Modern imaging techniques are enabling scientists to study the brain in ways once unimaginable and providing an understanding of the chemical processes that make memories.
With positron emission tomography and functional magnetic resonance imaging, for example, neuroscientists can pinpoint brain activity during different memory tasks.
At the National Institute of Mental Health, Alex Martin, chief of the section on cognitive neuropsychology, and colleagues James Haxby, chief of the section on functional brain imaging, and Leslie Ungerleider, chief of the Laboratory of Brain and Cognition, use these techniques to explore how the brain stores information.
The findings reveal a brain organized around the processes of learning, not the objects themselves, and by the way in which these objects are used. "A hammer, for instance, is stored in an area that involves motion, while the image of a cat is placed in a part of the brain that contains other visual shapes," Martin said.
To identify an object, he said, "we instantly retrieve information by the features that define it. What does it look like? What color is it? How does it move or how do we manipulate it, if it's a tool?"
For example, the studies found that verbs are stored in areas of the brain just in front of regions involved in the perception of motion. Colors of objects--the memory of a pencil's bright yellow hue, for example--are stored next to the perception of color.
Thus a hammer is stored three ways in the brain: once for its form, once for its function or motion and once for the memory of the motor skill needed to use it. "We store these bits of information about objects near their features," Martin said. "It's all very logical."
Memory Storage: a Multi-Site Process
But scientists are searching even deeper, seeking to unravel how individual brain cells store and share information. Last month, researchers at the University of Geneva in Switzerland announced that they had captured what appears to be the first electron micrograph image of the cellular changes involved in long-term memory. Reporting in the journal Nature, the team showed how the connections between two nerve cells in a rat's brain change significantly when long-term memory is established.
In an alteration that is believed to occur across nearly all species from rat to human, the team found significant changes in the spiny dendrites that form at gaps between nerve cells called synapses. Scientists believe that this change facilitates a host of associations linking a particular memory with experiences, thoughts, emotions, sights, sounds and smells.
