* A fifth-grader as she labors over her homework.
* A young man battling the demons of schizophrenia.
* An elderly man who keeps forgetting where he parked his car.
``For the first time, we can see the thinking, working, living human brain,'' said Steven Hyman, director of the National Institute of Mental Health, at a conference of 25,000 neuroscientists in Washington last month. ``We can talk about perception and intention - matters that could only be dealt with by philosophers in the past.''
The unfolding knowledge of the brain not only adds to scientists' understanding, but also makes it possible for researchers to develop treatments for afflictions such as strokes, mental illness and memory loss.
To their surprise, scientists have recently discovered that the physical structure of the brain changes as it responds to the environment and experience. Certain sets of brain cells, known as neurons, grow or shrink. New threadlike connections between neurons are established or old ones strengthened.
``It's as if there were little gremlins running around rewiring the circuits all the time,'' Hyman said.
The wiring of the brain is far more complex than that of the most powerful supercomputer. A normal adult has billions of neurons and many more supporting cells in his or her head. Each neuron may connect to more than 1,000 others.
``The magnitude of the task involved in wiring the nervous system is staggering,'' said Corey Goodman, a biologist at the University of California in Berkeley.
Here are some of the latest findings discussed at the conference of the Society for Neuroscience:
Vision. People have two main vision systems, according to Francis Crick, who became a neuroscientist 20 years ago after discovering the structure of DNA with James Watson in 1953.
One is a low-level, unconscious system that feeds visual signals directly from the eye to the motor systems of the brain, causing rapid, automatic actions such as reaching or flinching. ``Part of us behaves like a zombie,'' Crick said.
The other system is conscious seeing, in which higher levels of the brain take time to interpret primitive signals from the retina. Specific regions handle specific tasks, such as recognizing shapes, colors and motions. Others combine the patterns and extract their meaning.
``If you bounce a red ball, some visual systems see red, some see the ball, some see bouncing, and the brain puts it all together,'' said Thomas Carew, a specialist in learning and memory at Yale University.
Taste and smell. Taste and smell are entirely different, said Sue Kinnamon, a neuroscientist at Colorado State University in Fort Collins.
The taste buds in the tongue and throat can distinguish only four tastes - sweet, salt, bitter and sour - because that was all that our primitive ancestors needed to know.
Foods that tasted sweet or salty were good to eat, Kinnamon said. But a sour or acid taste meant the food was spoiled, and bitterness warned of toxins.
The nose is far more sophisticated than the tongue. Researchers have identified about 1,000 genes controlling neurons that respond to various smells.
``Some cells respond to jasmine, others to rose,'' said Richard Axel, a Columbia University neuroscientist. ``The perception of specific odors leads to appropriate thoughts, memories and behaviors.''
People recognize most flavors, such as chocolate, vanilla or cherry ice cream, mainly by smell, not taste.
Learning and memory. Neuroscientists distinguish two separate memory systems. The ``what'' memory holds facts, such as names, faces and events. The ``how-to'' memory preserves skills, such as reading or riding a bike.
Memories of both types are stored first in a short-term ``scratch-pad'' behind the forehead, where they remain for a few seconds or minutes. Later, some memories are converted to long-term storage, lasting days, months or years. ``What'' memories tend to fade, but ``how-to'' memories can last a lifetime.
Electronic PET scans show that, unlike the hardware in a computer, the brain is constantly changing as it learns and remembers.
The conversion of short-term into long-term memories requires the growth of new connections between neurons - ``an actual anatomical change in the brain,'' said Eric Kandel, a former president of the Society for Neuroscience.
For example, Leslie Ungerleider, a researcher at the National Institute of Mental Health, trained volunteers to pick out a tiny pattern against a confusing background on a computer screen. As the participants gained skill, a small section of their brains expanded.
The training ``recruited additional neurons'' into the network that handles visual processing, she said.
``Learning can create a new brain area, as well as modify an old one,'' said Martha Farah, a psychologist at the University of Pennsylvania. ``The size of a brain area grows or shrinks, depending on experience.''
Consciousness. Their new tools are finally allowing researchers to approach one of the ultimate human mysteries - the nature of consciousness or self-awareness. Lower animals can see, hear and smell, but none of them, with the possible exception of chimpanzees, are conscious of their individual existence.
``Philosophers worried about consciousness for 2,000 years with little success,'' Crick said. ``Until recently, it was considered a form of bad taste even to mention it among scientists.''
But now, ``consciousness can be studied at the level of wiring and synapses [connections] in the brain,'' said Pasko Radic, president of the Society for Neuroscience. ``We couldn't do that five years ago.''
Radic, a professor at the Yale School of Medicine, added, ``You can't understand the brain just by looking at its anatomy, genes or chemistry. You have to put it all together.''
Even human traits such as loyalty and imagination have ``a biological basis,'' according to Zach Hall, director of the National Institute of Neurological Disorders and Stroke.
The hatreds dividing the peoples of Rwanda, Bosnia and Northern Ireland may show that humans have inherited ``a biological need to divide the world into `them' and `us,' '' Hall said. ``We need to understand that so that we can perhaps modulate it.''
Mental disease. Researchers are applying new insights to treating brain disorders such as strokes, schizophrenia, Alzheimer's and Parkinson's diseases.
Until recently, Radic said, schizophrenia was considered a purely psychological disorder. But now it is known to have biological roots, such as an excess of certain chemicals in the brain.
There is still no cure for Alzheimer's, which destroys the memories of millions of aging people. ``A few years ago, we couldn't even discuss it intelligently,'' Radic said. ``Now we know at least four of the genes involved.''
Memory loss is becoming a major public health problem as the population ages.
``It's very likely that in the next five to 10 years, we'll have pills to help with age-related memory loss,'' Kandel said. Encouraging reports of two such memory-improving substances - ampakine and estrogen - were presented at the conference.
``Until recently, there was very little we could do for people with brain disease,'' Hall said. ``Now we have a treatment for stroke, and it's just the tip of the iceberg. Other treatments are coming along. We are making astonishing progress.''