Anatomy
• What is the difference between Structural Anatomy
and Functional Anatomy?
• What roles do each play in our understanding of the
brain?
Structural Anatomy
• Brain structures are identified in a hierarchical
fashion (with substantial randomness)
• Heispheres -> Lobes -> Sulci & Gyri
• Sulci and Gyri are all named
– but somewhat variable across individuals
Structural Anatomy
• Brodmann Areas defined by cytoarchitecture
– map of variations in cellular morphology
Connectivity
• Anatomists are concerned with brain regions and
how they are interconnected
• Interconnectedness occurs at various levels:
–
–
–
–
interneurons
cortico-cortical connections
thalamo-cortical and cortico-thalamic
afferent = “to” (e.g. sensory) and efferent = “from” (e.g.
motor)
Connectivity
• How do anatomists study connectivity?
– Retrograde Tracers (e.g. horseradish peroxidase) follow
axons back to where they came from
– Anterograde Tracers (e.g. dextran) follow axons to where
they are going
• Diffusion Tensor Imaging
(DTI)
– MRI Technique that traces long
white matter tracts
Connectivity
• “Ascending” and “descending” projections in sensory
systems
– estimate: for every ascending projection there are ten
descending projections
Connectivity
• “Ascending” and “descending” projections in sensory
systems
– estimate: for every ascending projection there are ten
descending projections
Why would we have descending projections?
Connectivity
• It is the inter-connectivity of the brain that (probably)
allows it to perform the vastly complex processes of
cognition
Structural and Functional
Imaging
• There are a number of well known techniques to
create images of brain anatomy
– CAT scan, MRI, X-Ray,
• Note however that structural and functional images
are not the same thing!
Structural and Functional
Imaging
• There are a number of well known techniques to
create images of brain anatomy
– CAT scan, MRI, X-Ray,
• Note however that structural and functional images
are not the same thing!
• Which is more useful?
Structural and Functional
Imaging
• This is a Functional MRI Image !?
Structural and Functional
Imaging
• This is a structural MRI image (an “anatomical” image)
Structural and Functional
Imaging
• What you really want is both images co-registered
Structural and Functional
Imaging
• What you really want is both images co-registered
• Why? What’s wrong with the functional image alone?
Structural and Functional
Imaging
• Functional images tend to be lower resolution and fail
to convey spatial information
Pixels
Structural and Functional
Imaging
• Structural images have finer (smaller) pixels
Pixels
Structural and Functional
Imaging
• Brain scans (CAT, PET, MRI, fMRI) are all made up of
pixels (stands for picture elements)
Pixels
Structural and Functional
Imaging
• “Slices” are assembled into “volumes”
Pixels
Structural and Functional
Imaging
• Volumes are composed of “volume elements” or
voxels
Voxels
Structural and Functional
Imaging
• Another thing you want: the ability to tell other
people where something is
– “the activity was centered on voxel #653” will not work in a
scientific journal
Structural and Functional
Imaging
• MRI anatomical spaces
– Talairach Space:
• Based on detailed analysis of one elderly woman
• Talairach & Tournoux (1988)
– Montreal Neurological Institute Template (MNI)
• based on average of 152 different brains, each normalized to
Talairach space
• advantage: gyri and sulci are more representative
• disadvantage: it’s blurry
– MNI “Representative Brain”
• the one brain from the 152 in the MNI Template set that is most
like the average
• advantage: it’s not blurry
• disadvantage: it’s still just one brain
Structural and Functional
Imaging
• Reasons for normalizing to standard stereotaxic
space (templates)
– two levels: within-subject and between-subjects
Structural and Functional
Imaging
• Within-Subject Reasons:
1. structural and functional volumes may not be
coregistered due to
•
•
movement
distortion
2. results can be described in standard coordinates
3. data across sessions can be averaged
Structural and Functional
Imaging
• Between-Subject Reasons:
1. Volumes will not match because of variability across
individuals
2. results can be described in standard coordinates
3. data across participants can be averaged
Preprocessing of Structural
and Functional Images
•
Normalizing images to fit a standard template
(e.g. Talairach)
1. Define Coordinate System using easily
recognizable landmarks
•
•
•
•
Origin in the Anterior Commissure
y-axis connects AC and PC
x-axis perpendicular interhemispheric plane and
through AC
z-axis perpendicular to x and y
The Talairach Coordinate
System
PC
AC
The Talairach Coordinate
System
-y
AC - PC
line defines
y-axis
+y
The Talairach Coordinate
System
-y
-x
x-axis
perpendicular to
interhemispheric
plane
+y
+x
The Talairach Coordinate
System
+z
-y
-x
z-axis
perpendicular to
x-y plane
-z
+y
+x
Next time: Cortical Flattening