Until quite recently, anatomists desiring a peek inside the human brain had to content themselves with dissecting dead tissue or, when the rare opportunity arose, examining people who had traumatic skull injuries. Only in the past few decades has it been possible to glimpse, in real time, the working brain in action. First came computerized tomography or CT scans, which use a series of X-rays to create a three-dimensional picture. Next, positron emission tomography, or PET scanning, revealed blood flow and metabolism by tracing the path of an injected radioactive chemical. Magnetic resonance imaging (MRI), now gaining the edge in brain research, produces detailed views of both anatomy and brain activity by agitating the body's hydrogen atoms using magnetic fields. Result: a sudden wealth of clues about everything from the mechanisms behind mental illnesses to what happens when a memory is triggered or a skill is learned.
[Learn about more new developments in Secrets of Your Brain.]
"The details, shapes, and patterns in the brain are exquisite on aesthetic grounds alone. But then you realize this is the machinery of thought," marvels Carl Schoonover, a doctoral candidate in neuroscience at Columbia University who was so inspired by the imagery now possible that he created a coffee-table book, Portraits of the Mind, to give it center stage.
For all that these technologies promise to reveal, equally remarkable is the boost they already give physicians in practice. Victims of severe concussions are imaged quickly to pinpoint internal bleeding. Stroke patients are scanned to gauge whether clot-busting drugs will be effective. And surgeons can locate and map their way around territories vital to language, vision, and motor skills before plunging in the knife. Imaging is especially important for doctors working with the brain, says Joseph A. Helpern, professor and vice chairman for research in radiology at the Medical University of South Carolina in Charleston. "You don't want to have to cut into it to find out what's going on."
Even the lowly microscope, used to study tissue sliced out of an animal brain or a human brain after autopsy, has been making significant new contributions. The problem with microscopes that use light (remember your high school's equipment?) is that numerous structures, including many all-important synapses where neurons communicate, are smaller than a light wave and thus invisible. But treating the samples with dyes that can be switched on or off by pulses of light allows scientists to view ever-tinier structures. And microscopes relying on much smaller electron waves to magnify and illuminate brain tissue are powerful enough now to reveal even the minuscule spines, thought to store memory, that protrude from the tentacle-like dendrites of a single neuron.
One cutting-edge microscope can even view those spines in a living animal. New York University School of Medicine scientists recently used this "two-photon" microscope, which employs infrared light and fluorescent dyes, to probe the brains of mice before and after challenging them by, for example, changing something in their cages or speeding up their walkway. During the course of several weeks, the microscope revealed a correlation between these experiences and the creation of new spines. "We're documenting how daily sensory experiences leave minute but permanent marks in the brain," says study coauthor Guang Yang, assistant professor of anesthesiology.
It's the study of the living human brain, however, that has seen the most exhilarating leaps. In some cases, older technologies are offering up new secrets; PET scans, for instance, capture neurons reacting when a person is under stress, and electrodes inserted into the brain reveal which neurons fire when a person thinks certain thoughts. But many of the latest findings hail from two advances in magnetic resonance imaging, "functional" and "diffusion" MRIs.
Back in the early 1990s, scientists realized that MRI magnets could be used to get images of where blood and nutrients rush when an area of the brain becomes engaged—when, say, a person is instructed to think a certain thought. These functional MRIs can then be compared to pictures taken minutes earlier. "I refer to functional MRI as a 'mindoscope,' because it allows us to connect the intangibles of conscious experience with the structure and function in the brain," says Joy Hirsch, who directs the Functional MRI Research Laboratory at Columbia University Medical Center in New York and helped curate "Brain: The Inside Story," an exhibit running through August 2011 at the city's American Museum of Natural History. While fMRIs are imperfect instruments, not least because blood takes a while to respond whereas neurons fire in a flash, the fact that subjects don't need to be cut into or injected with radioactive substances has made them the tool of choice for many researchers.