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One late morning in a laboratory at the Yale School of Medicine, two neurologists, Sally and Bennett Shaywitz, are doing just that: watching the activity inside the brain of an engaging eleven-year-old boy named Keith. Through a plate-glass window, I can see Keith lying on his back with his head in the circular scope of an MRI scanner. He's reading a series of paired cues through a periscope—a word and picture simultaneously flashed on a screen ("fox" and the image of a box, "cow" and a bow)—then quickly pressing the yes or no button on a button box to indicate whether or not the two rhyme.

The Shaywitzes are probing the circuits in the brain involved in reading. Right now they are hovering over two computer screens, one displaying the set of constantly changing reading cues supplied to Keith; the other showing a monochrome image of his brain in cross section. The scanning results in structural images, revealing the finest details of brain anatomy, and also functional images, showing the location of the brain's activity.

An MRI scanner is safe and noninvasive, requiring no radiation or injections. A massive and powerful circular magnet, it looks, as Keith says, like a spaceship or a doughnut filled with milk. MRI scans can provide a detailed anatomical picture of the brain with a resolution of less than half a millimeter, explains Sally Shaywitz, fine enough to detect an artery the diameter of a hair deep in the center of the brain.

As Keith reads his series of cues, computers are also compiling data on the neurons activated in his brain. Functional MRI reveals brain regions active during the performance of specific tasks by monitoring the changes in oxygen and blood flow that accompany neural activity. The harder a particular region of the brain works, the more blood-borne oxygenated hemoglobin moves into the area. A "blush" of this hemoglobin registers on the MRI scan as a slight rise in signal strength. In this way, the scan produces pictures of the cell circuits that fire as we engage in a particular mental activity. After the data are compiled, the result is a series of color photographs showing different brain areas "lit up" in a rainbow of hues, a kind of moment-to-moment map of neural activity.

Neuroimaging is not without its critics, in part because of the time scale of the technology. Functional MRIs take pictures on a scale of seconds; neural firing occurs on a scale of milliseconds. Furthermore, the activity that shows up in an fMRI is not necessarily causal. The scans show which regions are active during cognitive tasks but not necessarily which are essential to that task.

Still, says Sally Shaywitz, "functional neuroimaging has revolutionized the way we see the working brain. It can take a hidden function—and dysfunction—and make it visible." Such studies have put to rest the myth that we use only a small portion of our gray mass, the proverbial 10 percent. In fact, most of the neuronal nooks and crannies of the whole buzzing, blooming thing are fired up in the course of a day—though not all at once. Different groves of neurons erupt into activity at different times, with different tasks. Scans have captured the brain in action, navigating, calculating, grasping language, recognizing faces and places, perceiving time, reading verbs.

Imaging studies by the Shaywitzes and others have pointed to very specific neural areas that grow active in reading. Among these are a phonological region in the rear brain, just above and behind the ear, used by novice readers such as my student Bryan to sound out words phoneme by phoneme; and behind this, a so-called word-form area in the occipitotemporal region of the rear brain, which allows the expert reader to recognize whole words extremely rapidly, in less than 150 milliseconds. As learning readers move from novice to pro, they shift from using their phonological region to relying primarily on their expert word-form area.

It is this skilled circuit that flashes with activity as you pore over your work. The occipitotemporal region also grows active in the brains of car experts as they look at various makes and models of classic cars, and in ornithologists when distinguishing between different species of warblers. In fact, this rear-brain area may be important for expertise of all sorts, says Bennett Shaywitz. "It seems to be good for learning expert tasks, for getting better and better at something."

***

You hope your rear-brain expertise circuits have kicked in during your morning work and that you are now primed for your presentation. Your meeting has begun, and you're feeling sharp and on your toes. Late morning is a peak time for certain kinds of mental activity, according to some chronobiologists. Studies show that alertness and memory, the ability to think clearly and to learn, can vary by between 15 and 30 percent over the course of a day. Most of us are sharpest some two and a half to four hours after waking. For early risers, then, concentration tends to peak between 10
A.M.
and noontime, along with logical reasoning and the ability to solve complex problems.

