Scientists do not completely understand how memory is stored in the human brain. Some researchers believe that short-term and long-term memories reside in separate regions of the brain. In this August 1997 Scientific American article, staff writer Tim Beardsley summarizes the results of studies that have sought to discover which areas of the brain are involved in short-term memory and how that memory is organized.
The Machinery of Thought
By Tim Beardsley
In a darkened basement laboratory on the campus of the National Institutes of Health in Bethesda, Md., volunteers earn $100 by lying for two hours with their head inside a huge magnetic resonance imaging (MRI) machine while they gaze at a screen reflected in a mirror. The screen periodically displays black-and-white pictures: some are faces, others scrambled blocks of light and shade. When a face appears on the screen, the subject signals by pressing buttons whether the face is a new one or the same as one that was shown a few seconds earlier as a "target" to be remembered.
As the test proceeds, the MRI machine bombards the volunteer's brain with radio-frequency waves that excite hydrogen atoms in the bloodstream, causing the atoms to emit signals of their own. Later, the machine transforms the resulting electromagnetic cacophony into color-coded maps of oxygen consumption levels throughout the subject's brain. Because increased oxygen consumption results from heightened neural activity, researchers can analyze these brain maps to learn what parts of the brain work hardest when a person recognizes a face.
With experiments such as these, researchers are beginning to fathom the neural processes underlying "working memory"—the limited, short-term store of currently relevant information that we draw on when we comprehend a sentence, follow a previously decided plan of action or remember a telephone number. When we bring to mind the name of Russia's president, for instance, that information is temporarily copied from long-term memory into working memory.
Psychological studies have demonstrated that working memory is fundamental to the human ability to reason and make judgments that rely on remembered contextual information. There are compelling humanitarian reasons for understanding working memory. Schizophrenia, one of the most devastating mental illnesses, is believed to be caused in part by a defect of this system. Studies of the molecular basis of working memory "have implications for drug treatment in mental illness," says Patricia Goldman-Rakic of Yale University, one of the most prominent investigators of working memory.
An intensive research effort has started to produce detailed information about the areas of the brain involved when we engage this vital intellectual faculty and is illuminating the patterns of neural activity that allow it to operate. The important role of specific brain chemicals in working memory is also becoming clear. Yet for all the progress, researchers have still to agree on how working memory is controlled and organized.
From Electrodes to Fast MRI
The prototypical test for working memory involves what is called delayed choice. An animal or a person signals where some specific cue was previously seen, before an imposed period of waiting. Thus, a monkey might be given a choice of two jars in separate positions and be rewarded for pointing to the one in which it previously saw food placed.
The task provides no clue to the correct response at the time of testing, so the monkey must rely on its recollection of the correct location. A related challenge rewards an animal for remembering which of several images it saw presented initially as a target. The NIH volunteers who were recalling faces were engaged in a variant of this test.
Technological advances have greatly enhanced researchers' ability to probe the neural underpinnings of such capacities. Investigators began studying cerebral activity in working memory some 40 years ago by inserting electrodes into individual neurons within the brains of monkeys. This method has its limits, however. Although monkey brains have clear anatomical similarities to human brains, the animals' behavior is vastly simpler, making detailed comparisons with human thinking problematic. Lacking language, the animals must be patiently trained over a period of weeks to master tasks that a person would pick up in a minute.
Electrode-recording techniques are also ethically unacceptable for use on people. Researchers try to learn which parts of our species' brain do what by studying the effects of damage caused by injury, disease or therapeutic surgery. Yet patients have different medical histories—and their brains vary in exact shape—so interpreting this clinical data is tricky at best.
Earlier this decade, positron emission tomography, or PET scanning, made enormous strides by showing which parts of the human brain are busiest when performing different tasks, such as hearing words or speaking. But PET requires exposing the human subjects to radioactive tracers, and to keep radiation doses within acceptable levels, researchers have to use techniques that can resolve brain areas only about a centimeter apart. Also, during a delayed-choice task, PET scans are too slow to distinguish between the neural activity pattern of a target being held in mind and the pattern that follows a few seconds later when the target is recognized.
