Intersubject Normalization for Group Analyses in fMRI 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 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: That’s TalAIRach, not TAILarach!

Rotate brain into ACPC plane Find anterior commisure (AC) Corpus Callosum Fornix Find posterior commisure (PC) ACPC line = horizontal axis Pineal Body “bent asparagus” Note: official Tal says to use top of AC and bottom of PC but I suspect few people actually do this 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 ACPC=0 y>0 y<0 z y AC=0 y>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 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?!!! L R R L Neurologic (i.e. sensible) convention left is left, right is right L R x = 0 - + Note: Make sure you know what your magnet and software are doing before publishing left/right info! Radiologic (i.e. stupid) convention left is right, right is left R L 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) white matter (axons) FUNDUS BANK GYRUS SULCUS pial surface gray/white border SULCUS FISSURE GYRUS 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 Advantages surfaces are topologically more accurate 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 Movie: unfoldorig.mpeg http://cogsci.ucsd.edu/~sereno/unfoldorig.mpg Source: Marty Sereno’s web page

Cortical Flattening Source: Brain Voyager Getting Started Guide 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

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 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