Researchers using sophisticated imaging technology have for the first time photographed the process of memory formation, providing new insight into the normal functioning of the brain.
The study, in which neurologists used X-ray-like images of the brain to observe the process of recalling recently seen words, provides confirmation of the long-held belief that a central portion of the brain called the hippocampus plays the crucial role in processing events into memories.
But it also produced some unexpected findings. The researchers, from UC San Diego and Washington University in St. Louis, reported Monday that some processing of memories also occurs at other, previously unsuspected sites in the brain.
Finally, they observed that one exposure to an image, such as a word or a specific picture, reduces the time required for the brain to recognize the same image on a second exposure, a phenomenon called priming.
The results, presented at a meeting of the Society for Neuroscience in New Orleans, are providing a remarkable new scenario for the learning process, an insight scientists hope may lead to improved treatments for individuals with damaged brains.
Perhaps even more important, the use of the imaging technique, called positron emission tomography or PET scanning, is allowing researchers for the first time to investigate the mental processes of healthy individuals.
In the past, researchers have had to infer from behavioral studies that humans whose hippocampus had been damaged in accidents could no longer store new memories. They also observed this in animal studies.
“Now we are able to carry the study to normal people and study normal behavior, and that is very exciting,” UCSD neuroscientist Larry R. Squire said in a telephone interview Monday. With new techniques, he added, researchers are able to watch the brain in action to see which areas are involved in any specific function.
Researchers hope eventually to use PET scanning and other new techniques to make a comprehensive map of the brain that will chart the function of each specific area. “The long-term goal of scientists is to figure out exactly what happens in what regions, and how all the tasks are orchestrated together to produce the mind as we know it, said neuro-biophysicist John W. Belliveau of Massachusetts General Hospital in Cambridge.
The studies depend on the fact that portions of the brain doing the most work consume the greatest amount of glucose in the blood. As a consequence, blood flow to those regions is increased. That flow is monitored by PET scanning.
In the technique, researchers inject into the subject’s bloodstream a small amount of water tagged with a radioactive isotope of oxygen. Radiation detectors placed around the head measure the amount of radiation produced in different areas of the brain. The site where the greatest amount of radiation is detected has the largest blood flow, and thus is most active.
To monitor brain activity, the researchers produce repeated images of the brain over a short period of time. By identifying those areas of the brain where changes in radiation intensity change while the subject is performing a specific task, they are able to identify the part of the brain carrying out the task and to demonstrate communication between various parts of the brain.
Squire and Washington University neurologist Marcus Raichle studied 18 healthy volunteers. Using a computer screen, they showed the volunteers 15 four- to eight-letter words. Subsequently, they showed the volunteers lists of the first three letters of words, some of which were on the initial list and some that weren’t. They then asked the volunteers either to say the first word that came into their heads or to try to recall the specific word from the initial list.
When the subjects were forming memories, Raichle and Squire found that most of the brain activity was in the hippocampus, a banana-shaped structure deep within the brain. This part of the study confirmed conclusions that had been reached by studying damaged brains.
When the subjects were attempting to recall specific words that they had seen before, the researchers found extra activity not only in the hippocampus, but also in the frontal cortex, an area of the brain involved in thought processes. “This was not a surprise,” Squire said, “because the frontal lobes are thought to be important in searching for specific memories.”
Their third finding, which Squire termed “completely novel,” occurred when the volunteers were asked not to remember specific words but simply to say the first word that came into their heads. In this case, the bulk of brain activity occurred not in the hippocampus but in a region at the rear of the brain that is thought to be involved in processing visual information.
Furthermore, less energy is used the second time the volunteer views a word.
It appears, Squire said, that when an individual first views a word or an object, traces of that image remain in the visual processing center for a short while afterward, even though the volunteer has no conscious knowledge of its presence. When the object is seen for a second time, the volunteer can recognize and name it much more quickly and with less effort.