The new technique used at NIH and elsewhere, called functional MRI, can resolve the position of active neurons to about two millimeters and is fast enough to study activity before and after the brain recognizes a cue on a screen. The rapidly improving technique has over the past two years become the state of the art for functional brain imaging.
Experiments involving electrodes implanted in monkeys still provide crucial information, however, because they reveal in fine detail and on a millisecond-by-millisecond timetable what happens as these primates respond to cues and rewards. When animals perform such feats of working memory, several brain regions can play a role, but as Joach’n M. Fuster of the University of California at Los Angeles showed in the 1970s, one area that is always involved is the prefrontal cortex.
The prefrontal cortex is a layer of tissue that lies just behind the forehead. With neural connections to almost all the areas of the brain that process sensory information, it is well situated to maintain a flexible store of information relevant to any task at hand. It is also the part of the brain that has grown the most in humans, as compared with monkeys. Monkeys missing some parts of their prefrontal cortex preserve their long-term memory but perform miserably on delayed-choice tests. Humans similarly afflicted suffer a reduced attention span and ability to plan.
Fuster and, separately, Kisou Kubota and Hiroaki Niki of the Kyoto Primate Center made electrical recordings from a variety of neurons in the monkey prefrontal cortex, including some that apparently were active only while the animals were holding information in working memory. Subsequently, Goldman-Rakic and her colleagues have explored working memory in monkeys with more sophisticated tests. They established that prefrontal neural activity during a delayed-choice task indeed corresponds well to the functioning of working memory.
Goldman-Rakic and her associate Graham Williams have taken the analysis all the way to the subcellular level, showing that receptors for the neurotransmitter dopamine pivotally influence the responsiveness of cells in the prefrontal cortex and their actions in working memory. "There is no other example I know" of research that spans the gulf between behavior and subcellular function, Goldman-Rakic notes. She and her colleagues have recently shown that administering antischizophrenic drugs to monkeys for six months leads to specific changes in the numbers of two different types of dopamine receptors in that region, further evidence that schizophrenia—or its treatment—alters normal function there.
Research by other scientists supports the view that the prefrontal cortex could sustain working memory. Robert Desimone of the National Institute of Mental Health, along with Earl K. Miller, Cynthia Erickson and others, has discovered in the monkey's prefrontal cortex neurons that fire at different rates during the delayed-choice task, depending on the target the animal saw previously. Neurons in other parts of the brain generally "forget" the target when a distracting stimulus appears—their rate of firing changes. Prefrontal neurons detected by Desimone and his colleagues, in contrast, maintain their rate of activity during a delayed-choice task even after the animal is presented with irrelevant, distracting stimuli.
Activity in some prefrontal neurons, then, appears to embody directly the temporary working memory of the appearance of a target the animal is seeking. Other researchers have found prefrontal neurons that seem to maintain locations in working memory: Giuseppe Di Pellegrino of the University of Bologna and Steven Wise of the National Institute of Mental Health have found prefrontal neurons that are busiest when an animal has to remember where it saw a cue. Stimuli fail to excite the same frenzy unless they are in the location that is the current target for the task.
Neurons in the prefrontal cortex could thus apparently control how animals respond in a delayed-choice task. Fuster, one of the pioneers in the field, says the prefrontal cortex "serves the overarching function of the temporal organization of behavior" by driving networks that maintain currently important information in an active state. And neurons in the prefrontal cortex might exert their influence in more subtle ways, too.
Besides controlling directly the responses in delayed-choice tests, Desimone believes, the prefrontal cortex might tune the visual and possibly other perceptual systems to the task at hand. "What's loaded into working memory goes back to sensory processing," he suggests. Hundreds of experiments with both animals and people have shown that organisms are far more likely to perceive and react to cues relevant to their current needs than to irrelevant stimuli. This effect explains why we are more likely to notice the aroma wafting from a neighbor's grill when we are hungry than just after eating. If Desimone is right, the prefrontal cortex could be responsible for focusing an animal's attention and thus possibly steering awareness.
Imaging studies with PET and functional MRI corroborate the evidence from brain injuries that the human prefrontal cortex, like that of monkeys, is central to working memory. Several research groups have now imaged activity in the prefrontal cortex when people remember things from moment to moment. Different tasks may also require various other brain regions closer to the back of the head, but for primates in general, the prefrontal cortex always seems to be busy when target information is kept "in mind."
