Supplementary material:
Surgery.
An image fusion procedure was used to obtain the stereotactic co-ordinates for surgery.
The day before surgery, a 1.5-Tesla Magnetic Resonance Image (MRI) of the brain was
obtained for each patient. On the day of surgery, once the CRW (Cosman-Robert-Wells,
Radionics, Burlington, MA) frame had been put in place under local anaesthesia, a
computerized tomography scan of the brain was obtained and the image data was fused
with the MRI using the BrainLab software. The coordinates for the STN were
determined from T2-weighted MRI images in axial planes with the target placed 4-6
mm below and 12-13 mm lateral to the intercommissural line, and 2-3 mm posterior to
the mid-intercommissural point. The motor region of the STN was defined intraoperatively by micro-recording (200-600 KΩ platinum/iridium FHC microelectrodes
[Bowdoinham, ME]) and micro-stimulation.
Coherence analysis
Coherency is defined as the normalized cross-spectrum:
C1 2( f ) 
S1 2( f )
S1 1( f )S 2 2( f )
and coherence is the absolute value of coherency:
Co h
1 (
2 f )  C1 (
2f )
In order to detect significant values of coherence, we normalized the coherence values
into Z scores by computing their inverse hyperbolic tangent and dividing them by the
square-root of their variance.
at anh
Coh1 2( f )
var
1
2L
where L is equal to the number of disjointed sections used to estimate the coherence,
giving:
Zcoh 1 2( f ) 
a t anh(Coh 1 2( f ) )
1
2L
Neuroimaging: Location of the electrode contacts
The ventral tip and an entry point of the implanted electrode were determined in the
post-MRI to calculate the CRW arc (ARC) and ring (RING) angles relative to the ACPC plane (Zylka and Sabczynski, 1999). We modelled the artefact induced by the
Medtronic 3389 electrode based on the procedure described previously (Pollo et al.,
2004). Supplementary figure 1 shows the dimensions and the overlapping of the
artefacts induced by the stimulating contacts of the Activa 3389 electrode and the
electrode itself (a=1.4mm, b=1.16 mm, c=2.3 mm, d=1.5 mm, e=0.5 mm, f=1.27 mm).
The electrode and its artefact were implemented as an Analyze image containing 512
slices in the axial plane, each 0.1 mm thick (Voxel size 0.08 x 0.08mm). A different
intensity value was assigned to each contact of the electrode (i.e.: contact 0=50; contact
1=75; contact 2=100; and contact 3=125) in order to assess the co-ordinates of all the
points at each contact independently (Figure B and C). The origin of the image was
localized at the tip of the artefact. The artefact was rotated using the origin as the pivotal
point, where the ARC was the angle rotated about the x-axis and the RING was the angle
rotated about the y-axis. The co-ordinates of the origin of the rotated artefact became
the AC-PC co-ordinates of the tip of the DBS lead. The mean of the co-ordinates of all
the points belonging to the contact gave the AC-PC co-ordinates for each contact.