Repetitive transcranial magnetic stimulation (rTMS), which allows for direct activation of neurons, will play an ever-expanding role in depression and schizophrenia treatment, according to recent reports from Mark S. George, M.D., and Alan L. Schneider, M.D. George, who is distinguished professor of psychiatry, radiology and neurology, and director of the Brain Stimulation Laboratory at the Medical University of South Carolina College of Medicine, provided updates on rTMS research at the 2005 American Psychiatric Association annual meeting.
"We call this electrodeless electrical stimulation," George said at a symposium. "Electrical energy in a coil induces a magnetic field, and the field passes unimpeded through the skin and skull and induces an electrical current in the brain."
The physiological effects of TMS depend upon the site and frequency of stimulation (Ontario Ministry of Health and Long-Term Care, 2004). The frequency of cortical stimulation varies. Rapid-rate or repetitive TMS usually refers to the application of TMS for a train of minutes at frequencies >1 Hz and is commonly used in treatment studies. Transcranial magnetic stimulation at ≤1 Hz is referred to as slow or low-frequency TMS. The ability to stimulate the brain at either high or low frequency is important, because high-frequency rTMS (e.g., 20 Hz) may increase cerebral blood flow and neuronal excitability in the region of the cortex under the coil, but low-frequency rTMS (≤1 Hz) may have the opposite effect.
The magnetic pulse is further described by its intensity in proportion to the motor threshold (MT) of the individual. The motor threshold is the lowest intensity of stimulation that, when applied to the motor cortex, causes a standard contraction of a muscle in at least five of 10 consecutive trials.
While TMS devices can excite the surface cortex of the brain, George explained that blood-oxygen-level-dependent functional magnetic resonance imaging (BOLD fMRI) has shown that cortical stimulation causes trans-synaptic deeper effects. For example, 1-Hz TMS over the left prefrontal cortex was associated with increased activity at the site of stimulation (Li et al., 2004). Activity was also increased in connected limbic regions, including the bilateral middle prefrontal cortex, right orbital frontal cortex, left hippocampus, mediodorsal nucleus of the thalamus, bilateral putamen, pulvinar and insula (t=3.85, p
As a research tool, George described TMS as "a wonderful way to look into the brain, study neuropsychological effects and measure cortical excitability." Additionally, for brief periods during stimulation, rTMS can block or inhibit a brain function. He explained the concept further in an article for Scientific American (George, 2003). Applying rTMS over an individual's speech-control motor area, for instance, can make them temporarily unable to speak. This functional knockout capability (which immediately reverses) allows neuroscientists to reexplore which part of the brain controls a particular part of the body. When rTMS is coupled with imaging techniques, it allows researchers to noninvasively monitor circuit activity and behavior.