As we look at the world around us, images flicker into our brains like so many disparate pixels on a computer screen that change every time our eyes move, which is several times a second. Yet we don'tperceive the world as a constantly flashing computer display.
Why not?
Neuroscientists at The Johns Hopkins University think that part ofthe answer lies in a special region of the brain's visual cortexwhich is in charge of distinguishing between background andforeground images. Writing in a recent issue of the journal Neuron,the team demonstrates that nerve cells in this region (called V2) areable to "grab onto" figure-ground information from visual images forseveral seconds, even after the images themselves are removed from our sight.
"Recent studies have hotly debated whether the visual system uses abuffer to store image information and if so, the duration of thatstorage," said Rudiger von der Heydt, a professor in Johns Hopkins'Zanvyl Krieger Mind-Brain Institute, and co-author on the paper. "Wefound that the answer is 'yes,' the brain in fact stores the lastimage seen for up to two seconds."
The image that the brain grabs and holds onto momentarily is notdetailed; it's more like a rough sketch of the layout of objects inthe scene, von der Heydt explains. This may elucidate, at least inpart, how the brain creates for us a stable visual world when theinformation coming in through our eyes changes at a rapid-fire pace:up to four times in a single second.
The study was based on recordings of activity in nerve cells in theV2 region of the brains of macaques, whose visual systems closelyresemble that of humans. Located at the very back of the brain, V2 isroughly the size of a wristwatch strap.
The macaques were rewarded for watching a screen onto which variousimages were presented as the researchers recorded the animals' brainnerve cells' response. Previous experiments have shown that the nervecells in V2 code for elementary features such as pieces of contourand patches of color. What is characteristic of V2, though, is thatit codes these features with reference to objects. A vertical line,for instance, is coded either as the contour of an object on the leftor as a contour of an object on the right. In this study, theresearchers presented sequences of images consisting of abriefly-flashed square followed by a vertical line. They thencompared the nerve cells' responses to the line when it was precededby a square on the left and when it was preceded by a square on theright. The recordings revealed that the V2 cells remember the side onwhich the square had been presented, meaning that the flashing squareset up a representation in the brain that persisted even after theimage of the square was extinguished.
Von der Heydt said that discovering memory in this region was quite asurprise because the usual understanding is that neurons in thevisual cortex simply respond to visual stimulation, but do not have amemory of their own.
Though this research is only a small piece of the "how people see andprocess images" puzzle, it's important, according to von der Heydt.
"We are trying to understand how the brain represents the changingvisual scene and knows what is where at any given moment," von derHeydt said. "How does it delineate the contours of objects and howdoes it remember which contours belong to each object in a stream ofmultiple images? These are important and interesting questions whoseanswer may someday have very practical implications. For instance,how we function under conditions that strain our ability to processall relevant information - whether it be driving in city traffic,surveying a large crowd to find someone, or something else, maydepend in large part on what kind of short-term memory our visual system has."
Understanding how this brain function works is more than justinteresting. Because this study shows how the strength and durationof the memory trace can be directly measured, it may eventually bepossible to understand its mechanism and to identify factors that canenhance or reduce this important function. This could assistresearchers in unraveling the causes of - and perhaps evenidentifying treatment for - disorders such as attention deficitdisorder and dyslexia.
Source: Johns Hopkins University
Rudiger von der Heydt is a professor in Johns Hopkins University's Zanvyl Krieger Mind-Brain Institute.
(Photo Credit: Will Kirk)