Much depends on age, however. For teens and young adults, morning may feel a far cry from Rilke's "bright new page." Mary Carskadon, a chronobiologist at Brown University, has documented in longitudinal studies the physiological change in the body's biological clock during the adolescent years. Older teens shift toward a more owlish, or phase-delayed, pattern, secreting the hormone melatonin later in the evening and delaying their bedtimes. Yet they're forced to wake up early for the start of school. "Requiring older adolescents to attend school and attempt to take part in intellectually meaningful endeavors in the early morning may be biologically inappropriate," says Carskadon. Not only are these teenagers sleep deprived, "but they're being asked to be awake when the circadian system is in its nocturnal mode. The students may be in school, but their brains are at home on their pillows."

The relationship between the circadian rhythms of the body and mental performance is subtle and still a matter of debate. How well you do at a given mental task may be affected by a host of variables—boredom, distraction, stress, how confident you feel, how much sleep you got the night before, what you ate for breakfast, whether you had caffeine, your posture, the ambient temperature, air quality, noise, lighting, and other "masking" factors that have little to do with your circadian rhythms. "Time-of-day effects are intriguing but controversial," says Tim Salthouse, because they're difficult to isolate and replicate in scientific studies.

Still, there's evidence to suggest that the daily ups and downs of body temperature influence mental performance, with predictable peaks and troughs. Some studies have shown that the function of neurons is affected by brain temperature: Higher temperatures may result in faster transmission of impulses between neurons. Scientists at the University of Pittsburgh tested young adults over a thirty-six-hour period, taking their temperature every minute and measuring their performance every hour on various tasks of speed, accuracy, reasoning, and dexterity. The team found a significant time-of-day variation, with a nocturnal trough in performance close to the lowest body-temperature readings. On the flip side, researchers at Harvard reported finding a correlation between higher body temperatures and peak performance in alertness, visual attention, memory, and reaction time.

Two mental functions may be particularly susceptible to subtle circadian variations, according to Lynn Hasher of the University of Toronto and her colleague Cynthia May of the College of Charleston: decision-making and "inhibition"—the capacity to suppress distracting, irrelevant, or off-task information (such as the verbal content of those color words in the Stroop test). At "off-peak" times, people are more likely to have trouble suppressing distractions and to fall back on accessible, familiar decision-making routes rather than ones that demand analysis and evaluation. Work by Hasher and May suggests that these more subtle circadian effects vary with age. Young adults "are clearly bothered by distraction in the morning," say the researchers, "but, later in the afternoon, it is as if the distraction were invisible to them. The data for older adults show the opposite pattern."

Because inhibition is particularly difficult at one's "off' times, May recommends that people restrict to their peak hours those tasks requiring "focused attention (e.g., reading complex instructions), retrieval of exact information (e.g., recalling medication dosages), or careful control over responses (e.g., driving in heavy traffic)," or at least try to complete them in a setting with minimal distractions. On the other hand, as May points out, there may be some advantages to low inhibition. In tasks requiring creative problem-solving, less inhibition may allow people to consider more imaginative solutions.

Memory, too, may fluctuate with time of day. According to Hasher's work, older adults tend to experience what she calls "a substantial increase in forgetting across the day": Mornings, they forget an average of five facts; afternoons, about fourteen. The reverse is true for young adults.

In the past few years, scientists have begun to trace the role of circadian rhythms in learning and memory right down to the level of molecules, all with the help of a giant snail,
Aplysia californica.
If perchance you tried to master the material you needed for your meeting by staying up until the wee hours of the morning, performed well in the presentation, but then later found that your memory was vague on what you had learned, you're in good gastropod company.

