#692-T-PM
Sham TMS – A New Approach
Jens Sommer12, Andreas Jansen12,
Michael Deppe1, Caterina Breitenstein12, Stefan Knecht12
1 Dept.
of Neurology, University of Münster, Germany
2 IZKF, Münster, Germany
Introduction: Transcranial magnetic stimulation (TMS) is a means to stimulate the human cortex non-invasively. One of the
major concerns in TMS experiments is the lack of a good sham (placebo) condition to demonstrate the specificity of the effect.
Loo et al. [1] and Lisanby et al. [2] defined the following criteria for a good sham TMS:
1. Stimulation of the cortex should be avoided for sham stimulation,
2. identical coil positions,
3. identical acoustic artifacts, and
4. identical scalp sensations should be attained for sham and verum stimulation.
Here we present an economic solution to maximally fulfill these criteria.
criterion
cortex stimulation
Ident. coil positions
Ident. Acoustic artifacts
Ident. Scalp sensations
Coil tilted at 90°
Yes (½ of verum)
No
Yes
Sham weaker
Stim. at different site
Yes (same as verum)
No
Yes
Depends on the alternative stimulation
site
Special sham coil (Ruohonen et
al. [3])
No (center of coil)
Yes (at edges ½ of verum)
Yes
Yes
Approx. Yes
Special sham coil (e.g. Magstim
[4])
No
Yes (but exchange coil between
sham and verum stim)
No
No
Coil complex
described here
(1/8 of verum)
Yes
Yes
No (vibrations are comparable,
sensations can be achived by sync.
electric surface stim.)
Sham Condition
Methods: Two planar figure-of-eight coils are attached back-to-back (see Fig. 1).
Between the coils, a mu-metal shield is inserted and fixed with spacers (2 cm distance
from shield to each coil). For verum stimulation, the coil adjacent to the subject‘s head
is triggered and for sham stimulation the coil facing into the air. To probe the physical
properties of this coil complex, we measured the induced voltages (Upp) in a pair of
wireloops located in front of the verum coil, for the sham and verum stimulation with
and without the mu-metal shield (fixed stimulator output of 40% maximal output).
Results: In Figure 2 the induced voltages for the four
conditions are displayed. Without magnetic shield, the ratio of
sham to verum stimulation is 1:5. Using the magnetic shield
the ratio is reduced to 1:8. There is a slight decrease in
stimulation power during verum stimulation with the shield
(approx. 20% smaller) as opposed to verum stimulation
without shield.
Discussion: Sham TMS is a tradeoff between competing
quality criteria. So far, the applied sham conditions
accepted a reduced cortex stimulation effect to imitate the
sensations of verum stimulation. We here focused on the
development of a sham condition with only minor cortex
stimulation. This was achieved by using a mu-metal shield
between a double-coil arrangement. With two coils of the
same type and a rigid joint between them, the acoustic
artifacts and the vibrations felt on the skin are nearly
identical for sham and verum stimulation. The only
criterion not fulfilled is the identical scalp sensation arising
from scalp muscle stimulation under verum. However, this
can be realized by synchronous electrical stimulation of
the skin [5].
Figure 1: Verum-Sham-coil complex. The side
next to head defines the verum
stimulation side (e.g. beige coil for
verum, blue coil for sham stimulation).
Between the coils is a mu-metal shield
inserted (grey)
Figure 2: Induced voltages in probecoils in front of the verum side of the coil
complex (Upp² = (Uppx² + Uppy²), Uppz (not displayed) changes in a similar way.
The reduction of maximal Upp between the verum conditions result from change
of inductance of the coils due to the mu-metal shield.
Reference List:
1. Loo CK, Taylor JL, Gandevia SC, McDarmont BN, Mitchell PB, Sachdev PS. Transcranial magnetic stimulation (TMS) in controlled treatment studies: are some "sham" forms active? Biol Psychiatry 2000; 47(4):325-331.
2. Lisanby SH, Gutman D, Luber B, Schroeder C, Sackeim HA. Sham TMS: intracerebral measurement of the induced electrical field and the induction of motor-evoked potentials. Biol Psychiatry 2001; 49(5):460-463.
3. Ruohonen J, Ollikainen M, Nikouline V, Virtanen J, Ilmoniemi RJ. Coil design for real and sham transcranial magnetic stimulation. IEEE Trans Biomed Eng 2000; 47(2):145-148
4. Magstim Company, http://www.magstim.com/Coils.html#Placebo (70mm placebo coil)
5. Okabe S, Ugawa Y, Kanazawa I. 0.2-Hz repetitive transcranial magnetic stimulation has no add-on effects as compared to a realistic sham stimulation in Parkinson's disease. Mov Disord 2003; 18(4):382-388
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