Biomedical Engineering Reference
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test
8
[Hesse et al., 2003] and by [Astolfi et al., 2008].
An example of application of time-varying formulation of PDC was a study of a
foot movement task [De Vico Fallani et al., 2008]. Time-dependent MVAR param-
eter matrices were estimated by means of the RLS. The MVAR model was fitted
to the signals representing the current density on cortex found by solving the linear
inverse problem [Grave de Peralta Mendez and Gonzales Andino, 1999]. The dense
connections obtained between 16 regions of interest formed a complicated pattern
and to find the meaningful transmissions between the regions of interest the theo-
retical graph indexes [Newman, 2003] have to be applied. The directed connectivity
measures found by means of the projection of EEG activity on cortex represent some
causality measures, but they don't reflect the actual propagation of EEG waves, since
in the procedure of inverse projection the information on the phase difference be-
tween channels is lost. Taking into account the fact that DTF and PDC estimators are
practically not influenced by a volume conduction (which is zero phase propagation)
the operation of finding the current density on cortex seems to be unnecessary.
The determination of the dynamic propagation during performance of finger
movement and its imagination [Ginter et al., 2001, Ginter et al., 2005, Kus et al.,
2006] may serve as an example of the application of short-time DTF to the scalp
EEG. The established evidence [Pfurtscheller and Lopes da Silva, 1999] is that dur-
ing the movement the decrease of activity in alpha and beta bands (ERD) in the pri-
mary motor area corresponding to the given part of the body is observed, and increase
of beta activity called beta rebound follows. In the gamma band, brief increase dur-
ing movement was reported [Pfurtscheller and Lopes da Silva, 1999]. These findings
corresponded very well with the results obtained by means of SDTF.
Figure 4.21
shows the SDTFs as functions of time and frequency for a right finger movement
experiment. The gaps in the propagation of alpha and beta activity for electrodes
overlying left motor cortex (most pronounced for electrode C3) and subsequent fast
increase of propagation in beta band more frontally are compatible with spectrally,
temporally, and topographically specific ERD/ERS phenomena.
By virtue of selectivity of SDTF to phase difference (neglecting zero phase
or random phase dependencies) the evolution in the gamma band may be stud-
ied (after filtering of prevailing alpha activity). In
Figure 4.22
the EEG propaga-
tion in the gamma band during right finger movement and its imagination, ob-
tained by SDTF, is shown. In case of movement the short burst of gamma propa-
gation from C3 followed by propagation from frontal electrodes was observed. In
case of movement imagination this propagation started later and a cross-talk be-
tween different sites overlying motor area and other sensorimotor areas may be no-
ticed. (The dynamics of propagation may be observed in animations available at:
compatible with the notion that a more difficult task requires involvement of the sev-
eral sensorimotor areas and it is in agreement with neurophysiological hypotheses
8
In the Stroop test congruent and non congruent stimuli are compared, e.g., word blue written in blue with
the word blue written in red.
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