Up to now, maps of neural connections in the human brain have been based on one of two methods. Researchers have either concentrated on findings from dissected animal brains, or have monitored blood flow in the brains of live humans using positron emission tomography (PET) scanning while people carry out specific tasks. In the latter case, researchers assume that the lit-up regions are connected.

But Tomá`´s Paus and colleagues from the Montreal Neurological Institute have developed a way of studying human brain circuitry directly. The researchers use transcranial magnetic stimulation (TMS) to stimulate small individual areas of the brain which are responsible for a known function. They can then use PET to look at the rest of the brain to see which other areas are lit up. The team is the first to combine TMS and PET in this way, having overcome what they see as the crucial problem of shielding the instruments from each other.

TMS uses a coil placed next to the scalp to generate magnetic impulses that are targeted at a specific brain region. It can be used to elicit a motor response or to deactivate a brain region, for instance in the potential treatment of depression or stuttering . But Paus and his team used very low intensities to induce brain activity without any other effects, and this was then picked up on a PET scan.

The researchers studied six healthy volunteers, directing between five and thirty 200-microsecond pulses of magnetic stimulation at a region of their brains called the frontal eye field. This is known to control eye movement in tasks such as reading. The PET scans showed activity not only in the frontal eye field, but in several parts of the visual cortex quite far away from it--regions known from animal work to be anatomically connected to the stimulation site. Such a correlation between the results shows that the technique works, the researchers claim in the latest Journal of Neuroscience (Vol 17, no 9, 1 May 1997, p 3178). 

Peter Fox, a neurologist at the University of Texas Health Science Center in San Antonio, is also enthusiastic about the technique. "Now we will really be able to define human connectivity," he says. Fox has a paper under review that corroborates the Montreal findings, on a brain region which controls movement of the thumb.

Paus is now hoping to use the technique to examine the role of brain wiring in schizophrenia. "Studies suggest that something goes wrong when the brain is developing that affects the linking up of neurons," he says. Faulty links between different regions could account for the disease. This technique may reveal whether brain wiring in schizophrenics differs from that in people not suffering from the condition by stimulating individual areas and comparing the connections to the rest of the brain.

In fact, any team carrying out brain activation studies should be interested in this method, says Paus. For instance, researchers are eager to know how the brains of people with perinatal brain injury rewire themselves, or how rehabilitation works after a stroke. Many people regain some use of, say, their arm, despite the region responsible for motor control of the limb being damaged. Combined TMS and PET would be able to highlight any new pathways created in the brain to avoid the damaged area.