Why
Aplysia
? "It may not be a beautiful animal," says Eric Kandel, "but it's extremely intelligent and accomplished, with the largest nerve cells in the animal kingdom." A Nobel Prize-winning neurobiologist at Columbia University, Kandel has seen firsthand what the humble sea snail can tell us about what's happening in the brain when we glean new knowledge from our reading or absorb a lesson from a colleague or teacher.

"We humans are what we are because of what we learn and remember," says Kandel, "and in a way it's mind-boggling that we know what we know about what changes in the brain when we learn something new—how our minds are different at the beginning of a learning experience than they are at the end—because of studies of
Aplysia.
"

Kandel has been fascinated by the puzzle of learning and memory for more than a half century. Born in Vienna in 1929, he grew up in a morass of barbaric human behavior. He was taunted for being a Jew, watched his father seized by police, and at the age of nine witnessed the horror of
Kristallnacht,
which he remembers, he says, with a kind of flashbulb memory, "almost as if it were yesterday." In 1939 he and his family fled Vienna. Kandel spent the rest of his life asking questions about the nature of the mind: why people behave the way they do, how they hold on to the memories that shape them, and above all, how they learn. He believed that insight into the nature of our own being could arise through the study of lower organisms.

Indeed, from the nerve language of
Aplysia
Kandel garnered one of the great secrets of the human mind: Learning results from changes in the strength of the synapses, or junctures, between two precisely interconnected brain cells. In creating short-term memory, the brain strengthens already existing synaptic connections by modifying pre-existing proteins. In making long-term memories, it makes new proteins and grows new synaptic connections.

Though the process may be a good deal more complicated in humans than in marine snails, says Kandel, it appears to involve a similar set of mechanisms. At a much simplified level, it may go something like this: At any moment, your brain is alive with firings. A single neuron receives a stimulus and fires here, triggering another neuron to fire there. Much of the time nothing comes of the activity. The chemical message one neuron sends to its neighbor may be too weak or sporadic to set the neighbor alight and form a network. But when the mind is focused and attending, as it is during learning, that single neuron may send more frequent, stronger messages to its neighbor. The synapse on the neighboring neuron is then chemically altered by the exchange. If the first cell fires again, even weakly, it may trigger a synchronous response in the now more receptive second cell. This leaves both cells aroused and ready to fire again in the same pattern.

The upshot of all this may be merely a transient notion, flashed briefly across the mind, a memory trace that lasts barely a few seconds before it passes into oblivion. But if the stimulus is repeated, and the neurons continue to fire in synchrony, the synapses between them will strengthen. Eventually they are bonded, so that when one fires, the other does too. This process of bonding between neurons, known as synaptic plasticity, may be what underlies both learning and memory.

Once the process has taken place, Kandel's theory goes, signals blaze more easily along the pathway between the neurons, and the same signals produce larger responses. If the activity is repeated—the remembering of a word or concept or skill—the linking and lock-step firing continues and spreads to other neurons, forming a network of well-bound neurons, which fire together in the same pattern each time they are activated. The process draws together neurons involved in an event or idea. Hence the expression "Cells that fire together, wire together." With each repetition of a skill or activity, with each additional firing of the circuit, the synapses become more efficient and the learning more permanent.

"Practice makes perfect," says Kandel, "even in snails."

Aplysia
is in the learning limelight again, this time for what the gastropod tells us about circadian effects on learning and memory. In 2005, researchers at the University of Houston reported finding that the snail suffers forgetfulness when it pulls all-nighters. Like us,
Aplysia
is a diurnal creature that prefers daytime life. To probe the influence of circadian rhythms on its patterns of learning, the team examined its ability to absorb and remember lessons about noxious substances and inedible food. The study showed that
Aplysia
forms short-term memory of lessons equally well during day and night, but builds long-term memories only when it's trained during the day. At night, say the researchers, the biological clock seems to shut off the proteins involved in forming long-term memories—a lesson perhaps worth remembering.