Biomedical Engineering Reference
In-Depth Information
The relationship between the HF signal amplitude and the LF signal phase can be
seen more clearly in the phase-informed TF map in Fig.
9.3
c. Here, the same voxel
data from Region A has been used. The phase-informed TF map intuitively shows the
preference of the HF signal amplitude to a specific phase of the LF signal. The map
clearly indicates that the amplitude of the HF signal from 60 to 100 Hz significantly
increases near
150
◦
of the phase of the LF signal.
−
9.5.3.3 LPAC Temporal Trend and Comparison to Event-Related
Power Changes
Event-related power changes such as desynchronization (ERD) and synchronization
(ERS) are representative aspects of brain activities. Comparison between PAC tem-
poral trends and ERD/ERS should provide useful insights into brain information
processing. Using the lPAC signal from Region A, we computed modulation index
time courses with all 11 sub-bands of the LF signal. The HF signal was fixed to the
frequency band of 75
25 Hz. The results are shown in Fig.
9.4
a. In this figure, the
sizes of the circles represent the strength of the modulation index, and their colors
represent the LF sub-bands.
The results in Fig.
9.4
a show that LF signals at two sub-bands, 10
±
±
0
.
5 and
12
5 Hz, have significantly large modulation index values. The lPAC with LF
signal of 12
.
5
±
0
.
.
5
±
0
.
5 Hz reaches its maximum between
−
5 and
−
3s but the lPAC
±
.
−
.
+
.
with the LF signal of 10
5s.
This observation suggests the presence of multiple brain rhythm scales underlying
motor planning and execution.
A time-frequency representation of the broadband MEG signal from Region A is
showninFig.
9.4
b. In this figure, we can observe that the ERD whose center frequency
is approximately equal to 12.5 Hz starts near
0
5 Hz reaches its maximum between
0
5 and
0
−
3 s. On the other hand, Fig.
9.4
ashows
that lPAC with LF of 12
.
5
±
0
.
5 Hz starts decreasing around
−
3 s. Comparison
between Fig.
9.4
a, b shows that a strong 12.5 Hz ERS starts at
+
3
.
5 s at which the lPAC
with the LF signal of 12
5 Hz restarts. On the other hand, a stable lPAC activity
with the LF band between 9 and 11 Hz (marked by the dotted circle in Fig.
9.4
a) is
observed at the hand-movement period when a significantly strong ERD is observed at
the same frequency band near 10 Hz. Such coincidence between ERD/ERS activities
and lPAC time courses suggests that lPAC and ERD/ERS activities are interrelated.
.
5
±
0
.
9.5.4 Results of Analyzing Cross-Location PACs
Finally, the cross-location PAC (xPAC) was analyzed using the same hand-motor
MEG data. In this analysis, the LF signal is measured in voxels of the precentral
area; these voxels are labeled Voxel B in Fig.
9.5
a. The HF signal is measured at the
voxel near the primary motor area; the voxel is labeled Voxel A in Fig.
9.5
a. We first
