Dr Shock is skeptical about the possibilities of Transcranial Magnetic Stimulation (TMS) influencing the neural networks involved with depression. TMS can influence the cortex on the surface and probably a few centimeters beyond. In research TMS has been used to disrupt neural activity experimentally in studies of human cognition but relatively little is known about how TMS works.
Effects of TMS on brain functions:
1.Single burst TMS instead of repetitive TMS applied to the visual cortex interferes with visual imaging (repeated stimulation=rTMS has a longer lasting effect than a sinle burst)
2. Stimulation of the right spot in the motor cortex causes the tumb to twitch (intensity calibration of the TMS stimulus, this is used before repetitive stimulation to other parts of the scalp (rTMS))
3. Low frequency stimulation (1-5 pulses per second) depress brain activity, higher frequency stimulation (25 or more pulses per second) increases excitability.
4. Stimulation of the left prefrontal cortex with rTMS relieves depression in some patients who haven't responded to other antidepressant treatments
5. Low frequency rTMS to the temporoparietal cortex (above the ear) reduces auditory hallucinations in Schizophrenia
6. rTMS treatment on speech areas of the brain can make people name picture at a faster rate
7. rTMS applied to motor regions of the brain facilitates lightening fast movements
8. rTMS applied to the prefrontal cortex compared to sham rTMS enhances solving of analogy puzzles, rTMS might facilitate thinking. (Try rTMS before an exam). rTMS may raise baseline level of neural activity just enough so that neurons don't have to work hard to retrieve memory or problem solving strategies.
What is TMS?
rTMS influences neurons indirectly. It is a non invasive technique to stimulate brain tissue. Anesthesia is not required. Repeated pulses of electric current are sent through a metal wire, which is usually round or figure eight shaped. This electric current generates a perpendicular magnetic field. This magnetic field in return, generates another electric current in nearby material, in this case the current runs through brain tissue just below where the coil is placed on the scalp
How is it applied in Depression?
A depressed patients receives rTMS over the left prefrontal cortex for 20-30 minutes once a day for 2-4 weeks. It is unknowm if this is the right and effective combination of stimulation frequency, intensity, timing, and location.
In a recent article in Science: TMS elicits coupled neural and hemodynamic consequences, a more detailed look on how TMS might affect the neurons is presented.
TMS was applied to the cat visual cortex and the neural and hemodynamic consequences were evaluated.
Short TMS pulse train elicited initial activation (to 1 minute) and prolonged suppression (5-10 minutes). Oxygen concentration and hemoglobin levels were tracked simultaneously with the stimulation and the recordings. Oxygen consumption and hemoglobin are metabolic markers. Both mirrored the pattern of increase in firing (about 1 minute) by the neurons followed by a decrease in firing for several minutes after a stimulation by a TMS pulse train of a few seconds.
Neural activity elicited by flashes of white and black bars on a computer screen which can influence even anesthetized animals, had an altering effect on neural activation by TMS. Neural firing dipped sharply after TMS on the visual cortex and remained suppressed for several minutes during this challenge.
These findings can have important implications for the use of TMS in depressed patients:
The findings have implications for designing TMS therapies, says George. For depression therapy, for example, "we may need people to become sad in the chair while stimulating [them]," George says (Mark George, a psychiatrist at the Medical University of South Carolina in Charleston). "Alternatively, we might have them engage in formal cognitive therapy, thinking positive thoughts." Such considerations are important, he adds, as the Food and Drug Administration is considering approval for daily TMS of the prefrontal cortex to treat depression.
The new findings also suggest why the effects of TMS often vary, says Alvaro Pascual-Leone, a neurologist at Harvard Medical School in Boston. Pascual-Leone suggests that TMS results could be made more consistent by monitoring the physiological state of the brain using electroencephalography or functional magnetic resonance imaging.
The picture at the top of this article is from a website about neurostimulation in all it's forms
Boosting Brain Activity From the Outside In
Science 18 May 2001:
Vol. 292. no. 5520, pp. 1284 - 1286
Uncovering the Magic in Magnetic Brain Stimulation
Science 28 September 2007:
Vol. 317. no. 5846, p. 1846
Transcranial Magnetic Stimulation Elicits Coupled Neural and Hemodynamic Consequences
Elena A. Allen,* Brian N. Pasley,* Thang Duong, Ralph D. Freeman
Science 28 September 2007:
Vol. 317. no. 5846, pp. 1918 - 1921