Messages from the senses travel so swiftly through the brain that imaging machines such as PET and fMRI cannot keep up with them.
To track these messages in real time, scientists now use faster methodselectrical recording techniques such as MEG (magnetoencephalography) or EEG (electroencephalography).
These techniques rely on large arrays of sensors or electrodes that are placed harmlessly on the scalp to record the firing of brain cells almost instantaneously. Their data may then be combined with anatomical information obtained by structural MRI scans.
One of the first experiments in which structural MRI was used jointly with MEG produced a three-dimensional map of the areas of the brain that are activated by touching the five fingers of one hand.
A New York University research team headed by Rodolfo Llinás found this map to be distorted in the brain of a patient who had two webbed fingers since birth. A few weeks after the man's fingers were separated by surgery, however, parts of his brain reorganized and the map became almost normal.
The next generation of imaging technology will use functional MRI (fMRI) in various combinations with MEG and EEG, predicts John Belliveau, director of cognitive neuroimaging at the Massachusetts General Hospital in Cambridge.
Functional MRI shows activity deep in the brain with high spatial resolution. It is relatively slow, however, since it is based on the blood-flow response, which takes about 450 milliseconds.
"If you do a visual stimulation experiment, four to five different areas may have turned on within that time," Belliveau says. "We know where those areas are, but we don't know which one turned on first."
By contrast, EEG's spatial resolution is relatively poor, but because of its speed it may reveal the sequence of events. His group has already done some EEG recordings right inside the magnet of an fMRI machine, to get simultaneous measurements.
Together, such techniques will offer scientists a glimpse of how information from the senses is processed in different parts of the brain.
Building on the studies shown here, the new hybrids may then begin to tackle neural networks. They may help researchers examine how various parts of the brain exchange information andmost intriguinghow sensory information leads to thought.
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