Scientists use many methods to study the brain's structure and function. They take pictures of healthy brains and compare them to diseased brains. In addition, they examine brains taken from humans, primates and small mammals and try to understand how invertebrates' smaller nervous systems work. On a microscopic level, they also examine neurons.
Here are some tools used in brain mapping. These techniques take images of the brain:
- Computer axial tomography (CAT) scan X-rays the brain from many angles and show structural abnormalities.
- Structural magnetic resonance imaging takes advantage of water in the brain to create images with better resolution than a CAT scan.
- Diffusion tensor-MRI (DTI) images "tracts" of neurons that connect brain regions by following water movement in the brain.
These techniques examine brain activity:
- Electroencephalography (EEG) indicates electrically active locations in the brain using detectors implanted in the brain or worn on a cap.
- Positron emission tomography (PET) takes images of radioactive markers in the brain.
- Functional MRI (fMRI) shows images of brain activity while subjects work on various tasks.
- Pharmacological functional MRI (phMRI) shows brain activity as drugs are administered.
- Transcranial magnetic stimulation (TMS) noninvasively stimulates parts of the brain to trigger certain behaviors.
New methods allow researchers to see all the connections between neurons in an intact brain. This branch of study is called connectomics. The "wiring diagram" of a brain is called a connectome [source: Lichtman]. "Until recently, we've had no hope of getting these wiring diagrams," says Jeff Lichtman, a Harvard biologist who led the group that developed some of the new techniques. "We could see individual cells, but never all of them at once."
One such technique, known as Brainbow, labels every neuron in a live animal's brain a different color. By generating images of the animal's brain, scientists can see where and how neurons connect to each other. As the animal grows and ages, they can also watch how the neurons change connections.
Another technique uses the ATLUM, or automatic tape-collecting lathe ultramicrotome. This machine reads the wiring diagram of a brain. "We do something akin to paring an apple," explains Lichtman. "We essentially shave off a spiral cut as we rotate the brain on a lathe and put this ribbon of tissue onto a tape. We'll eventually get a hugely long tape, which is essentially the whole brain. Using an electron microscope, we will image that to see the structure of the wiring."
So far, Brainbow and the ATLUM are being used only to study animals with relatively small brains, like mice.
So, what's the point? What, if anything, can mapping accomplish? Learn what we can learn from mapping the human brain on the next page.