The Devil in the Details
Having shown that the prefrontal cortex is crucial to working memory, investigators naturally want to understand its internal structure. Goldman-Rakic and her associates at Yale have found evidence that when an animal retains information about a spatial location, the prefrontal activity is confined to a specific subregion. A separate area below it is most active when an animal is remembering the appearance of an object. These findings, together with observations of the anatomy of neural pathways, led Goldman-Rakic to propose that the prefrontal cortex is organized into regions that temporarily store information about different sensory domains: one for the domain of spatial cues, one for cues relating to an object's appearance and perhaps others for various types of cues.
There are, moreover, some indications that the human prefrontal cortex may be organized along similar domain-specific lines. A PET study reported last year by Susan M. Courtney, Leslie G. Ungerleider and their colleagues at the National Institute of Mental Health found that in humans, as in the monkeys studied earlier by Goldman-Rakic, certain brain areas are especially active during exercises that challenge working memory for visual details and for locations. Moreover, the most active brain regions lie in similar relative positions in both species.
Goldman-Rakic's proposal about the organization of the prefrontal cortex argues against the standard view of the various components of working memory. The British psychologist Alan Baddely proposed in 1974 that working memory has a hierarchical structure, in which an "executive system" in the prefrontal cortex allocates processing resources to separate "slave" buffers for verbal and spatial information. The memory buffers were supposed to be well behind the prefrontal cortex. But Goldman-Rakic is unconvinced that the brain's executive processes are confined to any particular location. Moreover, in the traditional model, memories organized by domain would lie somewhere behind the prefrontal cortex, not within it.
The high-speed imaging capability of functional MRI is now able to help resolve the question. A study that Courtney and Ungerleider and their colleagues published in April in Nature pinpoints the part of the brain that is liveliest while working memory holds an image of a face. That region—the middle part of the prefrontal cortex—has been fingered as the crux of working memory in a variety of studies.
Yet the face-recognition task Courtney and company used does not involve any obviously executive functions, Ungerleider notes. Their findings thus contradict the view that only executive functions reside within the prefrontal cortex, but they do fit with Goldman-Rakic's scheme. Similarly, Jonathan D. Cohen of Carnegie Mellon University and his co-workers found a region of the prefrontal cortex partly overlapping the one identified by Courtney that is active while subjects remember letters seen in a sequence. The more the subjects had to remember in the Cohen experiment, the more active their prefrontal regions. So Cohen's result also suggests that working memories are actually stored, in part, in the prefrontal cortex. Domain-specific organization "is the dominant view" of the prefrontal cortex, Wise says.
Wise himself does not subscribe to that dominant view, however. He points, for example, to a study reported in Science in May by Miller and his associates at the Massachusetts Institute of Technology. The researchers recorded from neurons in the prefrontal cortex of monkeys while they solved delayed-choice tasks that required them to remember information about both the appearance and spatial locations of objects. Over half the neurons from which Miller recorded were sensitive to both attributes, a result not expected if domain-specific organization prevails. "It argues against Goldman-Rakic's view that identity and location are processed in different parts of the prefrontal cortex," Miller says.
Goldman-Rakic responds that she and her colleagues have recently found hundreds of cells in part of the prefrontal cortex that respond selectively even in untrained animals to objects or faces—further evidence, she asserts, that the information in that area is organized in part by sensory domain. "We do feel the evidence is overwhelming that the functions of neurons in the prefrontal cortex are dictated in large part by the neurons' sensory inputs," she says. Moreover, Goldman-Rakic believes technical problems cast doubt on Miller's experiment. She maintains the targets he used were too close to the center of the visual field, which could produce spurious firings.
Michael Petrides of McGill University, another leading figure in the field, has mounted a different challenge to the standard view. Petrides's studies point to two distinct levels of processing, both within the prefrontal cortex. In his view the levels are distinguished primarily not by whether they maintain information about place or objects, as Goldman-Rakic holds, but rather by the abstractness of the processing they perform. The lower level in the hierarchy—physically lower in the brain as well as conceptually lower—retrieves data from long-term memory storage elsewhere. The higher "dorsolateral" level, in contrast, monitors the brain's processes and enables it to keep track of multiple events. This higher monitoring level is called on when subjects are asked, for example, to articulate a random list of each number from 1 to 10, with no repetition: a subject has to remember each digit already chosen.
