Transcript Document

459.5
Introduction
EEG Results
Combing Imaging Modalities
We study the transition from discrete to continuous
movements by investigating brain activity at 21 different
movement rates ranging from 0.5 to 2.5Hz. For this
purpose two separate EEG and MEG experiments were
performed using a continuation paradigm. One of the
subjects also participated in an fMRI study with a
similar experimental setup. Independent component
analysis (ICA) was used to separate the spatiotemporal
signals resulting in either two or three different response
types for EEG and MEG, respectively. Using the graywhite matter boundary extracted from an MRI scan as
constraint for the locations and directions of primary
currents we localized the origins of two of the three
MEG response types and found a division of the region
of activation identified with fMRI.
ICA decomposition consistently revealed two types of
spatiotemporal response patterns for all subjects. Samples for
each type are shown below for one subject.
Due to the columnar organization of the cortex, the primary
currents flow in a direction perpendicular to the boundary
between the gray and the white matter. This surface can be
extracted from T1 weighted MRI scans and serve as a constraint
for the calculation of inverse solutions.
Methods
Fullhead EEG and MEG recording: 84 electrode EEG system
SAM Technologies, San Francisco, CA), 143 SQuID MEG
system (CTF Inc., Port Coquitlam, BC);
Behavior was measured as pressure changes in an air cushion
placed under the right index finger;
Continuation paradigm: 20 cycles synchronization with an
auditory metronome, 20 cycles continuation at the same rate
without metronome;
21 different rates: 0.5-2.5Hz in steps of 0.1Hz;
4 subjects in each experiment;
Blind source separation was performed using the ICA Toolbox
from the Swartz Center for Computational Neuroscience at
UCSD;
Freesurfer was used to construct the surface that represents the
gray-white matter boundary;
fMRI from the same subject was used to identify sensory-motor
areas;
Regions of activity were computed as overlap between forward
solutions from the surface normals and patterns extracted from
MEG data;
First response type:
Peak latencies are consistent with
the well-known sequence of motor
evoked potentials. Peak amplitudes
are relatively independent of
movement rate.
different stimulus rates;
Left: The MRI scan is transformed such that the landmarks
Bottom: Grand average of the time series across all rates
on the subject’s head (nasion, and left and right preauriculars)
are located in the same slice);
(red) and movement profile (green). A black vertical line
indicates the point of peak finger flexion.
S3
S4
1.5
S1 SD
S2 SD
1
S3 SD
S4 SD
0.5
0
Produced Rate (Hz)
Produced Rate (Hz)
S2
2
S1
S2
2
S3
S4
1.5
0.5
1
1.5
2
Required Rate (Hz)
2.5
3
Second response type:
Active brain areas found in fMRI for the subject shown in the
MEG results during performance of a similar continuation task
(left). Activity is seen in sensory-motor cortex and SMA. On
the right activation is constraint to the gray-white matter
boundary. The same data overlaid on a 3d-reconstruction of
this surface is shown below.
Similar type one and type two responses are found in MEG
shown on the left and right, respectively.
S4 SD
0.5
1
1.5
2
locations and directions of the primary currents in the cortex.
MEG Results
S3 SD
0.5
Middle: The boundary between the gray and white matter;
The amplitude of this pattern
shows a systematic dependence on
movement rate. Strong activity
exists only at low movement
frequencies and the pattern
vanished at higher rates.
S2 SD
1
0
Type III
Right: The normal vectors for this surface, representing the
S1 SD
0
0
A strong correlation
between the patterns
identified as MEG
responses I and III
with the forward
solution on the gray
white matter surface
determines possible
source locations. The
figures on the left
indicate a lateralmedial separation of
the type I and III
responses. Note that
together the areas of
activation
coincide
with the regions
identified in fMRI.
Conclusions
ICA is an effective method for separating temporally
and spatially overlapping responses in complex
behavioral experiments with consistency across subjects
and modalities.
The dropout of slow wave activity seen in response type
II at higher rates indicates a simplification of the
movement related activity and may be associated with a
transition between discrete and continuous movements.
Using the gray-white matter surface as a constraint for
the location and direction of source currents produces
anatomically meaningful areas of activity that are
consistent with results from fMRI.
The distinct split between the origins of response type I
and III, and the difference of their activation profile in
time shows that combining imaging modalities strongly
enhances the spatial and temporal resolution of brain
activity recordings.
References
2.5
S1
x
Middle: Color coded amplitude of the average time series at
3
2.5
Type I
Steps towards the combination of modalities:
MEG Continuation
3
y
Top: Spatial pattern for each type of response;
Task Performance
EEG Continuation
Origin of MEG Responses Type I and III
2.5
3
Required Rate (Hz)
Produced versus required rate during continuation. All subjects
performed the task adequately, providing at least 50 good cycles
for averaging at all rates. Good cycles are those within an interresponse interval of two standard deviations from the mean at
each rate.
In MEG a third type of response is
found in all subjects. The dipolar
pattern
is
shifted
medially
compared to the type one response,
the
amplitude
is
relatively
independent of movement rate and
a peaks during the extension phase
of the movement cycle.
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Acknowledgement
Work supported by NINDS (grant NS39845), NIMH (grants MH42900 and 19116)
and the Human Frontier Science Program.