Biomedical Engineering Reference
In-Depth Information
M
P
is used as a measure of the strength of the PAC. The surrogate-data method,
of
This
M
P
.
9.5.3 Results of Local PAC Analysis
9.5.3.1 Time Course Estimation and Spatial Mapping
of the Modulation Index
We first analyzed local PAC (lPAC), and computed the modulation index time courses
for all the sub-bands of the LF signal at all voxel locations. The bandwidth of the
HF signal was fixed to 75
25 Hz. The resultant time courses were classified using
K-means clustering analysis, which thereby found three types of clusters. The first
kind of the cluster, called type I, consists of the time courses that peaked before the
onset of the hand movements. The time course averaged across all the type I time
courses is shown in Fig.
9.2
a. We computed
±
M
P
in Eq. (
9.6
) using the type I time
courses, and assessed the statistical significance of
M
P
at all voxel locations. Voxels
containing statistically significant lPAC activity were found with the LF signal of
12
5 Hz near the primary motor area. These voxels are shown in Fig.
9.2
b.
The second kind of the modulation index time courses, called type II, peaked
immediately before or during the hand movements. The mean time course of this
type is shown in Fig.
9.2
c. Voxels containing statistically significant
.
5
±
0
.
M
P
(computed
using the type II time courses) were found with the LF signal of 10
5 Hz near
the lateral part of the precentral area, as shown in Fig.
9.2
d. The third kind of time
courses, type III, peaked after the hand movements. The mean time course is shown
in Fig.
9.2
e. Voxels containing significant
.
5
±
0
.
M
P
(computed using the type III time
courses) were found with the LF signal of 12
5 Hz near the postcentral or
central sulcus areas. These voxels are shown in Fig.
9.2
f.
In summary, we found significant lPAC activities near the contralateral precentral
area before the execution. However, following the execution, lPAC activities were
found near the postcentral area and central sulcus. These findings may indicate that
a certain relationship exists between these three types of lPAC activities, and the
preparation and rebound of the motor and sensory systems.
.
5
±
0
.
9.5.3.2 Amplitude-Phase Diagram and Phase-Informed TF Map
An amplitude-phase diagram was derived using a voxel time course in the area
labeled as Region A in Fig.
9.3
a. Note that Region A shows significant lPAC activity
showing the type I time course. In this derivation, the LF signal was set to 12
.
5
±
0
.
5 Hz, and the HF signal to 75
±
25 Hz. The data in the time window from
−
4
.
5
to
2.5 s (when the type-I lPAC was nearly maximum) was used. The resultant
amplitude-phase diagram is shown in Fig.
9.3
b in which each bar indicates the mean
−