Petrides finds that both humans and monkeys with lesions in the dorsolateral part of the prefrontal cortex are crippled in their ability to monitor their own mental processes: they perform badly on special tests he has devised that require subjects to remember their earlier responses during the test. He also cites PET studies of healthy humans that find heightened activity in the same region when subjects are performing the tasks he uses. The finding is the same whether the tasks involve spatial cues or not. "The material does not seem to matter—the process is crucial," Petrides says.
Other researchers have found evidence to support the notion that the higher parts of the prefrontal cortex are key for self-monitoring. In an experiment by Mark D'Esposito and his associates at the University of Pennsylvania, volunteers performed either one or both of two tasks that, separately, did not require working memory. One task required subjects to say which words in a list read aloud were the names of vegetables, whereas the other asked them to match a feature of a geometric figure seen in different orientations. Functional MRI showed that the dorsoventral prefrontal cortex became active only when subjects attempted both tasks simultaneously. And in April at a meeting of the Cognitive Neuroscience Society, D'Esposito presented a meta-analysis of 25 different neuroimaging studies. The analysis supported Petrides's general notion that tasks involving more computation involve higher regions of the prefrontal cortex. "It was amazing that this came out," D'Esposito says.
D'Esposito's analysis also confirmed earlier indications that humans, far more than monkeys, represent different types of information in different halves of the brain. The meta-analysis did not, however, detect the upper/lower distinction between spatial and object working memory that Goldman-Rakic espouses.
Asymmetry of the human hemispheres is becoming apparent to other researchers as well. John D. E. Gabrieli and his colleagues at Stanford University have used functional MRI to study the brains of volunteers who were solving pictorial puzzles such as those often found on intelligence tests. The puzzles were of three types. One group was trivial, requiring the subject simply to select a symbol identical to a sample. A second group was a little harder: people had to select a figure with a combination of features that was absent from an array of sample figures. The third group contained more taxing problems that required analytical reasoning.
Gabrieli's study sheds some light on the debate over the organization of the prefrontal cortex. When volunteers pondered the intermediate class of tasks, which most resembled the tasks other investigators have used when studying working memory, the right side of the higher part of the prefrontal cortex was prominently active. Moreover, the activity was in areas that other researchers have found to be used when cues about spatial location are stored. This result fits Goldman-Rakic's idea that working memory for spatial location is stored in the higher regions of the prefrontal cortex, because these intermediate tasks all demanded that subjects visualize features in different locations.
When the volunteers in Gabrieli's experiment worked on the hard problems, however, the prefrontal cortices of the subjects became even more active, on the left as well as the right side. The added complexity produced a pattern of activation like that Petrides has found during his tests of self-monitoring.
Gabrieli's data thus provide some support for Petrides's theory of a higher executive level in the prefrontal cortex, as well as for Goldman-Rakic's view that domain-specific regions exist there. "There are definitely domain-specific places," Gabrieli says. "And there are others that rise above that." In other words, both sides in the debate over domain-specific organization of the prefrontal cortex may have a point. Yet in June, Matthew F. S. Rushworth of the University of Oxford and his colleagues reported in the Journal of Neuroscience that monkeys with large lesions in their lower prefrontal cortex could still perform well on delayed-choice tests. The finding casts new doubt on the theory that object working memory resides there and seems to support Petrides.
It may take years before the outstanding questions about the prefrontal cortex are settled and the operation of the brain's executive functions are pinned down to everyone's satisfaction. "If you put a theory out, people will attack it," Goldman-Rakic muses. "Everyone is contributing." And the modus operandi of the brain's decision-making apparatus is slowly becoming visible. "We are getting," Goldman-Rakic observes, "to the point where we can understand the cellular basis of cognition."
Source: Reprinted with permission. Copyright © August 1997 by Scientific American, Inc. All rights reserved.