This research finding, announced today in the British scientific journal Nature, shows that the prefrontal cortex may not just act as a waystation to storage places for information in working memory, but may, as animal studies have suggested, be the actual site of storage of such information.
Using non-invasive functional magnetic resonance imaging, Carnegie Mellon psychologist Dr. Jonathan Cohen and his collaborators were able to test this theory for the first time in human subjects.
"We believe this is important new information about the prefrontal cortex, establishing an important link with neurophysiological studies in animals," Cohen said. "It is the first time that we have pictures of such activity in humans. Our pictures show that the prefrontal cortex seems to have more of a role in the maintenance of information in working memory than influential theories have previously suggested."
Working memory is central to our higher cognitive functions, including language, planning and problem solving. It plays a central role in letting us keep several matters in mind simultaneously, and coordinating our activities.
Aspects of memory are scattered throughout the brain, however until now, it was unclear exactly what role the prefrontal region--just behind the forehead--played in memory storage. One prominent theory held that this area was involved exclusively in coordinative activities, for example, deciding what information to remember, but not the actual storage of the information itself. Neurophysiological studies have suggested otherwise--that information such as a phone number after you get it from directory assistance is actually temporarily stored in the prefrontal cortex. This had never been tested in human subjects.
Through a series of experiments asking subjects to perform a sequential memory task involving letters of the alphabet, the researchers examined whether the prefrontal cortex became less active after each letter was presented -- once its work of sending information to other storage sites was complete -- or whether it remained active, indicating that it was playing a role in maintaining this information.
Non-invasive functional magnetic resonance imaging of subjects done at the University of Pittsburgh Medical Center while subjects were performing the memory task, showed a sustained pattern of activity in the prefrontal cortex, suggesting that it is closely involved in the storage of the information.
Cohen has a joint research appointment at the University of Pittsburgh and several members of the research team are on the faculty of the Pitt Medical Center. His study was also conducted in collaboration with researchers at the University of Michigan.
Functional magnetic resonance imaging permits scientists to visualize safely the anatomy and function of the normal human brain by detecting changes in blood oxygenation. Because of this, fMRI is able to pull signals directly from the changing brain tissue without being invasive.
"Our research findings challenged the strong view that there is a clear dissociation between executive and maintenance processes in working memory, with the prefrontal cortex housing the former and not the latter," Cohen said.
"At the very least, it suggests that the relationship between executive functions and active maintenance of memories may be more complex than originally thought."
The next step for researchers is to determine exactly what information the prefrontal cortex may be holding onto, and how it is organized.
"Characterizing the type of information actively maintained in the prefrontal cortex may be functionally more relevant than a distinction between executive control and maintenance," Cohen added. "For example, the prefrontal cortex may be hodling onto the actual content of information, such as the phone number itself. Alternatively, it may be holding onto the goal of maintaining this information, or how it will be used, rather than the information itself. These are the types of questions that must now be answered."
Cohen said his research is another step forward for cognitive scientists who are seeking ways to better understand how the brain processes, stores and retrieves information.
And in the case of Cohen's other research interest in schizophrenia, the findings give some clues about how parts of the brain function than can go terribly awry in psychiatric illness.