When you open your eyes and reach out to shut off the alarm clock, two distinct brain systems are activated: one recognizes the clock, and the other guides your hand. Neuroscientists have long been aware of this “dissociation” between the recognition and guidance systems, but they had not been able to observe both in action. Now Lior Shmuelof and Ehud Zohary of Hebrew University in Jerusalem have used functional magnetic resonance imaging to see the duality in action in human volunteers.
The subjects watched videos of hands entering the screen from one side and grasping objects on the opposite side. Most previous studies of the two visual systems, Shmuelof explains, “were of people who had had brain damage. Those suffering from agnosia could not recognize objects, whereas those with ataxia could identify objects but could not guide their hands to grab them.” He says his work with Zohary is the first to test individuals for whom both systems operate normally.