Intersubject Normalization for Group Analyses in fMRI

1
Intersubject Normalizationfor Group Analyses in
fMRI
Jody Culham Department of Psychology University
of Western Ontario
http//www.fmri4newbies.com/
Last Update November 29, 2008 fMRI Course,
Louvain, Belgium
2
Brains are Heterogeneous
Slide from Duke course
3
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
4
Talairach Coordinate System

• Talairach Tournoux, 1988
• made an atlas based on one brain
• any brain can be squished or stretched to fit
  hers and locations can be described using a 3D
  coordinate system (x, y, z)

from an alcoholic old lady
Note Thats TalAIRach, not TAILarach!
5
Rotate brain into ACPC plane
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
Source Duvernoy, 1999
6
Deform brain into Talairach space

• Mark 8 points in the brain
• anterior commisure
• posterior commisure
• front
• back
• top
• bottom (of temporal lobe)
• left
• right

Extract 3 coordinates
7
Do We need a Tarailalch Atras?
8
Smoothing and Averaging
Large activations across multiple subjects are
more likely to show common activation than small
ones --gt Need to smooth (esp. for RFX analyses)
UNDER CONSTRUCTION Need to make a slide that
shows this with real data
9
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 dont fit well)
• activation foci can vary considerably other
  landmarks like sulci may be more reliable

10
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/matt
hew.brett/abstracts/MNITal/mniposter.pdf
11
Left is what?!!!
Note Make sure you know what your magnet and
software are doing before publishing left/right
info!
Note If youre really unsure which side is
which, tape a vitamin E capsule to the one side
of the subjects head. It will show up on the
anatomical image.
12
Brodmanns Areas
Brodmann (1905) Based on cytoarchitectonics
study of differences in cortical layers between
areas Most common delineation of cortical
areas More recent schemes subdivide Brodmanns
areas into many smaller regions Monkey and human
Brodmanns areas not necessarily homologous
13
Anatomical LocalizationSulci and Gyri
gray/white border
pial surface
SULCUS
FISSURE
GYRUS
Source Ludwig Klingler, 1956, in Tamraz
Comair, 2000
14
Variability of Sulci
Source Szikla et al., 1977, in Tamraz Comair,
2000
15
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

16
Cortical Surfaces
segment gray-white matter boundary
inflate cortical surface
render cortical surface
sulci concave dark gray gyri convex light
gray

• Advantages
• surfaces are topologically more accurate
• alignment across sessions and experiments allows
  task comparisons

17
Cortical Inflation Movie
Movie unfoldorig.mpeg http//cogsci.ucsd.edu/ser
eno/unfoldorig.mpg Source Marty Serenos web
page
18
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
4) correct for the distortions so that the true
cortical distances are preseved
Source Brain Voyager Getting Started Guide
19
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
subjects data to template Cite latitude
longitude of spherical coordinates
Movie brain2ellipse.mpeg http//cogsci.ucsd.edu/
sereno/coord1.mpg Source Marty Serenos web page
Source Fischl et al., 1999
20
Spherical Averaging
Source MIT HST583 online course notes