Cover Story

Advances in medical imaging allow neuroscientists to track "functional" changes in the brain--shifting patterns of blood flow or electrical activity as people perform learning tasks.

Over the next few years, scientists hope to combine magnetoencephalography (MEG), which provides excellent temporal resolution--"real time" brain activity--and functional magnetic resonance imaging (fMRI), which provides outstanding spatial resolution-pictures of where brain activity happens.

The combination could produce what some call "brain movies." Other scientists hope to perfect techniques that are less expensive and more portable than the cumbersome magnetic machines used today. Here's a rundown of the latest functional imaging technologies:

Positron Emission Tomography (PET)

Measures: Blood flow, metabolic reaction rate and biochemical synthesis.

Using: A positron emission tomograph that tracks positively charged electrons, or positrons, detected via a radioactive dye. After the dye is injected, a person lies flat and enters the tomograph head first.

Pluses: High-end spatial resolution

Minuses: Limited temporal resolution.

Functional Magnetic Resonance Imaging (fMRI)

Measures: Brain blood flow.

Using: A giant magnet that detects signals from protons in the brain while people lie still inside it. The signals indicate changes in blood oxygenation due to brain activity.

Pluses: Excellent spatial resolution over the entire brain.

Minuses: Limited temporal resolution, sensitive to movement.

Magnetic Resonance Spectroscopic Imaging (MRSI)

Measures: The brain's processing of specific chemicals during metabolism.

Using: A modified MRI scanner.

Pluses: Moderate spatial resolution, tracks chemicals used in specific types of learning tasks.

Minuses: Poor temporal resolution.

Magnetoencephalography (MEG)

Measures: Changes in weak magnetic fields tied to the brain's electrical currents.

Using: A helmet connected to Superconducting Quantum Interference Devices.

Pluses: Excellent, up to the millisecond temporal resolution over the whole brain.

Minuses: Limited spatial resolution, sensitive to noise.

Near-Infrared Imaging (nIR)

Measures: Brain blood flow.

Using: A sensor that monitors how infrared light interacts with oxygenated hemoglobin. The sensor is bound to a person's head and attached to a computer.

Pluses: Good temporal resolution, portable, relatively inexpensive, noninvasive. Doesn't require that people sit still.

Minuses: Poor spatial resolution, can image only one brain part at a time.

Event-Related Optical Signal (EROS)

Measures: Movement of photons through brain tissue.

Using: Near-infrared light and a helmet connected to a small, box-like computer.

Pluses: Noninvasive, relatively inexpensive, portable. Decent spatial and temporal resolution.

Minuses: Still in experimental phases, spatial resolution not as good as with fMRI. Looks only at localized brain areas.