘seeing’ the color change as being further along its trajectory.
Sundberg and colleagues recorded neuronal responses in an area that lies halfway along the pathway that processes visual information in the brain. With the stimulus sequence moving from left to right, they first mapped the exact area on the computer screen where a color change would elicit a response in a brain cell in this area. They then made the same measurement when the stimulus moved in the opposite direction.
If the cell simply encoded the physical location of the color change, the response would be the same because the color change occurred at the exact same physical location. “But what we found instead was a shifted profile,” reports Sundberg.
“It is as if the cell reached out to intercept the moving stimulus. As a result of this shift, we perceive the color change further along the motion trajectory, helping to compensate for processing delays,” explains Reynolds, assistant professor in the Systems Neurobiology Laboratory.
The Salk Institute for Biological Studies in La Jolla, California, is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine all but eradicated the crippling disease poliomyelitis in 1955, opened the Institute in 1965 with a gift of land from the City of San Diego and the financial support of the March of Dimes.