Last year, Alan Cowey and his colleagues atOxford identified an area in the macaque justbehind V4 that seemed to be responsible for colorvision.
Now Hadjikhani and her team believe they havelocated an analogous area in humans--V8.
They obtained their data by showing subjectsmoving wheels of color while monitoring whichareas of their brain the colors activated.
The researchers employed a technique calledfunctional magnetic resonance imaging, which looksat the tiny magnetic changes that result fromaltered levels of blood flowing in the brain.
To back up their data they studied the areas ofthe brain activated by color afterimages, theillusory images of complementary colors thatpeople experience after staring at a certain colorfor a long time.
In a result that Hadjikhani described as"cool," they were able to demonstrate that V8 wasalso activated during color after images.
But Zeki and his colleagues are up in arms.They have submitted a letter to NatureNeuroscience arguing against the findings. Atissue--in what Cavanagh described as a case of"physiological infighting"--are the true locationsof V4 and V8, and what their functions are.
"Zeki is almost certainly wrong," Cavanaghsaid. "Pioneering and fundamental, but wrong."
Like a Black and White Movie
The discovery has already had some practicalimplications, helping to explain the symptomsexperienced by people who lose their color visionin accidents.
Many people--almost 8 to 10 percent of the malepopulation--are born without normal color vision.This form of color blindness is due to defects inthe retina.
But it is a far smaller number ofpeople--probably about 20 or so in the entirehistory of neuroscience--who have lost their colorvision due to lesions caused by accidents. Thesepeople are known as spinal achromats. Toachromats, the world looks like a black and whitemovie.
In a separate paper published alongside theHadjikhani findings in Nature Neuroscience,Cavanagh reported that this condition is a resultof a lesion in V8.
The paper focused on the degree to whichinformation about color is separated frominformation about motion and brightness that isalso processed in the brain. The researchers foundthat achromats were able to distinguish the motionof certain color stimuli just as those without thedisorder.
This indicated that the pathway from the retinato the motion analysis areas was independent ofthe color areas damaged by the disorder