MAGNETOENCEPHALOGRAPHY ( MEG ) also relies on magnetism to examine the brain. In this case, it’s the body’s ambient magnetic fields, not those generated by an external machine, that form the basis of brain imaging. These magnetic fields are extremely weak-perhaps only a billionth of the power that causes a compass needle to point toward the north magnetic pole. Yet, when read by sensors placed on the skull, MEG scans reveal the electrical currents created by neural discharges. The resolution is as fine as a thousandth of a second and as small as a cubic centimeter. The MEG scan and EEG are the only observational techniques capable of anything approaching real-time revelations. When a patient thinks a specific thought, it shows up, in progress, on an MEG.

Mental functions also can be localized with a technique called positron-emission tomography, or PET. A radioactive isotope is injected into a patient. Because all radioactive atoms decay into stable atoms at a known rate, the decay of the isotope, which is usually paired with glucose, is recorded and turned into images with computer programs. Like MRI and CT scans, PET scans let observers localize activity inside the brain.

The array of brain-imaging techniques serves like the variety of hammers, saws, and other tools in a mechanic’s toolbox. A scientist observing the brain chooses the right tool based on what kind of information is needed. A CT or MRl scan would be the choice if a doctor suspects the growth of a tumor or physical damage to part of the cerebrum. A PET scan might be the appropriate choice for investigation of deficiencies associated with language or reason. And lack of oxygen use in stroke- damaged sections of a brain would call for a functional MRI.

A patient receives a PET scan to pinpoint regions of the brain that are most active.

Seeing Thoughts
A patient receives a PET scan to pinpoint regions of the brain that are most active.

True to the rational and observational methods of Descartes and Willis, science has made great strides in describing how the brain’s parts, both large and small, function. But understanding any organ that is “wider than the sky” is not as easy as toting up small pieces of information. The brain is an integrated unit, with its complexity arising from the synergy created by the simultaneous functioning of its billions of neurons and trillions of synapses in nonlinear ways. Science has learned much about movement, sensations, emotions, and the sense of self. Yet much is yet to be gleaned about the most complicated object in the universe. There will always be more to learn about the brain.

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