Rapid Self-Paced Event- Related Functional MRI: Feasibility and Implications of Stimulus- versus...
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Transcript of Rapid Self-Paced Event- Related Functional MRI: Feasibility and Implications of Stimulus- versus...
Rapid Self-Paced Event-Related Functional MRI: Feasibility and Implications of Stimulus- versus Response-
Locked Timing
Maccotta, Zacks & Buckner, 2001
Outline
This is a methods paper Problems with other fMRI methods A new Method! Rapid self-paced
event-related fMRI Empirical test Conclusions
Blocked designs
All trials in a given block are of the same type
Can’t see what happens on individual trials
Strategies could be employed Some questions require that events
happen at unpredictable times
Event-Related Designs
Allow you to examine single trials Stimulus onset is locked to
beginning of a TR (repetition time) Require a lot of down-time for
BOLD response to return to baseline
Rapid Event-Related Designs
Dale and Buckner, 1997 Bold signals are additive You don’t need to wait for the
BOLD signal to return to baseline before the next trial
But trials still began at the beginning of a TR
Joseph et al. (1997)
In the context of event-related designs, the stimulus does not have to be time locked to image acquisition
Gives temporal resolution better than TR
Over-samples the hemodynamic function
The questions here
Can a self-paced event-related fMRI:• successfully detect task correlated
activation during trials that are fully self-paced by the subject?
• Precisely characterize the hemodynamic response?
The Subjects
17 right handers 8 males no history of significant neurological
or psychiatric problems What exactly qualifies a NON-
significant neurological or psychiatric problem?
Imaging Stuff
Scanner: 1.5 T Coil type: Circularly polarized head
coil Restraints: Thermoplastic face
mask and foam cushions
Structural Images
Resolution: 1.25 x 1 x 1 mm T1 weighted TR (Repetition time) = 9.7 ms TE (Time to echo) = 4 ms flip angle = 10º TI = 20 ms TD = 500 ms
Functional Images
Echo-planar asymmetric spin-echo sequence
TR (repetition time)= 2.36 s TE (Time to echo)= 37 ms Resolution: 3.75 x 3.75 mm Flip angle 90º
Functional Images
Each image acquisition consisted of 16 contiguous, 8 mm-thick axial images parallel to the anterior-posterior commisural plane
There were 128 image acquisitions per run
Each run took 5 minutes (4 runs per subject)
The Cognitive Paradigm
Mental rotation A pair of human stick figures were
presented left and right of fixation Each figure was rotated by some
amount (in 30º increments) Figures could be same or mirror
images
The Cognitive Paradigm (cont’d)
A left or a right hand keypress was required (counterbalanced across subjects)
Stimuli remained on-screen until a response was made
750 ms later, the next trial began
Logic
The task is not important The main thing here is test the
feasibility of doing this Left vs. Right respond hand should
light up known areas Manipulating the amount of rotation
will manipulation response time
Functional Data Preprocessing
Rigid-body rotation and translation was performed to correct for motion (Snyder, 1996)
Images were translated into standardized atlas space (Talairach & Tournoux, 1988)
Over-Sampling
Stimulus onset could happen at any point in a TR (0 - 2360 ms from onset)
This means you get over-sampling of the hemodynamic response function
Usually stimulus onsets occur at integer multiples of TR
Over-Sampling
This means your smallest temporal bin that’s useful is TR
But if stimulus onsets can occur anywhere over TR, you can get smaller temporal bins
With rapid presentation and smaller temporal bins, you should be able to get better temporal resolution
Proportion of stimulus onsets/bin
0
0.2
0.4
0.6
0.8
1
0 - 590 591 - 1180 1181 - 1770 1771 - 2360
Time from beginning of TR (ms)
Prop
ortio
n of
st
imul
us o
nset
s
Proportion of stimulus onsets/bin
0
0.2
0.4
0.6
0.8
1
0 - 590 591 - 1180 1181 - 1770 1771 - 2360
Time from beginning of TR (ms)
Prop
ortio
n of
st
imul
us o
nset
s
Over-Sampling
To take advantage of this, each trial was rebinned to the time bin closest to the true trial onset
Bin size was varied from TR, TR/2 to TR/4 to see what would happen
Statistical Map Generation
Trials were sorted and averaged according to response hand (Left vs. Right) and response time (Slow, Medium, Fast)
Statistical Map Generation
For each voxel, difference time courses between trial types were generated (Left - Right)
This was then regressed with a set of idealized hemodynamic response curves
Statistical Map Generation
This regression represents a difference between conditions
Because conditions only differ in response hand (left vs. right) activation is expected in motor areas
Regions of Interest
Six regions of interests were located:• right and left motor cortex;• right and left supplementary motor
area (SMA), and;• right and left cerebellar cortex
Regions of Interest
ROIs were identified by 19 or more suprathreshold (Z > 3.3) 8 mm3 cubic voxels
Most significant peaks in 12 mm radius were kept
ROIs were defined to include all voxels within 12 mm of a peak
Regions of Interest
Time courses for each trial type were extracted for each region of interest, averaging across all voxels within the region
Time course differences between trial types were then generated, which gave the BOLD hemodynamic response associated with each trial type comparison
The Hemodynamic Response
Three parameters were estimated:• Amplitude• Time-to-onset• Time-to-peak
Sig
nal
Cha
nge
Time
Time-to-onset
Time-to-peak
Amplitude
Behavioural Results
518 mean correct responses/session 92 % correct (72 - 98 %) RT(Rhand) = 1303 ms SD = 666 ms RT(Lhand) = 1348 ms SD = 792 ms
Imaging Results
Initial analysis was done to see if the procedure could get accurate maps of task correlated activation
Left - Right and Right - Left trials were examined, with bin sizes of TR, collapsed across response speed
Imaging Results
Observed activation corresponded to the left and right motor networks (motor cortex, SMA and cerebellar cortex), as expected
Results for time-to-onset (2 s) and time-to-peak (4 - 6 s) are similar to those from fixed-pace fMRI studies
Temporal Sampling of BOLD
Was the over-sampling of the hemodynamic response even across the TR?
Yep.
Temporal Resolution
Does using smaller bins (TR/2 or TR/4) still allow for estimates of hemodynamic response?
Yep. And the precision is better, because the bin size is smaller
Regardless of bin size, the hemodynamic response function can be modeled by a gamma function
Hemodynamic Response Timing and Behavioural
Response Timing What happen to the hemodynamic
response (for motor cortex) as the time taken to perform the task increases?
Slow responses are associated with longer time-to-onsets, time-to-peaks and smaller amplitudes
Response-Locked Timing
You can measure time-to-onset, time-to-peak and amplitude of the hemodynamic response from both the stimulus onset and the response onset
This can tell you if a manipulation has it’s effect before or after activation reaches a site
Response-Locked Timing
Remember that trials were binned according to response time
Slower trials were probably caused by a requirement for more mental rotation
This processing should occur before neural activity hits motor cortex
Response-Locked Timing
If this is the case, hemodynamic response parameters should be invariant across behavioural response times when response-locked
This more or less happened, but there was some differences still
Summary
You can get robust hemodynamic response estimates from rapid, arbitrarily timed events in an event-related fMRI paradigm
Multiple regions known to be active during motor response execution were showed significant activation
Summary
This was true at the group and individual level
Results paralleled those from fixed-pace studies
presentation rate is not dictated by TR
Presentation rate can be less than TR
Summary
Self-paced paradigms produced even sampling of the hemodynamic response across TR, which allows for better temporal resolution (resolution better than TR) of the hemodynamic response