Friday, March 30, 2007

A primer on, and reservations about, TMS - transcranial magnetic stimulation

O'Shea and Walsh offer a brief introduction to TMS, and its use in cognitive neuroscience:
As any schoolboy with a toolkit or a broken toy soon appreciates, to find out how a machine works you need to take it apart, and to put it back together again, you need to know how it works. The next lesson is that, no matter how hard you try, you always end up with a handful of leftover nuts and bolts. These remaining components can be informative: will your machine still work without them? The same logic applies to one approach to understanding human brain function: by investigating the effects of lesions in animals and accidental brain damage in humans we can ask which parts are necessary for specific functions. Over the past twenty years, it has become possible to interfere with human brain functions safely and reversibly, and to control when and where the interference is induced. The technique, known as transcranial magnetic stimulation (TMS), has become a mainstay of cognitive neuroscience.
A PDF verion can be downloaded HERE.

There is controversy over the ethics of using this techniques on humans, it can cause seizures or psychosis in some subjects. This is the subject of a letter to the editor in the March 23 issue of Science from Leslie Sargent Jones of the Univ. of S. Carolina:
When Science publishes research using healthy human subjects, one assumes there was minimal risk and/or vital clinical value. This does not appear to be the case for the work by D. Knoch and colleagues ("Diminishing reciprocal fairness by disrupting the right prefrontal cortex," Reports, 3 Nov. 2006, p. 829). Their results on the dorsolateral prefrontal cortex's role in judgments of fairness and self-interest are interesting, but they largely validated what was already suspected.

Experimental subjects received repetitive transcranial magnetic stimulation (rTMS) for 15 min to produce "suppression of activity in the stimulated brain region." The rTMS generated an electric maelstrom powerful enough to disrupt all activity for 7 min. Animal rTMS research (with overexposure as in LD50 drug toxicity studies) shows that anything studied (e.g., receptor levels) is modified. For rTMS in humans, known risks range from headaches to, more rarely, seizures or psychosis (1). Long-term occult changes and self-reported symptoms in healthy subjects have not been studied, and rTMS continues to be used for studies both fascinating and frivolous (just check the literature).

The use of rTMS on healthy subjects does not meet the definition of "minimal risk" (45 CFR section 46.102: risks… "not greater … than those … encountered in daily life"). We know that healthy subjects don't risk seizures or psychosis in their "daily life." What we don't know is what the residual effects of this activity-swamping tsunami of electrical current are. The Report demonstrates a naiveté about the possibility of rTMS having long-term or negative consequences. Oddly, some of these authors have used rTMS to treat neuropsychiatric disorders on the basis of its long-lasting effects (2). Roentgen's technology was also once thought harmless, and x-rays were used to check shoe sizes (3). We know better now.

  1. K. Machii, D. Cohen, C. Ramos-Estebanez, A. Pascual-Leone, Clin. Neurophysiol. 117, 455 (2006).
  2. C. M. Miller, Newsweek, "Minds and magnets," 11 Dec. 2006 (
  3. J. Duffin, C. R. R. Hayter, Isis 91, 260 (2000).

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