Transcript Variability of HRF - Department of Psychology

Jody Culham
Department of Psychology
University of Western Ontario
http://www.fmri4newbies.com/
Intersubject Normalization
for Group Analyses in fMRI
Last Update: November 29, 2008
fMRI Course, Louvain, Belgium
Brains are Heterogeneous
Slide from Duke course
How can we define regions?
Talairach coordinates
• Example: The FFA is at x = 40, y = -55, z = -10
Anatomical localization
• Example: The FFA is in the right fusiform
gyrus at the level of the occipitotemporal
junction
Functional localization
• Example: The FFA includes all voxels around
the fusiform gyrus that are activated by the
comparison between faces and objects
Kanwisher, McDermott & Chun,
1997, J Neurosci
Talairach Coordinate System
Talairach & Tournoux, 1988
• made an atlas based on one brain
… from an alcoholic old lady
Note: That’s TalAIRach, not TAILarach!
• any brain can be squished or stretched to
fit hers and locations can be described
using a 3D coordinate system (x, y, z)
Rotate brain into ACPC plane
Corpus Callosum
Fornix
Find anterior commisure (AC)
Find posterior commisure (PC)
ACPC line
= horizontal axis
Note: official Tal says to use
top of AC and bottom of PC
but I suspect few people
actually do this
Pineal Body
“bent asparagus”
Source: Duvernoy, 1999
Deform brain into Talairach space
Mark 8 points in the brain:
• anterior commisure
• posterior commisure
• front
• back
• top
• bottom (of temporal lobe)
• left
• right
Squish or stretch brain to fit in “shoebox”
of Tal system
y<0
AC=0
y
y>0
z
y>0
ACPC=0
y<0
x
Extract 3 coordinates
Do We need a Tarailalch Atras?
Variability between Japanese
and European brains, both male
(red > yellow > green > blue)
Variability between male and female brains,
both European
(red > yellow > green > blue)
Source: Zilles et al., 2001, NeuroImage
Smoothing and Averaging
Large activations across multiple subjects are more likely
to show common activation than small ones
-->
Need to smooth (esp. for RFX analyses)
UNDER CONSTRUCTION: Need to make a slide that shows this with real data
Talairach Pros and Cons
Advantages
• widespread system
• allows averaging of fMRI data between subjects
• allows researchers to compare activation foci
• relatively easy to use
Disadvantages
• not appropriate for all brains (e.g., Japanese brains don’t fit well)
• activation foci can vary considerably – other landmarks like sulci
may be more reliable
MNI Space
• Researchers at the Montreal Neurological Institute created a better
template based on a morphed average of hundreds of brains (not just
one brain like Talairach)
• The MNI brain is more representative of average brain shape; however,
it does not provide Brodmann areas
• The MNI alignment is more complex than Talairach: SPM uses it but
many software packages still use Talairach
• CAVEAT: The MNI and Talairach coordinate are similar but not identical - careful comparison requires a transformation
Source: http://www.mrc-cbu.cam.ac.uk/personal/matthew.brett/abstracts/MNITal/mniposter.pdf
Left is what?!!!
Neurologic (i.e. sensible) convention
• left is left, right is right
L
R
-
Note: Make sure you know what your
magnet and software are doing before
publishing left/right info!
+
x=0
Radiologic (i.e. stupid) convention
• left is right, right is left
R
L
+
x=0
Note: If you’re really unsure which
side is which, tape a vitamin E
capsule to the one side of the
subject’s head. It will show up on
the anatomical image.
Brodmann’s Areas
Brodmann (1905):
Based on cytoarchitectonics: study of
differences in cortical layers between areas
Most common delineation of cortical areas
More recent schemes subdivide
Brodmann’s areas into many smaller
regions
Monkey and human Brodmann’s areas not
necessarily homologous
Anatomical Localization
Sulci and Gyri
gray matter
(dendrites & synapses)
BANK
white matter
(axons)
FISSURE
FUNDUS
Source: Ludwig & Klingler, 1956,
in Tamraz & Comair, 2000
Variability of Sulci
Source: Szikla et al., 1977, in Tamraz & Comair, 2000
Variability of Functional Areas
Watson et al., 1995
- motion-selective area, MT+ (=V5) is quite variable in stereotaxic space
- however, the area is quite consistent in its location relative to sulci
- junction of inferior temporal sulcus and lateral occipital sulcus
- see also Dumoulin et al., 2000
Cortical Surfaces
segment gray-white
matter boundary
render cortical surface
inflate cortical surface
sulci = concave = dark gray
gyri = convex = light gray
Advantages
• surfaces are topologically more accurate
• alignment across sessions and experiments allows task comparisons
Cortical Inflation Movie
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Movie: unfoldorig.mpeg
http://cogsci.ucsd.edu/~sereno/unfoldorig.mpg
Source: Marty Sereno’s web page
Cortical Flattening
2) make cuts along
the medial surface
(Note, one cut
typically goes along
the fundus of the
calcarine sulcus
though in this
example the cut was
placed below)
1) inflate the brain
3) unfold the medial
surface so the
cortical surface lies
flat
Source: Brain Voyager Getting Started Guide
4) correct for the
distortions so that the
true cortical distances
are preseved
Spherical Averaging
Future directions of fMRI: Use cortical
surface mapping coordinates
Inflate the brain into a sphere
Use sulci and/or functional areas to match
subject’s data to template
Cite “latitude” & “longitude” of spherical
coordinates
QuickTime™ and a
YUV420 codec decompressor
are needed to see this picture.
Movie: brain2ellipse.mpeg
http://cogsci.ucsd.edu/~sereno/coord1.mpg
Source: Marty Sereno’s web page
Source: Fischl et al., 1999
Spherical Averaging
Source: MIT HST583 online course notes