Biomedical Engineering Reference

In-Depth Information

at the hot-spot again and rotated the coil to different orientations where we perform

the motor threshold estimation. Again, to reduce the effects of stress and varying

vigilance, we use steps of 20
and 10
to not unnecessarily prolong the session,

although we are able to use very small coil rotation steps with the robotized TMS

system. With our setup, each session lasts approximately 1.5 h. For session 1, we

rotate the coil clockwise from 0
to 80
in mixed steps of 20
and 10
, resulting in

orientations of 0
,20
,30
,40
,50
,60
and 80
. In this case, 0
denotes the

reference (left-to-right) coil orientation. For session 2, we use orientations of 160
,

180
, 200
, 210
, 220
, 230
, 240
and 260
, where 180
denotes the right-to-left

coil orientation, used as reference for session 2. The coil orientations are ran-

domized for each stimulation.

For realization of the experiment, an robot operator is responsible for accurate

coil placement with the robotized TMS system and an investigator performs the

actual stimulation with MEP recordings. A double-blind experiment is performed

for MT estimation with only the robot operator knowing the actual coil orientation.

Subject and investigator have no knowledge of the orientation. Therefore, the

investigator sits in reverse to the TMS robot.

3.1.1.3 Further Analysis

Due to biphasic stimulation [
24
], we can expect having a sinusoidal relation with two

minima between coil orientation a and motor threshold. This sinusoid should have

period p (as opposed to 2p which would be trivial) with the global minimum roughly

at p (standard orientation) and the second minimum approximately at 0 (left-to-right

orientation). Due to different slopes of the coil current pulse, the MT at p should be

smaller than the MT at 0. Therefore, a second sinusoid with period 2p should be

added to express the change of the amplitude which is orientation dependent.

Therefore, the sinusoidal relation should have the form

MT
ð
a
Þ¼
a
þ
b
cos
ð
2
a
þ
c
Þþ
d
cos
ð
a
þ
e
Þ
;

ð
3
:
1
Þ

where a
;
b
;
c
;
d
;
e are constant factors. We therefore fit the experimental data to this

sinusoidal relation with nonlinear regression and estimate the error of the fit. The

fitting is performed using MATLAB (The MathWorks, Inc., Natick, MA, USA).

As a quantitative measure for the goodness of the sinusoidal fitting, we use the

coefficient of determination R
2
. It is defined as:

R
2
¼
1
P
y
i
f
i

Þ
2

ð

ð
3
:
2
Þ

P
y
i
y

Þ
2
;

ð

where y
i
represents the es
ti
mated MTs for a given coil rotation i, f
i
is the value of

the sinusoidal fit at i, and y symbolizes the arithmetic mean of the estimated MTs.

Minimal thresholds and thresholds at standard orientation were compared with

a repeated measures t-test using MATLAB. Similarly, the optimal coil orientation

is compared to the standard coil orientation.

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